Monday, 6 August 2018

The Churnet Valley - Part 2

Barytes crystals in Hollington Formation sandstone

In the first part of the exploration of the Churnet Valley with the Sheffield U3A Geology Group, we encountered various exposures of the Hawksmoor Formation/Chester Formation, with its distinctive beds of conglomerate but, walking up Rakes Dale to Alton, this is succeeded by the younger Helsby Sandstone Formation/Hollington Formation

Vernacular architecture in the village of Alton

In the village of Alton, although many of the old buildings are built out of red brick, several houses provide good examples of Hollington sandstone, which varies from buff to red in colour and is commonly mottled. Although used largely for historic buildings in the West Midlands, and for restoration, its reputation made it worthy of inclusion in the Triton Stone Library

An exposure of Hollington sandstone on Toot Hill

Continuing to Toot Hill, the Hollington sandstone dry stone walling is seen to contain small crystals of the mineral barytes, which has weathered proud of the surrounding sandstone, and a good example of the same stone is found at the precipitous edge, which overlooks the Churnet Valley below and from which the ruins of the Alton Towers stately home can be seen. 

Alton Towers

Walking back down towards the Churnet Valley, various outcrops of the conglomeratic Hawksmoor Formation were encountered, including some that contained a distinctive pattern of honeycomb weathering

An outcrop of conglomeratic sandstone with honeycomb weathering

The close proximity of these outcrops to the tracks that we were following provided a good opportunity for the members of the group to closely study the pebbles and the distinctive cross bedding and other sedimentary structures in the rock, which provide clues to the formation and origin of these rocks.

An explanation of cross bedding © The Churnet Valley Geotrail

Although the Sheffield U3A Geology Group tries to introduce its members to a varied geology as possible on its field trips, the nearby Carboniferous rocks of the Pennines region – especially the limestone and gritstone of the Peak District National Park – tend to be visited time and time again and it was refreshing change for everyone to see a very different type of landscape. 

An old brick pit

The last stop on the field trip, before we returned to Oakamoor, was a brief visit to an old brick pit, which is marked by a shallow depression in the landscape that is now filled with water. The Triassic sandstones of England often contain beds of impermeable marl – a calcium carbonate rich mudstone laid down as lacustrine deposits – that was sometimes thick enough to be exploited 
for local usage or commercially.

An inscription by Thomas Patten and Company

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