Homes Under the… Microscope

May 7, 201511:57 amNovember 12, 2015 12:11 pm

Thanks to the crusading efforts of Beth and, in particular, generous donations of stone by a host of homeowners, we now are starting to build up a useful collection of stone samples from buildings, notably of Old Red Sandstone used in the Bromyard Downs area. Having been ground down to less than the thickness of a human hair and mounted as microscope slides, these are now enabling us to investigate the subtle variations that may distinguish one quarry’s stone from another’s.

The main lesson we’re learning thus far is that those variations are indeed very subtle. On many petrological criteria (e.g. grain size, sorting, cementing minerals) we are seeing just as much range in a single quarry as across the whole set of samples. In many ways this is to be expected when considering the nature of Old Red Sandstone past environments and sedimentation. Sandstones were deposited in shallow streams subject to flash floods, rapid channel migration and drought. This means that deposits with markedly different grain sizes will often occur in close association with one another. Added to that is the fact that calcrete (a type of limestone that forms beneath the surface in arid to semi-arid soils) formation may proceed more quickly between channels, uninterrupted by sedimentation. Thus the calcite cements common in these sandstones are also highly variable laterally.

We are now moving to look at the mineralogical makeup of the sand grains within the rocks as this is something which might be more conserved within a given layer. The material being supplied to streams and rivers is determined far more by the geology, erosion and weathering in mountain regions of the rivers’ catchments than by the locally variable mechanics of flow at the site of deposition.

That said it remains to be seen whether the variations required in climate, bedrock outcrop or drainage patterns if the upper catchment existed and, if so, whether they can be meaningfully distinguished in the rock. Nonetheless there are promising signs that certain minerals such as feldspar and mica do show variation between slides. Furthermore, even if nothing conclusive can be determined in this way we will have learned something about what can be achieved by these methods.

Written by Elliot Carter

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