Surface geology:

Exposed dolomitic limestone bedrock at the top of the Pit Lane

The bedrock in the colliery yard consists of flaggy yellow-brown dolomitic limestone largely composed of granular aggregates of yellow dolomite crystals but with a scattering of other aggregates with a dark purple to black colouration  which gives it a slightly speckled effect.

Sample of dolomitic limestone bedrock from Pleasley Colliery

The dolomite grain size appears bi-modal:  larger grains about 90 - 100 mm in size and smaller ones at 5 - 10 mm. There are also a large number of mm sized inter-stitial calcite crystals which appear to cement the larger crystals together, although rather poorly. The smaller grains do not fill the inter-stitial cavities, giving the rock a fairly high porosity.

The dark grains have a pearly lustre and vary in size from a few mm up to about 30 mm. In some of the larger grains the colouration is zoned from whitish translucent at one end to dark semi-translucent purple at the other. Under crossed polars the darker translucent parts show bi-refringence (Calcite).

Hand samples fracture under the hammer quite easily and the conchoidal looking edges are very friable.

Removal of the shallow remains of the colliery yard, buildings and other surface features in 2006 revealed a number of fissures in the underlying bedrock varying from a few inches to a couple of feet wide. The top few feet of some of these had a filling of loose flaggy bedrock rubble whilst others were capped with brown (boulder) clay. At least one fissure contained a large plug of concrete.  The general appearance of the fissures suggested a conjugate alignment similar to that seen in the railway cutting to the West but with a somewhat different orientation.

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25 Nov, 2011