Tuesday, 22 June 2021

A Field Trip in Conisbrough - Part 3

A detail of a glaciofluvial deposit at Constitution Hill

Continuing with my recce for a proposed field trip in Conisbrough with the Sheffield U3A Geology Group, I left North Cliff Quarry and started to make my way down the path along the east end of the area known as The Crags, before stopping to photograph an outcrop of massive, well bedded dolomitic limestone of the Cadeby Formation. 
An outcrop of dolomitic limestone at The Crags

When visiting North Cliff Quarry with the first group of Year 7 KS3 students at XP School in Doncaster, where the Doncaster Geodiversity Assessment had been used to select suitable sites for a geology field trips, I hadn’t noticed any rocky outcrops because it was covered in trees and it made me wonder if the area might be worth further investigation at a later date. 
A headstock on Doncaster Road

Continuing down the hill to Doncaster Road, I crossed the road and stopped briefly at the old headstock that commemorates the former Cadeby Main Colliery that, with the nearby Denaby Main Colliery, once dominated the landscape here and employed thousands of people. 
Sculptures produced for the former Earth Centre

Crossing over the railway at the entrance to the site of the former Earth Centre, where sculptures entitled “Fund Raising Tree” and “Egg” are reminders of this ill conceived and money wasting Millennium Commission project, I crossed the bridge over the River Don and continued past the site of the colliery railway marshalling yard until I reached Cadeby Cliff. 
Cadeby Cliff

In February 2007, when I last visited this site, numerous bryozoan reefs stood out against the general background of the escarpment but, in late September, when the vegetation had reached the height of its growth, these are much less obvious. 
A view of Constitution Hill in February 2007

Walking eastwards past Constitution Hill Bridge, I was disappointed to see that one of the most interesting rock exposures that I had seen during my survey of several hundred potential RIGS (Regionally Important Geological Sites) in South Yorkshire, in 1996/1997, was in danger of becoming overgrown. 
A view of Constitution Hill in September 2020

Above an angled retaining wall to the north side of the old railway line, which I have always assumed was built to support it, the 6 m x 2.5 m exposure of well cemented Quaternary glaciofluvial rock is becoming engulfed by trees and shrubs. 
Mapped glaciofluvial deposits on Constitution Hill

The British Geological Survey map marks an area of about 3.5 hectares of Quaternary glaciofluvial sand and gravel, most of which is covered by the gorse that thrives on the sandy soil formed here, and it is very briefly described in the 1948 memoir - Geology of the Country Around Barnsley.
A detail of glaciofluvial sandstone at Constitution Hill

Scrambling up to the outcrop, I had a further look at the various sediments here. The upper part comprises grey/brown, well cemented shingle, with large angular and sub-rounded limestone clasts and flattened sandstone pebbles, which in places are imbricated to the east. 
A section through the outcrop

This overlies a pink/orange gritty sandstone, which does not contain blocks of limestone and contrasts in colour with the overlying rock on both weathered and fresh surfaces. In places, the stone is covered in flowstone, which suggests that the sediments have been laid down in fissures or rifts in the limestone, where they have been cemented with calcite that has dissolved and percolated through the rock from above.
An example of flowstone

During my surveys of geological sites in Doncaster, I encountered other glaciofluvial sand and gravel deposits, but they were unconsolidated deposits that lie immediately above the Chester Formation and some have a silty fine grained matrix that could be derived from reworked till. 
Mixed sandstone pebbles and limestone blocks

I am not an expert in Quaternary geology but a former curator of Natural History at Doncaster Museum, Colin Howes, has long since advocated further research along the Don Gorge and, based on my experience of undertaking geological surveys in Doncaster, I have always shared his views and have similarly expressed these to Doncaster MBC.
A section through the outcrop

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