Geology Corner

1st - 31st December

Geology Corner

Many thank to Dr Doug Robinson and Professor Chris King for this month’s instalment.

Carboniferous building stones (360–300 Ma) 

The Carboniferous period had two main environments when sediments were laid down, with the Lower Carboniferous (360 – 330 Ma) being dominated by clear tropical seas, and the Upper Carboniferous (330 – 300 Ma) by river deltas topped by swampy forests. The limestone rock type that dominates the Mendips formed in the Lower Carboniferous while mudstones, sandstones and coal formed in the Upper Carboniferous. The limestone contains many fossils, which along with palaeomagnetic evidence, show that the region lay south of the equator, at about 15°S. This is similar to the present-day position of the city of Cairns in Queensland, Australia near the Great Barrier Reef. Like the offshore Cairns area today, the setting was a clear tropical sea full of sea life in which lime (calcium carbonate) was laid down as lime sand and mud. Fossils in these rocks are a record of the abundant sea life, particularly of corals and shelly animals, which can be found in some blocks in the Palace buildings and gardens. 

The limestone often occurs in layers of fine-grained lime sand and mud as found in the gatehouse wall (walk over the drawbridge into the gardens and turn right to find the block shown in the photo). These massive and hard layers of limestone were difficult to work with medieval tools and so it is not a common building stone in the Palace. These blocks have a typical pale greyish colour when they are weathered, but when a broken fresh surface is visible they have a much darker grey colour as in the photo. 

The Carboniferous limestone is the main rock type of the Mendip Hills, as exposed on the southern slopes of Pen Hill in the Stoberry area, and in Tor Woods immediately to the east of the Palace.  It is also the rock that forms the crest of much of Mendip as well as Cheddar Gorge and Burrington Coombe.

You can spot field walls of Carboniferous limestone right across the Mendip Hills, adding to the typical Mendip scenery. Upper Carboniferous rocks are not present on Pen Hill, but there is a small outcrop at Deer Leap, near Wookey Hole. They are mostly found in the eastern and northern Mendip area, where there are many former coal mining villages (for example Pensford on the Bristol road, with its Pensford Miners Welfare Institute building – now the Miners Coffee shop).

These rock types are not very suitable as building stones, but one of the sandstones (Pennant) has been widely used for flagstones across the region. These blocks often show fossil ripple marks indicating current movement as the sand was laid down on a beach or in a shallow sea.

Cross the bridge into the Spring Gardens and about 50 m to the north east there is a paved area around an old sluice from which the photo showing ripple marks formed on the ancient Carboniferous beach is taken.