Wednesday, 13 May 2020
Clays and soils of Devon.
The sourcing of early southwest Neolithic pottery has depended largely on identifying inclusions but not the clays. Archaeologists have agreed generally that local clays would be sourced, not more than 3km, (Gosselain, The source, accessed on line) for making pots from the site. However, the only definite way to source clays is to test them through SEM and NAA-Neutron activation analysis, XRF analysis. These are the normal tests to determine chemical analyses of clays and archaeological sherds. This produces statistical and chemical analysis data which is used for a finger print match from samples taken from sherds, and it has to be considered whether the extra cost of this type of research is worth the gain in statistical enhancement of the current state of knowledge. If this information was available it would be possible to write definitive clay paste recipes for the Neolithic, not just speculative ones. The clays and soils are mapped below, in Figs 12 and 13 ((Harries et al 2004 Gallois, 2004) and the maps are relevant only to the study and prospection areas, of east Devon.
I. Sampling the clays, quartz and sands.
Appendix II provides a list of the sources of clay and tempers which were sampled, prior to work starting on the quartz temper project.
Quartz is available within most types of geology of the region. The quartz used in this project was sourced from The Plym estuary to The River Otter, at Hams Cottage, Honiton, see Fig.11, and Budleigh Salterton beach and estuary. The quartz pebbles from Budleigh were very distinctive in their uniform size and shape, however, and although Budleigh has been cited as the most likely source (Quinell and Taylor, forthcoming) of quartz used for pottery from sites in east Devon, pebble beaches are protected which prevents collection on a large enough scale for a project of this size when other sources are easily available. Pebble and vein quartz is readily available on beaches such as Whitsand Bay, nr Plymouth, in the river sands on the River Otter, Creedy and Yeo. Additionally, vein quartz is widespread in the environment. It is possible to say that it is found everywhere. From the samples prospected for the purposes of this project, this would appear to be the case. A lot of vein quartz ends-up as river smoothed pebbles of all sizes, both crystalline and non-crystalline. There are a lot of large, outcrops of crystalline quartz on Dartmoor both east and west. There are small rounded grains from granite river sands and Upper Greensand river sands (personal observation). It is also possible to find vein quartz which contains well-formed crystals.
The geology of the region has featured heavily in this project. The soils and subsoil particularly, not least for prospecting for good potting clays. As the project has proceeded the geographical area which should be looked at has increased to include Cornwall. However, when the focus of the project changed to production issues where sourcing clays was no longer the focus, the scope and timing of the project have limited the prospection for Cornish clays within the study area and only small samples of the east Devon clays have now been collected. I still haven’t managed to find a freely available source of Permian breccia clay.
The articles which have been really useful in locating some of these resources are published by the Ussher Society, Geoscience in the South-West England, which, among other types of research, run geo-commercial prospecting projects testing the stability of soils and subsoil in advance of civil engineering works. All articles are free access and available on online. www.ussher.org.uk. In fact the Ordnance Survey soil maps have not been referenced as they are not as up to date now as the Ussher Society proceedings.
The clays of the Exeter and east Devon region consist generally of the Namurian clay, which is found all over the region, see Fig 12 above, it is the weathering product of the shale-slate beds from the upper carboniferous Crackington formation; as well as the Devonian red sandstone clays which have not been prospected or used in this project. The Permian breccia, which would probably be relevant to the Raddon enclosure pottery, formed along a fault in the Crackington formation in the Permian era which is called the Crediton Trough, (Harries et al 1995). The Gault or keuper marl clays which are formed on the Upper Greensand which were sourced at Branscombe and Monkton, nr Honiton and extend beyond Lyme Regis would, perhaps, be relevant to the pottery at High peak and Donkey sanctuary.
Fig.11… River Otter at Hams Cottage, Honiton, Devon. Showing the natural sieving action of sand and gravel on meanders and river banks. Author’s photograph.
Although these two clays are very distinct, the Branscombe sample being a much darker brown, The Monkton Greensand sample is likely to be the same clay type as that found three kilometres away at Hembury Fort, see fig 13 below,. Hembury is covered in the exact same clay (personal observation, Todd, 1984)) and both these clays are weathered in situ. Both clays are extremely sticky and would likely make goodpotting clays. Initial plasticity tests for these clays was positive. With levigation and sieving, their plasticity can be increased but it is not definitively possible to say anything about the potential of clay until it has been fired to temperatures of 800˚, 1000˚,1150˚, 1280˚ Celsius. These are the traditional temperatures used to determine at which temperature clay matures and if it melts at any of these temperatures, it is not possible to use beyond that temperature and thus limits the potential of the clay.
Fig. 12. Geology of east Devon. Showing the major geological formations and nomenclature. This is the source for naming the yellow clay used in experiments as Namurian. (Harries et al 2004)