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The Long-Term Conservation of the Australopithecus -bearing Member 4 Excavation Walls at the Sterkfontein Caves, South Africa

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The Sterkfontein Caves UNESCO World Heritage site represents one of South Africa’s most valuable cultural heritage resources and is one of the world’s most prolific palaeoanthropological sites with its fossiliferous deposits spanning the last 3.5 million years. One of the most famous fossil-bearing deposits at Sterkfontein is the 2.5 million-year-old Member 4. This is the world’s richest Australopithecus-bearing deposit and has yielded iconic fossils like StS 5 – Mrs Ples, StW 53, two partial skeletons, and two species of Australopithecus. After 80 years of research, Member 4 continues to provide crucial evidence for human origins research. Over the last 35 years, since excavation of the Member 4 started exposing the walls of the deposit, their deterioration has been accelerating. The implications of this deterioration and impending collapse are severe, not only from a palaeoanthropological perspective but also a heritage management point of view. This article focuses on our efforts to conserve the deteriorating areas of the Member 4 excavation site. The project required the development of a comprehensive set of strategies that had to be adapted to the specific requirements of the national and local heritage management agencies and remain sensitive to ongoing research programmes. The strategy developed included: multiscale integrative documentation of the exposed deposits; comprehensive, independent but cohesive stabilization of the different components of the deposit while maintaining visibility for ongoing and future research; and installation of stabilization infrastructure that could be adapted to the long-term conservation excavation plan while maintaining deposit integrity and site safety.
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Studies in Conservation
ISSN: 0039-3630 (Print) 2047-0584 (Online) Journal homepage: http://www.tandfonline.com/loi/ysic20
The Long-Term Conservation of the
Australopithecus-bearing Member 4 Excavation
Walls at the Sterkfontein Caves, South Africa
Dominic Justin Stratford & Matthew V. Caruana
To cite this article: Dominic Justin Stratford & Matthew V. Caruana (2018) The Long-
Term Conservation of the Australopithecus-bearing Member 4 Excavation Walls at
the Sterkfontein Caves, South Africa, Studies in Conservation, 63:4, 201-214, DOI:
10.1080/00393630.2017.1307635
To link to this article: https://doi.org/10.1080/00393630.2017.1307635
Published online: 04 Apr 2017.
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The Long-Term Conservation of the Australopithecus-bearing Member 4
Excavation Walls at the Sterkfontein Caves, South Africa
Dominic Justin Stratford and Matthew V. Caruana
School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Johannesburg, Gauteng, South Africa
ABSTRACT
The Sterkfontein Caves UNESCO World Heritage site represents one of South Africas most
valuable cultural heritage resources and is one of the worlds most prolific
palaeoanthropological sites with its fossiliferous deposits spanning the last 3.5 million years.
One of the most famous fossil-bearing deposits at Sterkfontein is the 2.5 million-year-old
Member 4. This is the worlds richest Australopithecus-bearing deposit and has yielded iconic
fossils like StS 5 Mrs Ples, StW 53, two partial skeletons, and two species of
Australopithecus. After 80 years of research, Member 4 continues to provide crucial evidence
for human origins research. Over the last 35 years, since excavation of the Member 4 started
exposing the walls of the deposit, their deterioration has been accelerating. The implications
of this deterioration and impending collapse are severe, not only from a
palaeoanthropological perspective but also a heritage management point of view. This article
focuses on our efforts to conserve the deteriorating areas of the Member 4 excavation site.
The project required the development of a comprehensive set of strategies that had to be
adapted to the specific requirements of the national and local heritage management
agencies and remain sensitive to ongoing research programmes. The strategy developed
included: multiscale integrative documentation of the exposed deposits; comprehensive,
independent but cohesive stabilization of the different components of the deposit while
maintaining visibility for ongoing and future research; and installation of stabilization
infrastructure that could be adapted to the long-term conservation excavation plan while
maintaining deposit integrity and site safety.
ARTICLE HISTORY
Received August 2016
Accepted March 2017
KEYWORDS
Sterkfontein; heritage;
Member 4; Australopithecus;
excavation; conservation
Introduction
The Sterkfontein Caves (Figure 1) represents one of the
most prolific palaeoanthropological sites in the world
with its fossiliferous deposits spanning the last three-
and-a-half million years. It is the longest running
palaeoanthropological excavation and remains an
active research site after 80 years of scientific investi-
gation. In 1999, an area described as Sterkfontein,
Swartkrans and Environswas awarded UNESCO
World Heritage status, highlighting a need to preserve
the landscape and sites for the global benefit of society.
The 47,000 Ha area that lies about 50 km northwest of
Johannesburg, South Africa (which later included the
fossil hominin sites of Makapansgat and Taung), is
known as the Cradle of Humankind World Heritage
Site (COHWHS). The Sterkfontein Caves (henceforth
Sterkfontein) is the oldest and largest research site in
the COHWHS and is divided into two entities: the
tourist concession a small museum, café, and tour
around the underground portion of the caves; and
the research site encompassing several underground
excavations and the main excavation located on the
surface (Figure 2). The surface excavations have
focused on the highly fossiliferous deposits known as
Member 4 and Member 5 (described in more detail
below).
Sterkfontein Member 4 is the worlds richest Austra-
lopithecus-bearing deposit and has yielded iconic
fossils like StS 5 Mrs Ples (Broom 1947), StW 53
(Hughes and Tobias 1977; Tobias 1978), two partial
skeletons (StS 14 and StW 431) (Broom, Robinson,
and Schepers 1950; Toussaint et al. 2003), and two
species of Australopithecus (Au): Au. africanus (rep-
resented classically by cranial specimen StS 5 Mrs.
Ples) and Au. prometheus (represented by cranial speci-
mens StW 505 and StW 252) (e.g. Clarke 1985,1988,
1994,2013). Yielding the vast majority of the >750
hominin fossils from the Sterkfontein Caves, Member
4 documents over 600,000 years of landscape, faunal,
environmental, and hominin evolution (Pickering
et al. 2010). Hominin fossils from Member 4 are a
point of reference for every Plio-Pleistocene palaeoan-
thropological research project and are utilized on a
daily basis for a wide range of morphological and taxo-
nomic research. In addition, this area of the site pre-
serves the transition from Australopithecus-bearing
Member 4 to the Oldowan stone tool-bearing
Member 5, one of the crucial time periods in human
© The International Institute for Conservation of Historic and Artistic Works 2017
CONTACT Dominic Justin Stratford dominic.stratford@wits.ac.za School of Geography, Archaeology and Environmental Studies, University of the
Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg 2050, Gauteng, South Africa
STUDIES IN CONSERVATION, 2018
VOL. 63, NO. 4, 201214
https://doi.org/10.1080/00393630.2017.1307635
evolution studies. There is little doubt that Member 4
represents an invaluable resource for palaeoanthropol-
ogy and has remained a symbol of South African heri-
tage for over half a century.
Over the last 35 years, since excavation of the
Member 4 started exposing the walls of the deposit
(described below), they have been gradually deterior-
ating. The implications of this deterioration are
severe, not only from a palaeoanthropological perspec-
tive but also a heritage management point of view.
These implications were set out by the COHWHS, Site
Management Plan Guidelines (February, 2003: p. 14)
prepared by the Mahube Consortium.
Site instability poses threats to site significance. The
main ones are:
(1) Collapse of fossiliferous material, rendering it
unrecoverable.
(2) Loss of information on site stratigraphy
(3) Covering of crucial stratigraphic sections
(4) Loss of witness sections
(5) Making certain areas too dangerous or too inac-
cessible to excavate.
Expanding on this brief description, the decay of the
walls causes the downward movement of both fossil-
bearing fine sediments as the walls gradually decalcify,
and collapse of larger clasts and breccia blocks loo-
sened by the removal of fine sediments. Numerous sig-
nificant collapses have already taken place in the north
wall (Figure 3) of the excavation distributing large dolo-
mite blocks to the base of the excavation (Figures 4 and
5). Cumulative decay of the walls will also eventually
lead to massive collapses, causing extensive damage
to the site and the interred fossils. Without interven-
tion, these decay processes are unquantifiable,
uncontrollable, and accelerating. Fossils falling 9 m
from the upper reaches of the Member 4 walls travel
over 600,000 years in time (Pickering et al. 2010) and
are mixed into the fine sediment accumulating at the
base of the excavation.
Figure 1. The location of the Sterkfontein Caves in relation to the Cradle of Humankind and other major fossil localities and its
location in South Africa.
202 D. J. STRATFORD AND M. V. CARUANA
From a heritage management perspective, Member
4 represents an invaluable South African resource of
global importance and must be preserved not only
for future research but also for public education and
awareness. Sterkfontein remains the only research
site in the Cradle of Humankind that is open to the
public through the Maropeng tourism management.
This article focuses on our efforts to conserve the dete-
riorating areas of the Member 4 excavation site.
Research history of Sterkfontein Member 4
The Member 4 deposit is integrally tied to the history of
palaeoanthropology and has hosted some of South
Africas pioneering scientists such as Robert Broom,
Philip Tobias, C. K. Brain, and Revil Mason. This
started in 1936 with the discovery of the first adult Aus-
tralopithecus (TM1511) by Broom (1936). This discovery
would inspire the next 80 years of research at Sterkfon-
tein and in the area now known as the Cradle of
Humankind. It was only two years later that the first
Paranthropus specimen was discovered at the nearby
site of Kromdraai (Broom 1938). TM1511 was partially
blasted out of an area of the site known as the Type
Siteby lime miners removing calcium carbonate
deposits from two massive speleothems which
formed against the north wall of the site (Figure 2).
The large voids left by the mining were filled with
dumps of calcified sediments (breccias) that had
been displaced as a bi-product of the mining. It is
from these dumps that the first part of TM1511, the
brain endocast, was recovered. These dumps (later to
be named D18 and D13 by Tobias [Tobias and
Hughes 1969]) were to yield a large assemblage of
hominin fossils. During his continued work on the
exposed calcified and highly fossiliferous sediments
in the Type Site, Broom recovered exceptional
hominin fossils including StS 5 Mrs Ples, and StS 14,
the first partial Australopithecus skeleton (Broom,
Robinson, and Schepers 1950). At this point, the size
and depth of the Type Site deposits were unknown
and early stratigraphic work by Cooke (1938) con-
strained the fossiliferous sediments to the upper part
of a deposit that extended into the lower cave(now
the exit stairs of the tourist route).
Later, in 1956, stone tools were found in the western
area of the site by Brain (Robinson 1957) and the
Extension Sitewould be defined and built into a
three-stage stratigraphy by Robinson (Robinson 1962)
with the Australopithecus-bearing sediments being
the oldest lower breccia. Tobias then initiated a new
phase of work with Alun Hughes in 1966 (Tobias and
Hughes 1969). Their work started extensive in situ exca-
vation of the exposed breccias, working from the
western Extension Siteeastwards. In 1976, in a decal-
cified infill in a solution cavity on the southern extre-
mity of the site, close to the stone tool-bearing
deposits, Hughes discovered StW 53 (Hughes and
Tobias 1977). Excavations continued eastwards and in
1982 Hughes encountered a very large area of decalci-
fied, highly fossiliferous breccia. Using spades and
picks, Hughes excavated this enormous area in the
centre of the site (light red area in Figure 2), stopping
where the decalcified sediments became too heavily
calcified. Between 1982 and 1991, Hughes excavated
an enormous fossil assemblage from this area, yielding
Figure 2. Aerial photograph of the Sterkfontein Member 4 and Member 5 excavation site. Light red area represents Hughesexca-
vation of the in situ Member 4 deposit (between 1982 and 1991) and the north and east excavation walls. Insert [after Kuman and
Clarke (2000)] shows the site in relation to major fossil-bearing deposits (Member 4, Member 5, the Type Site and Extension Site)
and the excavation grid. Areas are described in detail in text. Photograph by DJS.
THE CONSERVATION OF THE AUSTRALOPITHECUS-BEARING MEMBER 4 EXCAVATION WALLS, STERKFONTEIN 203
the only example of fossil wood yet known in the
Cradle of Humankind (Bamford 1999), as well as abun-
dant Australopithecus fossils, including the partial skel-
eton StW 431 (Toussaint et al. 2003). The faunal
assemblages have been the subject of extensive
research from micromammal analysis to taphonomy
(e.g. Vrba 1976; Bamford 1999; Avery 2001; Kibii 2004;
ORegan and Reynolds 2009). A consequence of the
comprehensive excavation of the decalcified sedi-
ments was that Hughes exposed three, very tall (up
to 11 m) vertical walls in the north, east, and south of
the excavation (Figures 35). Partridges(1978) strati-
graphic work placed these exposed deposits and
those exposed in the Silberberg Grotto into a
member-based framework called the Sterkfontein
Formation (Partridge 1978). In this system, the
Figure 3. The north wall of HughesMember 4 excavation. At the highest point, the wall is 8 m from top to the base of the exca-
vation. The upper 2 m of the deposit is very heavily calcified and impermeable to water. Part of the intact cave roof lies on top of the
north wall deposits and prevents water from penetrating the deposits from behind the exposed walls. Eight metres to the north of
the north wall exposure is a deep, open karstified joint which links to another cave system (The Lincoln Cave). This too limits the
movement of water behind the exposed, vertical excavation wall.
Figure 4. The east wall of HughesMember 4 excavation (A). At the highest point, the wall is 11 m from top to the base of the
excavation. Insert (B) shows a closer view of the area within the dashed line and focuses on the undercutting and major cracks
in the soft, decalcified sediments exposed in the lower east wall. The upper 4 m of the deposit is very heavily calcified and
impermeable to water. Insert photograph is taken at an oblique angle across the east wall to show how the cracks infiltrate the
sediments along the steep bedding strata of the deposit. Note the large block at the base of the excavation which collapsed
from the north wall (also seen in Figure 5).
204 D. J. STRATFORD AND M. V. CARUANA
Australopithecus-bearing deposits exposed in the Type
Site and in Hughesexcavations were classified as
Member 4. The stone tool-bearing deposits to the
West, in the area of the Extension Site, were classified
as Member 5 (Figure 2).
In 2014, one of the authors (DJS), the Cradle of
Humankind World Heritage Site Management Auth-
ority (COHWHSMA; administrators of UNESCOs World
Heritage Site preservation and conservation policies)
and the University of the Witwatersrand, undertook
measures to stabilize and preserve these important
scientific and heritage resources with the guidance of
SAHRA (South African Heritage Resource Agency).
This article describes our work to address the deterio-
ration of the Member 4 walls.
Condition of the walls
Over the last 14 years, periodic site inspections, con-
ducted by several parties associated with the
COHWHSMA and SAHRA, have identified the
deteriorating condition of the Member 4 walls. An
increase in the rate of deterioration is implied from
the frequency and focus of the reports on features in
the walls.
2002 Phase 2a report by the Mahube Consortium
for the COHWHS Cultural Resources Management
Project identified no serious structural issues in the
surface pitexcept an overhanging browat the top
of the southern wall. It was recommended that this
was removed prior to any work being conducted at
the bottom of the excavation. It is important to note
that the report incorrectly identifies the north and
south walls the northern wall (Figure 3) actually has
a dangerous overhanging brow. The picture in the
report with the dangerous overhangis the north
wall, not the south.
February 2003 Site Management Plan by the
Mahube Consortium for the COHWHS Cultural
Resources Management Project identified the same
problems with the same incorrect labelling of the
north and south walls.
May 2007 Sterkfontein fossil site inspection report
prepared by COHWHSMA for SAHRA identified high
and unprotected abandoned excavation faces, exca-
vation edges, witness sections and sampling(p. 15)
and specifically collapsing walls of the Member 4 exca-
vation and a fracture in the north wall of Member 4
(p. 31) and recommended removing the overhanging
block(p. 31) and continued monitoring of this area
of the site.
May 2008 Sterkfontein fossil site inspection report
prepared by COHWHSMA for SAHRA identified the
same issues as the previous year but recommended
further monitoring of the widening crackin the
north wall of Member 4.
November 2008 Sterkfontein fossil site inspection
report prepared by COHWHSMA for SAHRA identified
Site safety within the main excavation area of the
grid [The area of the Member 4 excavation] as a
concern. Specifically, they identify that Excavation
over the years has created several steep unprotected
drops. In places, overhangs have resulted and unstable
overlying material has collapsed(p. 6). The report
further adds that the situation emphasises the need
to bench deep excavations and for SAHRA to build
this requirement into the permitting process(p. 6).
May 2009 Sterkfontein fossil site inspection report
prepared by COHWHSMA for SAHRA identified a large
crack in the eastern excavation face(p. 6) and rec-
ommended photographic monitoring.
September 2012 Site Safety Inspections Final
Report prepared for COHWHSMA by consulting engin-
eers SRK provided a more detailed description of the
instability of the Member 4 walls and identified
tensile cracksand undercutting high wallin the
east and north wall of Member 4 that may lead to
major failure of the high wall which would bury any
Figure 5. Eastern region of the south wall of HughesMember
4 excavation. At the highest point, the wall is 11 m from top to
the base of the excavation. The upper 4 m of the deposit is very
heavily calcified and impermeable to water. Note the large
block at the base of the excavation which collapsed from the
north wall (also seen in Figure 4).
THE CONSERVATION OF THE AUSTRALOPITHECUS-BEARING MEMBER 4 EXCAVATION WALLS, STERKFONTEIN 205
people present at the base of the walls. Specifically
they classify the danger as high wall collapse. SRK con-
sulting also recommended the excavation of benches
during excavation (all quotes are from page 5).
Site visits with consulting engineers SRK in 2014
expanded upon the 2012 report and we identified fea-
tures in the walls that suggested deterioration was
reaching a level that would cause major and potentially
imminent collapse of breccias. These are described in
the following section.
North wall
The north wall of the Member 4 excavation (Figures 2
and 3) spans an E-W length of approximately 13 m
and measures, at its highest point, 8 m from the base
of the excavation to the landscape surface. This
profile preserves within it the transition from Australo-
pithecus-bearing Member 4 to the Oldowan stone tool-
bearing Member 5, one of the crucial time periods in
human evolution studies. The in situ sediments
exposed in the profile range from large poorly sorted
and chaotically organized clasts with very high void
spaces in the lower reaches to matrix-supported
mostly decalcified (with small pockets of more and
less decalcified sediment) sand-loam sediments in the
upper. The upper 1 m of sediments remain heavily cal-
cified and represent a significant load issue for the
below decalcified and loose, friable sediments. This
heavily calcified sediment provides a hard capto the
deposit which prevents water penetrating the walls
from above. Below this hard capthe sediments are
soft. Several large cracks (Figure 3) indicate slumping
and subsidence of the upper, heavier sediments and
provide conduits for rainwater and plant growth,
thereby significantly accelerating the decay process.
This has facilitated localized undercutting in the wall
which had formed a dangerous browthat was ident-
ified in early site inspections (described above). This
brow was removed by Professor Clarke. The outer
surface of the exposed decalcified sediments shows
clear mud rivulets and small erosion channels down
the face of the wall. The lower sediments are soft and
friable enough to break easily by hand. Several very
large blocks have already collapsed from this profile
to the base of the deposit (seen in Figures 4 and 5).
East wall
The east wall (Figure 4) spans N-S about 8 m and is at
its highest point 11 m from the base of the excavation
to the surface of the deposit. In a similar way to the
north wall, the uppermost sediments remain heavily
calcified and the lower have decalcified. Sedimentolo-
gically, the deposit is similar in texture, sorting, and
organization from bottom to top and is generally
matrix-supported and poorly sorted with occasional
very large dolomite blocks. The deposit surface
remains heavily calcified and provides a hard cap
which prevents water penetrating the walls from
above. Below this hard cap, the sediments are rela-
tively soft. In the case of the east wall, water has infil-
trated the deposit along its natural, steeply sloped W-E
strata. As water has exploited these natural weaknesses
in the exposed sediments, the lower, softer sediments
have been eroded, undercutting the upper, cemented
sediments by several metres. Several ominous over-
hangs of breccia (Figure 4) and large cracks (>4 m
wide, 10 cm high, and of an unknown depth) can be
seen in the middle of the wall (Figure 4). In addition,
several deep, vertical erosion features are visible.
Fossils have been found within a small talus of
eroded and collapsed sediments that has accumulated
at the base of the east wall. Their provenience is
obviously irretrievable.
South wall
The south wall (Figure 5) spans E-W about 13 m and is
at its highest point 11 m from the base of the exca-
vation to the surface of the deposit. It is directly oppo-
site the north wall. The south wall sediments are similar
to those exposed in the east wall and strata can be seen
sloping steeply W-E. These sediments have fared better
over their period of exposure and no cracks or localized
decalcification can be seen. The sediments from
bottom to top remain intact and well cemented.
The obviously concerning condition of the north
and east walls and visible ongoing erosion provoked
us to prioritize finding solutions for the short-term
and long-term preservation of the north and east
profile while ensuring that high-resolution documen-
tation was carried out in case of collapse. It should be
noted that in all of the walls, water can penetrate
only the deposits through direct contact with the verti-
cal exposed excavation walls. Due to the heavily calci-
fied caps, surface water is not able to remove
sediments from behind the walls. The most vulnerable
sediments are those softsediments exposed below
the calcified caps.
Conservation strategies
This project required the development of a compre-
hensive set of strategies that had to be adapted to
the specific requirements of the site, research, and heri-
tage environment as dictated by the research director
(DJS), SAHRA, and the COHWHSMA who administer
UNESCO guidelines on the conservation and preser-
vation of World Heritage sites. The conditions for the
conservation strategies were as follows:
(1) The site in general and all sensitive areas must be
documented in detail prior to any conservation
206 D. J. STRATFORD AND M. V. CARUANA
mitigation; the walls, their sediments, and strati-
graphic features have to remain visible and acces-
sible for both research and education purposes.
(2) The area directly below the walls and the base of
the excavation must be accessible for non-invasive
research.
(3) All elements in the deposits must be indepen-
dently secured so as to create a combination of
mutually reliable and supportive stable structures.
(4) If collapse does happen, the strategies must control
the movement of material and guarantee the
safety of anyone working in the vicinity.
(5) The stability of the walls must be maintainable
through multiple phases of conservation focused
re-excavation. The following strategies were
devised to address and satisfy these conditions.
Documentation
Prior to the implementation of any structural stabiliz-
ation or conservation intervention, it was crucial that
the site was comprehensively documented. This
ensures that there is a permanent, high-resolution
record of the profiles and the stratigraphic structures
and features exposed in them. From these records,
we can perform a number of in situ stratigraphic ana-
lyses including fabric analysis from exposed clasts
and interpretation of deposit mixing from exposed sol-
ution cavities. The stratigraphic information derived
from the 3D documentation of the walls can also be
combined with reconstructed artefact and fossil distri-
bution data (Stratford, Merlo, and Brown 2016),
enabling us to reconstruct the stratigraphy of the
space excavated already. A lack of stratigraphic docu-
mentation during the excavation of Member 4 has
impressed serious limitations on our ability to elucidate
and associate intra-deposit environmental, faunal, and
taphonomic evidence. Four techniques were used to
facilitate the documentation of the Member 4 walls
and general excavation space. The techniques were
chosen to provide a multiscale high-resolution 3D
record of the site that could be used for a range of cul-
tural heritage management, outreach, education, and
research purposes. All four techniques had to be inte-
grative and be spatially managed within the site and
national geospatial framework (South African Lo27
coordinate system [see Stratford, Merlo, and Brown
(2016) for details]). We chose a combination of close
range photogrammetry, 3D terrestrial laser scanning,
UAV-based photogrammetric modelling, and high-res-
olution photomosaics of the excavation walls. The
specific techniques and instruments used for each are
described in the methods section below. Each
method records different information (e.g. point
cloud data for laser scans versus raster surfaces for
photographs) and so we felt that it was necessary to
apply multiple methods in order to enable a diverse
range of data processing (e.g. integration of point
clouds into a GIS) or visualizations.
Fine sediment stabilization
The sediments in the exposed walls vary greatly from
matrix-supported to clast-supported with large void
spaces. In areas where the sediments have been decalci-
fied or not calcified at all, exposed to rain, vegetation,
nesting, and burrowing animals, there is a strong poten-
tial for displacement of soft, fine sediments. This is clearly
taking place in both the north and east wall where small
rivulets of mud are seen as sediments are eroded from
the faces. The north wall, which faces south, receives
very little sunlight and so during Johannesburgswet
season (August to March) the wall can remain damp
and shaded for weeks. As described above, fossils con-
tained within the matrix of the deposit can be displaced
and washed to the base of the excavation with each rain.
Where extensive fine sediments are removed, larger
clasts are destabilized and may cause more significant
collapse. Water generally affects the lower parts of the
deposit, softening, eroding, and undercutting the more
calcified upper sediments encouraging more signifi-
cant collapse. It was decided that a dedicated strategy
needed to be focused on securing the fine sediments
to curtail these processes.
Coarse sediment stabilization
The removal of fine sediments through direct contact
with precipitation gradually destabilizes clasts and
pockets of calcified sediments, which in turn collapse.
Two major collapses have already taken place in the
north wall when two very large dolomite blocks
(1 m
3
) and associated sediments were dislodged
during a period of particularly heavy rains. In other
areas, extensive undercutting has caused a serious
risk of major collapse of calcified upper sediments. It
was clear our conservation strategy would also need
to provide dedicated stabilization of clasts of a wide
range of sizes from 10 cm to large portions of calcified
wall >4 m
2
.
Long-term strategies
No matter how well we stabilize the walls of Member 4,
they will decay. At up to 11 m in height, the potential
for collapse is great as is the danger to anyone
working around or underneath them. For this reason,
a long-term conservation-focused excavation needs
to be carried out to reduce the maximum vertical
height of any one area of the walls. This can be most
easily accomplished by excavating large steps
(benches) into the walls. This strategy will provide
safer access to the different levels of the profile for
future research. If one part of the wall collapses for
THE CONSERVATION OF THE AUSTRALOPITHECUS-BEARING MEMBER 4 EXCAVATION WALLS, STERKFONTEIN 207
any reason, then damage is limited to specific level and
debris contamination is kept to a minimum. Any col-
lapse is also far less dangerous when the maximum ver-
tical height of walls is limited to less than 3 m.
There are, however, both space and practical limit-
ations to the excavation of steps. Ideally, maximum ver-
tical height of a deposit wall should not exceed 2 m, as
per the SAHRA guidelines. Each 2 m step should also be
2 m wide. If this is the case at the highest point of the
Sterkfontein Member 4 walls, we would need to
increase the size of the excavation by 10 m east and
west, effectively enlarging the excavation by over
90%. Space restrictions on the property just will not
allow this. Practically, 1 m
3
of calcified sediment may
take between six months and one year to process
(excavate and clean individual fossils contained in the
sediments), depending on the richness of the exca-
vation. SAHRA guidelines also restrict the volume of
sediment removed from a site to amounts manageable
within a three-year period. For the sake of this project,
special permission was provided by SAHRA to excavate
more than this. We, therefore, decided that we would
first start by excavating a 3 m step in the east and
north walls, reducing the maximum height to 8 m
and removing the enormous weight of the heavily cal-
cified upper sediments. Specific excavation techniques
will be described in the methods section below.
Methods
Photogrammetry
Photogrammetry is a method of developing 3D models
from photographs and has recently been developed
into a common tool for the documentation of exca-
vations (e.g. Douglass, Lin, and Chodoronek 2015), arte-
facts, (e.g. Sumner and Riddle 2008) and heritage
management (e.g. Yilmaz et al. 2007). The triangulation
of common features in overlapping photographs is
used to generate point clouds in an X, Y, Z coordinate
system with mesh overlays created from features cap-
tured by the camera lens in high resolution. This pro-
vides relatively simple and efficient control over
irregularly sized and shaped features and volumes.
An advantage of photogrammetry is that it records
sediment, clast, and surface textures and colours at a
photographic quality. For this project, two photogram-
metric models of different scales were developed.
The first used close range photography to develop a
model of the Member 4 excavation site and walls
(Figure 6). Both models used the same 10 megapixel
Olympus XZ-1 digital camera. Two hundred and thirty
five close range photographs were taken using mul-
tiple focal ranges and angles with at least 30% frame
overlap to create a highly model. The model was geor-
eferenced using the site datum points and achieved a
maximum error of 0.5 m in any direction.
The second model used a UAV-based camera to
develop a site and proximal landscape model
(Figure 7). Six hundred and twenty photographs were
taken from a 3DR X8+ octocopter at two different alti-
tudes of 15 m and 30 m with a short period at 50 m to
cover an area of 48,750 m
2
(195 m N-S, 250 m E-W). The
flight plan was partially managed by Droidplanner soft-
ware and partially manually flown by DJS. Photographs
were taken every two seconds and managed by a
photopix intervalometer. The model was georefer-
enced using six surface datums [see Stratford, Merlo,
and Brown (2016) for details] identified on the photo-
graphs and achieved a maximum error of 0.22 m in
any one direction.
Laser scanning
Laser scanning generates dense point clouds (X, Y, Z
coordinate format) for recording and georeferencing
Figure 6. Three perspectives of the Member 4 excavation rendered from the photogrammetric model. Perspective (A) shows a
general view of HughesMember 4 excavation site with the Type Site to the north. Insert (B) shows the north wall as seen in
Figure 3. Insert (C) shows the east wall, as seen in Figure 4.
208 D. J. STRATFORD AND M. V. CARUANA
purposes. It has been used in the conservation and
research of sites of archaeological and architectural
importance (e.g. Barton 2009; Rüther et al. 2009,
respectively). The combination of laser and photogram-
metric techniques for comprehensive documentation
of shapes, textures, and colours has been used to
great effect (e.g. Lerma et al. 2010). We followed the
same procedure as Lerma et al. (2010) here. The scan-
ning process was facilitated by Group 5 Projects Ltd.
with the goal of producing a high-resolution 3D laser
scan of the Sterkfontein excavation site with special
attention being paid to the Member 4 walls
(Figure 8). Scanning was conducted using a FARO
Focus 3D X330. At highest resolution, 183,399,725
points were recorded. Scan data were georeferenced
in FAROs proprietary SCENE software which automati-
cally identified target boards that were situated within
the Sterkfontein spatial framework with a Nivo 5C total
station. Georeferenced cloud data achieved errors with
a mean of 0.0041 m (minimum error 0.000 m;
maximum error 0.0154 m). The cloud point data
from the scan can be integrated with the photogram-
metric models and also the GIS framework to provide
highly detailed and interactive record of the site and
its features, prior to any conservation mediation,
deposit movement, or further excavations.
Photomosaics
Additional strategic georeferenced photographic
mosaics were also developed of the Member 4 walls
prior to any intervention. Photographs were taken
with the Olympus XZ-1. Each overlapping corner of
each photograph was plotted using a Nivo 5C total
station and these references were used to georefer-
ence the photographic mosaics. These mosaics
provide a highly detailed record of the exposed sedi-
ments, clasts, and features with no digital photograph
shape and size manipulation carried out during the
photogrammetric modelling process.
Fine sediment stabilization
Fine sediments exposed in the wall of Member 4
needed to be secured in place while preserving their
integrity and visibility. The application of a styrene
acrylic copolymer emulsion was chosen to stabilize
the walls of Member 4 for several reasons. It should
first be noted that the use of resin coatings can be pro-
blematic if water saturates the deposit from behind
and cannot evaporate through a non-permeable
coating, potentially causing further collapse of the
Figure 7. UAV-based photogrammetric model of the Sterkfontein Caves site. HughesMember 4 excavation site can clearly be seen
in the centre of the image.
Figure 8. High density cloud point image of HughesMember
4 excavation site rendered from the laser scan data.
THE CONSERVATION OF THE AUSTRALOPITHECUS-BEARING MEMBER 4 EXCAVATION WALLS, STERKFONTEIN 209
walls. As such, the first step in ensuring stabilization
was preventing water from penetrating into the
deposit. Several factors relating to the geological
context of the Member 4 deposit naturally prohibit
water saturation. The Sterkfontein palaeo-cave system
was formed within the Malmani dolomites, which dip
from N-S in this region. The Member 4 deposit sits on
a gently north-sloping side of the hill and any water
penetrating the dolomites is naturally drained away
from the excavation along impenetrable, interbedded
chert bands.
Second, the northern part of the Member 4 exca-
vation is partially detached by a large karstified joint
running E-W to the north of the Type Site (Figure 3).
Along with the dip of the dolomite host rock, this
joint creates a natural drainage system leading water
away from Member 4 area. Finally, the top of the
Member 4, Member 5, and Type Site areas are exten-
sively covered by breccia formations that are cemented
from the top down by calcium carbonates. This has
created a capthat is impenetrable by water (see
above). Thus the Member 4 deposit is affected only
by hydrological erosion from the face of the walls,
approximately 4 m from the top of the excavation
walls where cracks have formed (see Figure 4). To
ensure that the penetration of water and organic
matter into established cracks was limited in the
north and east walls, we filled the cracks with a
water-based adhesive grout. We also used the grout
to block the opening of the cracks and divert any
running or pooling water.
In terms of stabilization, styrene acrylic copolymer
emulsion has been demonstrated to be a superior con-
solidation agent when compared to other resin-coating
options. This was extensively tested by Bishop,
McAplin, and Jones (1998) who compared the strength
of styrene acrylic copolymer emulsion with polyvinyl
homopolymer resin in the stabilization of earth roads
under wet and dry conditions. Sand and soil cores
were extracted from earth roads and treated with the
two resin types when dry and also after four hours of
submersion in water. Cores were then measured in
Megapascal units (MPa) using tensile stress testing to
compare compression strength. Results showed that
in dry conditions styrene acrylic copolymer emulsion
had higher compression strength (2.8 MPa) when com-
pared to polyvinyl resin (1.9 MPa). Furthermore, styrene
acrylic copolymer emulsion showed higher com-
pression strength in wet conditions (0.9 MPa) when
compared to polyvinyl resin (0.1 MPa), and the former
also demonstrated a higher propensity to prevent
water uptake after its application (Bishop, McAplin,
and Jones 1998).
Acrylic polymer resins have wide-ranging appli-
cations in the conservation of archaeological and
palaeontological materials. In particular, studies on
acrylic resins used to preserve fossil bone for curation
have demonstrated the importance of using consoli-
dants of this type for protecting fossil materials from
long-term decay (Johnson 1994; Howie 1995; Shelton
and Johnson 1995; Elder et al. 1997; Fernández Jalvo
and Monfort 2008; López-Polín 2012). Consolidants
are typically rated according to stability, reversibility,
and compatibility and acrylic polymer resins, such as
Paraloid® B-72 and Primal®, have been shown to
satisfy these requirements (Lazzari and Chiantore
2000; Bracci and Melo 2003; Cocca et al. 2004; López-
Polín 2012). In terms of stability, the use of acrylic
polymer consolidants has been shown to restrict the
artificial ageingof fossil bone in museum settings.
This has been extensively tested by comparing conso-
lidated and unconsolidated fossils to thermal treat-
ment, demonstrating that fossils consolidated with
acrylic polymer resins show substantial resistance to
heat-induced weathering (Lazzari and Chiantore 2000;
Bracci and Melo 2003; Cocca et al. 2004; López-Polín
2012). Also, acrylic polymer resins are completely
reversible through the application of acetone, which
does not alter fossil bone in any manner (López-Polín
2012).
On the basis of the above-mentioned research, we
applied a clear styrene acrylic co-polymer emulsion
called Suncryl 583, a resin designed to stabilize fine
silt to sand-sized particles on mining spoil heaps to
secure the fine sediments while maintaining their visi-
bility. Diluted to 40% Suncryl, 60% water proportions,
the clear styrene resin penetrates sediment through
interstitial voids to a depth of 5 cm and dries quickly
to form a transparent water and alkali resistant
coating. Good penetration of the Suncryl resin was
noted because of the porosity of fine sediments and
void spaces in the clast-supported matrix. To address
the stability of the Suncryl 583 resin, we tested multiple
coats on modern and fossil bone cortical surfaces prior
to its application on the Member 4 walls. The resin
formed a pliable but water-tight film over the bone
surface but did not stick to it and no modification or
effect of the bone surface could be seen by naked
eye or at 10x magnification. The Suncryl was applied
using an electric pump and hose (Figure 9) to dry
walls that had not been exposed to rain for at least
two weeks. Four coats were applied to the walls to
ensure they were comprehensively covered and
sealed. The clear resin decays very slowly and may, in
five years, require another coat or localized patching.
Finally, dissolution of the Suncryl coating can be
achieved through acetone and water if needed.
Figure 10 shows the deposit walls before and after
the application of the Suncryl.
Coarse sediment stabilization
Stabilizing the larger components of the Member 4
deposits was more complex and required a combination
210 D. J. STRATFORD AND M. V. CARUANA
of strategies to be developed based on the requisites for
the site and its continuing research and long-term con-
servation plans. In collaboration with SRK civil engineers
and Underground Support Solutions Ltd., a strategy was
developed to use a combination of double layered steel
mesh to secure the small to large individual clasts
(10 cm 2 m maximum length) exposed in the vertical
walls. Horizontal 34 mm steel cables, spaced at regular
intervals of 1.5 m down the vertical faces of the north
and east Member 4 walls (Figures 11 and 12), provide
security for the large boulders or major structural fail-
ures. The steel cables were placed over the mesh and
secured with 24 × 750 mm steel bolts that were drilled
into the corners of the deposits and secured using
32 × 400 mm FASLOC resin capsules. To ensure the
mesh would conform to the shape of the deposit and
control the movement of both small and large material
if there was collapse, shorter bolts (500 mm) secured the
steel cables along their length and in areas of recog-
nized instability. The tension of the steel cables is
managed by turnbuckles and can be adjusted according
to the stretch of the cable caused by seasonal tempera-
ture changes. The vertical position of the cables was
chosen to enable access for excavations while ensuring
the lower deposits remain secure and stable. As exca-
vations deepen in horizontal levels, to create the 3 m
steps (described above), the mesh is simply folded
down over the next lowest cable, exposing the sedi-
ments to be excavated and providing extra cover and
security to the lower sediments.
Excavation
In order to ensure the long-term preservation and con-
servation of the Member 4 area of the Sterkfontein site
Figure 10. The North wall of HughesMember 4 excavation
before the application of the styrene resin (A), and after four
coats of the resin had been applied and dried (B).
Figure 11. The fully stabilized east wall of HughesMember 4
excavation. The resin-sealed face is covered by a double layer
of steel mesh and secured in place with four horizontal, ten-
sioned steel cables. The tension of each cable is adjustable
using the turnbuckles. As the excavation of the 3 m deep
step progresses, the cables can be loosened, the mesh folded
down and the cables tightened again to ensure the stability,
preservation, and safety of the lower walls.
Figure 9. First applications of the clear styrene acrylic resin
(Suncryl 583) to secure and preserve the fine sediments in
the north wall of HughesMember 4 excavation. Although
the resin that is applied is white, it dries to a clear finish
(Figure 10). Four coats were applied to the north, east, and
south walls.
THE CONSERVATION OF THE AUSTRALOPITHECUS-BEARING MEMBER 4 EXCAVATION WALLS, STERKFONTEIN 211
and preserve the integrity of the fossil- and artefact-
bearing sediments for the future, 3 m by 3 m steps
will be excavated into the currently exposed high
walls (as described above). From an archaeological per-
spective, it is always better to excavate a sensitive area
of a site under controlled conditions, than to recover
material from the base of an excavation after collapse.
The Member 4 walls represent an exceptional example
of this where, as described above, collapses essentially
transport fossils 600,000 years as they fall to the bottom
of the excavation.
The upper sediments in both the north and east walls
of Member 4 are heavily calcified and very difficult to
excavate using high-resolution spatially controlled
techniques. Often and traditionally, excavation of this
cemented sediment requires comparatively heavy
duty techniques relative to soft sediment archaeologi-
cal excavations. Irregular blocks of calcified sediment
are usually broken off the main deposit using drills,
feathers, and wedges. The spatial context of the block
is recorded prior to removal and the block is then
taken to the laboratory for preparation, which involves
the manual excavation of individual fossils using an air-
scribe (a pen-like device with a vibrating pneumatic tip).
Excavation and processing of heavily calcified
sediments is extremely laborious and requires great
experience, patience, and skill. The implementation of
a new system of geospatial control at the site (Stratford,
Merlo, and Brown 2016), allows several higher resol-
ution spatial documentation and excavation techniques
to be used for the new phase of conservation-focused
Member 4 excavations. For example, exposed fossils
can be individually excavated from in situ sediments
and recorded with GIS integrated photogrammetry
and millimetre resolution total station surveying. If
blocks of sediments need to be removed, then each
block can be modelled and mapped in GIS and any
fossils excavated from that block in the laboratory can
be spatially constrained within the controlled volume
of the block. Importantly, sediments and sediment
samples can be taken with high-resolution spatial
control something that was not possible in the past.
Results and discussion
Figures 11 and 12 show the final fully stabilized walls of
the Sterkfontein Member 4 excavation site. These walls
now satisfy the requisites that were set out by the
researchers, SAHRA, and the Cradle of Humankind Man-
agement Authority. As per these conditions, the site has
been documented at a range of scales using spatially
integrative techniques (photogrammetry, laser scan-
ning, photomosaics). The fine sediments have been
independently secured together with small (<10 cm)
clasts with four coats of clear styrene resin. Cracks that
focus water flow and plant growth have been filled,
blocked by grout and sealed by the resin. Medium-
sized clasts (10100 cm) have been secured with a
double layer of steel mesh. The areas that have recog-
nized potential for major collapse of sediments and
clasts have been secured behind the resin and mesh
with tensioned steel cables. The combination of these
strategies ensures that each component of the deposits
is secured behind an all-inclusive conservation scheme.
These strategies also provide us with the safety and
adaptability to excavate the upper 3 m of the north
and east walls carefully with minimal influence or
damage to the lower portions of the walls. Visibility of
the sediments ensures that stratigraphic observations
and documentation can continue. In terms of mainten-
ance, the system is very simple. The clear resin decays
very slowly and may, in 5 years, require another coat
or localized patching. The mesh does not rust and will
stay in place for over 10 years. If areas of sediment are
exposed through the excavation process, then mesh
patchescan be applied. Over several seasons, the
mesh and cables may stretch, in which case the cables
can be re-tensioned by hand using the turnbuckles
which are placed at one end of each cable. Significant
quantities of spare mesh and resin are kept at Sterkfon-
tein for these eventualities and several permanent
members of the research and excavation team have
Figure 12. The fully stabilized north wall of HughesMember 4
excavation. The resin-sealed face is covered by a double layer
of steel mesh and secured in place with three tensioned steel
cables. The upper cable is secured over the top of the sedi-
ments and is not visible. The tension of each cable is adjustable
using the turnbuckles. As the excavation of the 3 m deep step
progresses, the cables can be loosened, the mesh folded down
and the cables tightened again to ensure the stability, preser-
vation, and safety of the lower walls. Person is for scale.
212 D. J. STRATFORD AND M. V. CARUANA
been trained in their application. The maintenance of
this conservation strategy is, therefore, sustainable
and cost effective in the long term.
Although this strategy satisfies all the requirements
stipulated by both COHWHSMA and SAHRA and the
research team, there are always limitations that need
to be acknowledged. For example, although visibility
of the profiles has been retained, visibility of the smal-
lest details of the exposed sediments is a little obscured
by the mesh. Another limitation is the movement of
water through the sediments from the landscape
surface. The grout and resin significantly limit pen-
etration of water from the immediate vicinity of the
vertical walls from direct rain or very localized precipi-
tation run off. It is clear that a policy of consistent moni-
toring of the walls is still necessary in order to identify
any sediment movement, subsidence, or the develop-
ment of new fluid conduits around the site. Further-
more, Suncryl 583 can be re-applied within the next
five years if its protective coating should significantly
degrade. When our excavation of the steps (benches)
has been completed, then the horizontal and new ver-
tical surfaces should be sealed with resin and mesh as
soon as possible to restrict the start of decay.
This area of the site is currently the most vulnerable,
and whereas decay of other exposed sediments is inevi-
table (e.g. the south wall), this work has significantly con-
tributed to the conservation of the structural, cultural,
and scientific integrity of Member 4 and Member 5
and Sterkfontein in general. As noted in the reports of
COHWHSMA in November 2008, incorporating respon-
sible excavation practices that include the stepping
and sealing of walls into peer-reviewed excavation
plans is ideal and should be made obligatory in all
archaeological or palaeoanthropological excavations
and enforced by SAHRA. Some of the techniques devel-
oped here may be of particular relevance to similar sites
around the world. We have found the application of
clear styrene resin to parts of freshly excavated walls a
particularly effective way of preserving stratigraphic evi-
dence and features in a wide range of contexts including
rock shelter and open-air sites.
Conclusions
Member 4 remains one of the most important
palaeoanthropological deposits in the world and over
the last 80 years has yielded more Australopithecus
specimens than any other locality. The long history of
research in the area of Member 4 at Sterkfontein is inte-
grally tied to the history of the science itself and so rep-
resents a key witness, not only of the late Pliocene to
Plio-Pleistocene geological, landscape, environmental,
faunal, and hominin evolution, but also of the develop-
ment of the science of palaeoanthropology. For these
reasons, this area of the Sterkfontein Caves is recog-
nized as one of South Africas most valuable cultural
heritage resources and symbolizes South Africas cul-
tural and scientific development and impact on a
global scale. The protection of this resource for the
public and research is, therefore, of great importance
for preserving the countrys cultural heritage. The
result has satisfied all the requirements of the
COHWHS Cultural Resources Management Authority,
which administers the guidelines set out by UNESCO,
the South African Heritage Resources Agency and the
researchers who continue to work on this important
site. The infrastructure installed provides the safety
and security for the next step of the longer term
efforts to preserve this area by excavating steps into
the high walls, thereby reducing the maximum
height of any one profile. These excavations have
started. The development of a multifaceted conserva-
tion approach to this deposit in documentation and
stabilization ensures that these walls are now protected
and secure for at least the next 15 years.
Acknowledgements
The authors extend warmest gratitude to the Cradle Manage-
ment Authority, particularly Lindsey Smith, for working hard
to help fund this project, and equally to Bruce Rubidge, Chris-
tine Steininger, and DVC Beatrys Lacquet for Wits Universitys
significant and crucial contribution. The efforts of these indi-
viduals have been fundamental to the preservation of this
invaluable resource at the Sterkfontein Caves. We would
also like to thank Alan Naismith at SRK civil engineers for
his involvement at the inception of the project and Chris
Smith at Underground Support Services Ltd. for his tireless
work in making this project happen.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The Cradle of Humankind World Heritage Site Management
Authority. The University of the Witwatersrand, Johannes-
burg. MVC is supported by the NRF-DST Center of Excellence
in Palaeosciences.
ORCID
Dominic Justin Stratford http://orcid.org/0000-0001-9790-
8848
Matthew V. Caruana http://orcid.org/0000-0002-2095-5360
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214 D. J. STRATFORD AND M. V. CARUANA
... Enfin, le dernier aspect à considérer est celui du facteur temporel. Au sein du Membre 4 de Sterkfontein, qui contient un assemblage très variable d'Australopithecus, les dépôts couvrent près de 600 000 ans (Pickering et Kramers, 2010 ;Stratford et Caruana, 2018). Cette fenêtre temporelle n'exclut pas la possibilité d'une seule et même lignée qui évolue et varie dans le temps. ...
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