The three major groups of debitage found in the Stonehenge Landscape are dolerites, rhyolitic tuffs (almost exclusivelyfrom Craig Rhosyfelin, now designated as Rhyolite Group A–C) and ‘volcanics with sub-planar texture’ now designatedas Volcanic Group A and Volcanic Group B. The only other significant debitage group, but only accounting for about5% by number, is an indurated sandstone now called the Lower Palaeozoic Sandstone.The Lower Palaeozoic Sandstone is a coherent lithological group with a slight metamorphic fabric and is afine-grained feldspathic sandstone with characteristic dark, mudstone intraclasts. Palynological (acritarch) dating ofthe sandstone suggests that it is Late Ordovician or younger whilst the petrography suggests that it is older and moredeformed than the Devonian (ORS) sandstones exposed in South Wales.
Spatially, as with all the major debitage groups, the Lower Palaeozoic Sandstone is widely and randomly distributedthroughout the Stonehenge Landscape; temporally, almost none of the debitage has a secure Neolithic context but somemay have later Roman associations. The debitage cannot be matched to any above-ground Stonehenge orthostat butmay be from one or two buried and, as yet, unsampled stumps.
The lithology is believed to be from an unrecognised Ordovician (or less likely Silurian) source to the north or
northeast of the Preseli Hills.Although there has been confusion within the archaeological literature between the ‘Devonian’ Altar Stone, Lower Old Red Sandstone (Devonian) Cosheston Group sandstone and the Lower Palaeozoic Sandstone, all three are very different lithologies with separate geographical origins.