The Roman gold mine of Las Médulas

Full text


I
Field Trip in Honour of the Work of Prof. Richard J. Lisle
THE ROMAN GOLD MINE OF LAS MÉDULAS
Aller, J., Bastida, F., Bulnes, M.T., Fernández Rodríguez, F. J. and
Poblet, J.
In the first stop of the day we will visit the rests of the most important gold mine of the
Roman Empire. In this text you will find something about the geology of the deposit and also
about the complex and sophisticated techniques that the romans used to exploit this ore.
GEOLOGY
In NW Spain there are two types of gold deposits:
1) Complex late Variscan ore deposits associated with fractures trending NE-SW (Spiering
et al., 2000).
2) Placer deposits originated from erosion of the Variscan gold. These have Tertiary or
Quaternary age.
The gold deposit exploited by the romans in Las Médulas is one of the second type. Gold
here is disseminated in Miocene siliciclastic series where two formations have been described
(Fig. 1):
The lower Toral Formation presents dominant silt and clay with some lacustrine limestones.
The upper Leitosa Formation appears in two different facies: the Santalla facies, which is
more distal and presents intercalations of conglomerates and sands, and the Las Médulas facies,
more proximal and with dominant conglomerates.
The gold is not homogeneously distributed along the series, but concentrated in the lower
part of the Santalla facies (60-300 mg Au/m3).
Fig. 1.- Stratigraphic column of the Las Médulas area after Martín-
González et al. (in press).


II
These facies prograde towards the East, and where deposited in an alluvial fan coming
from this direction. The Miocene series were deformed by faults during the Alpine orogeny
(Fig. 2).
Fig. 2.- Geological map of the Las Médulas area after Martín-González et al. (in press).


3
HISTORY OF THE GOLD EXPLOITATION
Before the conquest by the Romans, NW Spain was inhabited by several Celtic peoples
(cantabri, astures, gallaeci) among which the astures were the one dwelling in the Las Médulas
area. They obtained gold from river placers and used it for jewelry.
Conquest of NW Spain by Rome involved a bloody conflict (Cantabrian wars by the name of
the people that more resisted) that lasted from 29 to 19 b. C. This time span coincides with the
beginning of the August rule (27 b. C to 14 a. C.), which is the beginning of the Roman empire
after the civil wars. This is the time of the great reorganization of the Roman world, including a
new monetary system based in gold and silver coins minted by the state and giving a norm for
all the Roman world. They needed a lot of gold for this and they took it from anywhere it was.
In the new social order established by Rome, work in the mines was an obligation for the
subjugated people of the area. They had not in fact the status of slaves, but it is also improbable
that they received any wage for their work. They were forced workers.
In Asturia, the area of NW Spain inhabited by the astures, romans exploited gold in rock and
in placers (Fig. 3). In the first century, NW Iberia already was the most gold productive area of
the Roman world. In the book XXXIII of his Naturalis Historia Pliny (23-79) who was himself
a procurator (high status official linked directly to the emperor) in Spain and visited the mines
distinguishes three forms of gold exploitation:
Fig. 3.- Roman gold mines in rock and placers
in Asturia (Conventus asturum) (Fernández-
Posse et al., 2002).


4
1. Aurum fluminum: Gold from river sands.
2. Aurum canaliense: Gold following veins in solid rock.
3. Aurum arrugiae: Gold obtained from the arrugiae, underground excavations used to collapse
the rocks in a process known as ruina montium (collapse of the mountains).
It was the third type of exploitation the one used in Las Médulas. We will focus now in the
description of this technique that involved the transport of a great amount of water to the area of
exploitation and the construction of channels and reservoirs. We must note that there is a
consensus between the authors that describe this technique in the sense that the systematic use
of hydraulic force was used to produce the ruina montium, though as it can be seen below, this
point is not specifically indicated in the text of Pliny.
Channels bring water to Las Médulas from the eastern mountains and are in general 80-100
km long and up to 2 m wide, maintaining a constant slope between 0.2 and 0.4 %. The water
depth was up to 10 cm. In Fig. 4 we see the trace and profile of one of these channels. A small
collecting dam is found in the beginning of the channels and small tunnels and terraces are
found along them, but never aqueducts.
Fig. 4.- The higher channel in the area of Las Médulas (Fernández-Posse et al., 2002).
The bigger reservoir in the area was La Horta in Las Médulas and had a capacity of 18000
m3. It was located in the upper part of the mining area and water was taken from here for the
mining operations.
Once a rock mass containing gold had been identified through a thorough prospection, in a
first stage this rock mass was excavated with tunnels and wells as shown in Fig. 5-1. Then, a


V
great amount of water was let to fall from the reservoir into the tunnels and it produced a
collapse of the rock mass. More water thrown on the fallen material helped to send it to the
wash channels, where gold was separated by a system similar to that used in modern mines.
Only the heavier cobbles and boulders, which could not be sent to the wash channels because
they could obstruct or even destroy them, were separated by hand and piled in great
accumulations that are now called “murias” by the people of the area. The final topography
resulting is shown in Fig. 5-2 and Fig. 6.
Note that all the galleries in Fig. 5-1 are blind. They were made in this way to take full
advantage of the hydraulic force for the collapse of rocks. As a result of this, rock prisms
remained elevated near to the exploitation front after the ruina montium was conducted. They
are common in the area and show vertical cliffs intersecting in different directions that cannot
be explained by any geological process observed in the area, but only by the collapse of human
made galleries. In our visit, we will go inside La Cuevona, one of these galleries that did not
collapse. Another gallery is observed in Fig. 6 (indicated by a red arrow).
Fig. 5.- Exploitation system of the ruina montium (Sánchez-Palencia et al., 1996).
Fig. 6.- Landscape of Las Médulas. The red arrow indicates a gallery made during the mining
operations.


VI
Due to its high interest, we give here the full description of the ruina montium as given by
Pliny the Elder in the book 33 of his Naturalis Historia (translation slightly modified from Bird,
2004). Pliny saw himself the works in situ during his stay in NW Spain in 74, five years before
his death during the same eruption of mount Vesuvius that destroyed Pompeii and
Herculaneum:
[70] The third method has surpassed the achievements of the Giants. Mountains are hollowed out by
underground passages [cuniculi] driven over great distances by the light of lamps. The same light is the
measure of the shifts, and for many months daylight is not seen. This class of working they call arrugiae,
and collapses occur suddenly and crush the miners, and so arches [fornices] are left at frequent intervals
to support the mountains.
[71] Silex (quartzite?) occurs in both types of mine. They break it with fire and vinegar, but more
often, since that process chokes the galleries with fumes and smoke, they cut it to pieces with crushing
machines [fractaria] carrying 150 pounds of iron (aprox. 50 Kg), and they carry out the debris on their
backs through the gloom by night and day, passing it on to the next in line; and the last one sees the light
of day. If the silex seems too thick, the miner follows its edge and circumvents it, and yet working in silex
is considered easier, [72] for there is a kind of earth composed of a sort of clay mixed with gravel -they
call it gangadia -which is almost impossible to overcome. They attack it with iron wedges and the
hammers mentioned above.
When the operation has been completed, they cut through the necks of the arches [fornices] starting
from the furthest end. A subsidence gives the signal, and the only one who notices it is the watchman on
the top of the mountain. [73] He, by shouts and by signal, gives the order for the workmen to be called out
and at the same time himself flies down (from his station). The shattered mountain falls right away from
itself with a roar that cannot be imagined by the human mind, and with an equally incredible rush of air;
the conquerors observe the collapse of nature. And yet there is no gold so far, nor were they sure there
was to be any when they were digging, and it was reason enough for such dangers and expense to hope
for what they desired.
[74] There is another operation, equally laborious, and even involving greater expense. They have
previously led rivers flowing along the ridges of mountains for the washing of the collapsed material and
often from a hundred miles away (this should be 148 km; it is not real). They call (the channels) corrugi, I
believe from conrivatio. And this involves a thousand tasks: the fall of the water (at the mine) must be
headlong, so that it rushes down rather than flows, and so it is led from the highest places. Valleys and
gaps are bridged by built-up channels. Elsewhere impassable rocks are cut away and compelled to
provide a bed for hollowed-out wooden troughs. The man who does the cutting hangs from ropes, so that
to someone looking from afar he has the appearance not just of a wild beast but a winged one. [75] They
take the levels for the most part suspended by ropes and mark out the lines for the course, and where
there is not room for someone's feet to stand, streams are led by man. Also (there is) a fault in the
washing if the flowing stream brings down mud in its flow; this kind of earth they call urium. Therefore
they make the course through rocks or pebbles and they avoid the urium. At the head of the drop on the
brow of the mountain they dig out reservoirs, 200 feet on each side (59.2 m) and 10 in height (2.960 m).
In these they leave five outlets roughly three feet square (0.26 m2), so that when the tank is full and the
blocks are struck away a torrent shoots out with such force that it can roll boulders forward.
[76] Even now there is another task on the level ground. Ditches are dug out -they call them agogae -
through which the torrent of water can flow, and they are strewn at intervals with ulex. This bush is like
rosemary, rough and able to catch the gold. The sides (of the agogae) are enclosed with boards and the
channels are supported over broken ground, Thus the earth flowing onwards slides away into the sea,
and the shattered mountain is washed away, and owing to these causes Hispania has already moved its
earth from afar into the sea. The debris which is removed with tremendous effort in the previous method,
so that it does not choke the shafts [putei], in this method is carried away by water. [77] Gold sought by


VII
arrugia is not smelted, but has its own properties at once. Thus nuggets [massae] are found, not as in
shafts, and exceeding 10 pounds (3.29 Kg); they call them palagae, others palacurnae, and what is
smallest balux. The ulex (from the agogae) is dried, burned and its ash washed over grassy turf so that
the gold settles.
[78] Some have shown that Asturia, Callaecia and Lusitania yield 20,000 pounds (6.5 tons) in a
single year by this method in such a way that Asturia produces the most. And in no other part of the
world has there been this abundance over such a long period.
Another interesting aspect is the great amount of debris resulting from the washing
operations in the mine. This debris produced an obstruction in the valley that gave rise to the
Carucedo lake (Fig. 7).
Fig. 7.- Main interest points in the archeological site of Las Médulas (Sánchez-Palencia et al., 1999).
Fig. 7 shows a map with the location of the main archeological sites in the Las Médulas area.
These give a rich and detailed report of historical events occurred from the first centuries b. C.
to the III century. The older site corresponds to the San Juan de Paluezas castro, indicated by 3
in the northern part of the area. This is a typical castro (fortified village) of the astures and
shows how they lived previous to the Roman invasion, in small communities with a high grade
of autonomy and self-sufficiency. The decline of this world is observed in sites as the Borrenes
castro (9), a strong defensive wall built in century I b. C. probably as a reply to the beginning of
the Roman military operations. The Roman occupation is recognized in the areas with rests of
the gold mining operations, channels and debris accumulations, but also in some villages that
have been excavated. Near to Lago, number 30 indicates a domus (Roman house) with plenty


VIII
of comforts where the directive class lived (the remains found indicate that they even ate
oysters!). There are also miner villages at different points. Near to Orellán, number 12 indicates
a zone where the rests of the furnaces of a small iron industry can be observed near to a small
iron mine exploiting a ferruginous breccia of the basal Silurian.
Las Médulas mine was in operation from the beginning of the first century to the end of the
second or the beginning of the third century, when work ceased in all the gold mines of NW
Spain. The reason for this end is disputed among the scholars but all agree that the exhaustion of
the ores was not the cause. Calculations of the amount of gold obtained in Las Médulas mine
along this time span give c. 5 tons of gold (Fernández-Posse et al., 2002). As stated by the
Roman historian Florus (74-130), who wrote about the Cantabrian wars: “The astures, working
hard in the depths of the earth, began to know their wealth at the same time that they extracted
it for others.”
REFERENCES
Bird, D. (2004). Pliny’s arrugia: water power in Roman gold-mining. Mining History: The Bulletin of
the Peak District Mines Historical Society 15, 58-63.
Fernández-Posse, M. D., Orejas, A., Plácido, D., Ruiz del Árbol, M., Sánchez-Palencia, F. J. and
Sastre Prats, I. (2002). Las Médulas. Patrimonio de la Humanidad. Libro de la exposición en el
Real Jardín Botánico del CSIC (Madrid), 167 pp.
Martín-González, F., Heredia, N., Fernández, L. P. and Bahamonde, J. R. (in press). La mina romana
de oro de las Médulas (El Bierzo, Provincia de León, NO de España): Patrimonio de la
Humanidad como recurso docente para la enseñanza de las Ciencias de la Tierra. Revista de
Enseñanza de las Ciencias de la Tierra.
Sánchez-Palencia, F. J., Fernández-Posse, M. D., Fernández Manzano, J. and Orejas, A. (1999). La
zona arqueológica de Las Médulas , León. Instituto de Estudios Bercianos, 148 pp.
Spiering, E. D., Pevida, L.R., Maldonado, C., González, S., García, J., Varela, A., Arias, D. and
Martín-Izard, A. (2000). The gold belts of western Asturias and Galicia (NW Spain). Journal of
Geochemical Exploration 71, 89-101.