Glacial Traces in the South-West

This is an extract from Chapter 6 of THE BLUESTONE ENIGMA

 

There is great uncertainty about the nature of deposits on Salisbury Plain, in spite of the fact that archaeologists have been digging there for centuries. As indicated earlier, enigmatic “Clay-with-Flints” deposits are found here and there, and are generally deemed to be residual deposits which may be related in some way to periglacial or cold climate conditions but which have nothing to do with glacier ice. However, there is a danger that the term “Clay-with-Flints” is used as a catch-all term suggesting a degree of uniformity both in terms of physical characteristics and origins. We should perhaps learn a lesson from the manner in which the term “bluestone” has been somewhat unwisely used to cover a vast range of rock types. We should also be prepared to consider the possibility that some, at least, of the clay-rich deposits found in pockets on Salisbury Plain are ancient glacial deposits. They have never been properly studied. They may or may not carry far-travelled erratics. Further to the west, it would be reasonable to assume that there are glacial deposits (and maybe the remnants of a terminal moraine) somewhere beneath the thick peat layers in the Bridgwater - Glastonbury district and also somewhere near Bradford-on-Avon near the confluence of the Avon and Frome Rivers.

So what is the hard evidence for glaciation on the Bristol Channel coasts of Somerset and Avon, Devon and Cornwall? There are scores of sites where till and fluvio-glacial materials have been found, and scores more where erratic boulders have been found, particularly on or near the coast. These are just some of the key locations:

 

Trebetherick, Cornwall. Here there is a strange deposit which has been discussed by geologists and geomorphologists for many years. Exposures in the cliffs at Trebetherick Point show a “boulder gravel” which includes erratics of quartzite, slate, dolerite, gritstone and many other rock types, some of which are far-travelled. There are also other erratics in the vicinity. The consensus seems to be that this is a very old glacial deposit which has been changed by slope processes etc over many mellennia.

Porthleven. Here there is a giant erratic called the Giant’s Rock. It weighs about 50 tonnes, and it is still not known where it came from. It is washed by the sea at high tide. In the vicinity, resting on a wave-cut platform, there are many other erratic boulders made of slate, gabbro, granite and vein quartz. Some geomorphologists have argued that the erratics have been left here by ice-floes, but that explanation leaves something to be desired since sea-level must have been far lower then it is today when the norhtern hemisphere was cold enough for ice-rafting on a large scale. Almost certainly these erratics have come from very ancient glacial deposits that have since been eroded away.

Lundy Island. On the island plateau there are many erratics which are very different from the granite and slate which makes up the island’s bedrock. There is also evidence for ice moulding on the western side of the island. The ice which affected the island came from the west or north-west, and affected the island at least up to an altitude of 115m.

Ilfracombe. On the plateau between Ilfracombe and Berrynarbour there are abundant erratics carried by ice at least up to an altitude of 175m.

Valley of the Rocks (Lynmouth). This spectacular feature is difficult to explain except by reference to glacial meltwater at a time when ice was melting away along the coastline. There are other valleys at Hartland Quay, Damehole Point, Speke’s Mill Mouth, and Bideford that also suggest deep erosion by glacial meltwater.

Fremington. At Brannam’s Clay Pit there is a glacial deposit made up of clay with erratic stones and boulders including andesite, dolerite, granite, slate and vein quartz. In many ways it looks like the Irish Sea till found in West Wales and South-East Ireland. Another glacial deposit is found at Fremington Quay.

Croyde and Saunton. Here there are many more erratic boulders on the shore platform. Like the erratics at Porthleven, they are best interpreted as the relics of very old glacial deposits.

Scilly Isles. On the northern coasts of the Scilly Isles there are deposits of till and in places a clearly defined end moraine. These deposits appear to date from an advance of the Irish Sea Glacier around 19,000 years ago. To the south of the ice limit there are fluvio-glacial gravels and also erratics dating from earlier glacial episodes.

Westonzoyland. In this area masses of sand and gravel (referred to as “The Burtle Beds”) rise out of the Somerset marshes. These beds are now thought to represent interglacial estuarine conditions at a time of higher sea level. But there was at one time a great erratic here, known as “The Upping Stock.” Very inconveniently, it was smashed up to make road stone.

Greylake. This site, about 10 km to the SE of Bridgwater, is a classic location for the study of the Burtle Beds. Underneath the interglacial and intertidal muds and sands there is deposit with a clay matrix and stones of all shapes and sizes. It is now thought to be a till, similar in character and origin to the till at Kenn.

Bathampton Down, near Bath University. Here there are glacial erratics (probably very old) in limestone fissures on a plateau.

Holwell, near Nunney. Here on the eastern edge of the Mendip Hills, there is an enigmatic reddish brown clay full of rounded and sub-angular stones of many different types. It may well be related to the old glacial deposits of Bathampton Down.

Bleadon Hill, east of Weston-super-Mare. Here there are fluvioglacial gravels laid down when ice was melting away at the end of a glacial episode.

Kenn, Somerset. At Kenn Church there are fluvioglacial gravels with a channel cut into them, which was later filled with more recent deposits. So important is this site, and other related sites in the area, that geomorphologists now refer to “the glacial deposits of the Kenn Formation”.

Court Hill. Here there is the most unambiguous evidence for the glaciation of the Avon coastlands -- including till, fluvioglacial and glacial lake deposits.

Portishead Down. This ridge was overridden by ice which came in from the Bristol Channel and left behind till and other deposits, and an assortment of glacial erratics.

Kennpier. Here there is a till which appears to be very old, showing that ice pressed into the Avon coastlands and onto Sedgemoor.

Yew Tree Farm. Here, at the base of a sequence of deposits, there are coarse gravelly materials containing erratic cobbles. Again these seem to be derived from an ancient glacial deposit.

Weston-in-Gordano. At the bottom of a Quaternary sequence of deposits there are sands and gravels with abundant erratics that must have come from old glacial deposits.

The Punchbowl, Exmoor. Morainic material relating to a small valley glacier has recently been described. There are also signs that there was a small ice cap on Exmoor -- probably it covered the upland every time the climate was cold enough for the expansion of the Irish Sea Glacier.

Dartmoor. There are moraines in a number of places, suggesting that Dartmoor was also covered by a thin and short-lived ice cap on more than one occasion.



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