The Quarternary History of the Mendips
Notes from a lecture by Mr P Smart.
Since the Quarternary Period is only 2 million years old, the geology is very near geomorphology, the study of land forms.
The Mendip ridge is a plateau of Carboniferous Limestone, topping 260 metres with the valleys infilled with Dolomitic Conglomerate of Trias age. These, during the Quarternary have been re-eroded, so the present landscape resembles the Triassic one, but today is temperate and not, as then, hot desert. For during the Quarternary there have been 3 glaciations covering most of Britain. Elsewhere the Carboniferous Limestone has been scraped bare of soil, giving the limestone pavements of Northern England, but not so in the Mendips. Here there is a deep rich soil, which suggests that ice did not cover the top of the ridge, although there is a local till in the Gordano valley.
On the north side of Mendip, the problematical Rickford valley must be a periglacial overflow channel, marginal between Carboniferous Limestone and triassic marls.
The study of caves has revealed valuable information. The joints and bedding planes of the limestone channel the water, which in turn dissolves and enlarges them forming passages, and eventually underground river systems. With the lowering of sea level during glacial periods the water table is lowered, so the underground passages are extended, making a sequence of caves, the oldest ones being near the top surface. An underground river system is often smooth, tubular and meandering, just as a surface river.
From swallet holes on the surface cave systems get infilled with local debris of Old Red Sandstone, Limestone and Dolomitic Conglomerate, which in time get covered by stalagmites. Very often a sequence of infill sediments are preserved in this way.
On the top of Mendip is a dry valley network converging at Cheddar Gorge. These are typical of limestone country, so when rain or snow is released it drains away quickly into gorges or swallet holes with considerable energy. But during glacial times when permafrost conditions occurred, on thawing the top layer melts and slumps downhill taking much debris with it. This can block the swallet holes.
The top of Mendip shows very good periglacial phenomena so probably it stood above the ice sheets which were filling the valleys to the north and south.