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Sunday, 5 June 2016

Ice Age events in Dorset

Accumulated sarsen stones -- remnants of a duricrust associated with an ancient Tertiary rock cover now largely eroded away (credit: Dorset Life magazine).

I came across this article by John Chaffey in Dorset Life magazine.  Worth sharing, as it is relevant for much of southern England.

The Geology of Dorset: the Quaternary period

In the last of his series, John Chaffey looks at the Quaternary period in Dorset

Published 2013

Dorset Life Magazine

The Quaternary Period includes the older Pleistocene (Ice Age) deposits, ranging from 1.8 million years to 10,000 years old. In Pleistocene times, much of Britain was under the influence of ice sheets, but this glaciation did not extend as far south as Dorset. During Pleistocene times Dorset experienced a periglacial climate, similar to the tundra conditions of present-day northern North America, Europe and Asia. No ice sheets were present in Dorset, but extensive snowfields covered much of the higher land.
During the Pleistocene, Dorset was under the influence of permafrost, where the upper layers of rock were frozen to some depth. Near the surface was the so-called active layer, where some thawing occurred in the summer, with consequent freezing in the winter. After the spring thaw the active layer became waterlogged, and moved slowly downslope, a process known as solifluction. These soliflucted deposits can sometimes be observed in coastal sections. A particularly good example lies at the seaward end of Scratchy Bottom, where the buff-coloured periglacial material can be seen in the cliffs resting on the vertical Chalk strata. Terraces and lobes of soliflucted material are common on the sides of many valleys in Dorset.
Alternative freezing and thawing of rocks, which occurs under periglacial conditions can produce shattered angular debris. On a large scale, cliffs that existed in the Pleistocene can be disrupted and broken by this action and large quantities of debris would have accumulated at their base. It seems likely that the cliffs of Portland Limestone at St Aldhelm’s Head were subjected to freeze-thaw action in the Pleistocene, and the large screes at their base, now covered thickly with lichens, may have originated at that time.
Where there were accumulations of snow in hollows on hillsides during periglacial times, it is possible that the process of nivation (expansion through snow action) may have been important in expanding these hollows. Small hollows on the southern side of the Purbeck Hills may well have contained accumulations of snow. Freeze-thaw action around the edge of these hollows could have led to their enlargement, with solifluction carrying the resultant debris downhill. One hollow on the western side of Godlingston Hill demonstrates these features remarkably well, with a clear section at the base of the hill showing the soliflucted debris that has been removed from the well-marked hollow above. Similar hollows are found just to the west of Corfe Castle.

In west Dorset the unusual Valley of Stones in the Chalk downland to the west of Black Down displays a larger number of boulders, known as sarsen stones, in the valley bottom. These sarsen stones are flinty conglomerates or silicified sandstones that originated as part of a ‘duricrust’, an indurated crust that developed under tropical conditions, (similar to those in present day Australia), on a past cover of Tertiary rocks on the surrounding Chalkland. These relict fragments of the ‘duricrust’ could have been moved downslope by solifluction into the valley bottom, where they are found today. Valley sides in the Chalk are often mantled with Chalk debris, known as ‘coombe rock’ which may contain flints: such debris has been carried downslope by solifluction. Soliflucted material is also known as ‘head’: the valleys leading down to the coast in the south of the Isle of Purbeck have thick mantles of head, containing angular fragments of the local Portland Limestone. It is likely that most valley sides, and many dry valley bottoms carry a cover of periglacial head.

During Pleistocene times Dorset’s climate varied between very cold spells and warmer interludes. As temperatures began to rise towards the end of the colder spells, thawing caused melting of the snowfields on the higher parts of the landscape. Flow in the streams increased, enabling the rivers to carry much greater loads of debris, particularly flinty material from the Chalk. This debris was laid down as huge spreads of gravel in the main river valleys such as the Piddle, the Frome and the Stour, and possibly the ancient Solent River, which originally ran from what is now the entrance to Poole Harbour eastwards along the Solent. Later, it appears to have changed its course to flow in a more southerly direction just east of the present Isle of Purbeck. From time to time these rivers incised their courses, leaving the gravel spreads as important terraces above the new and lower course of the rivers.

Today these gravel terraces dominate the scenery of the lower valleys of the three main Dorset rivers. In the Frome and Piddle valleys they first appear around Cattistock and Puddletown respectively, and become increasingly more important elements in the landscape downstream. Successive incisions by the rivers have produced a staircase sequence of terraces in these valleys, becoming increasingly higher away from the present river. This staircase effect is particularly well seen on the long straight road from Gallows Hill to Wool as it descends towards the Frome valley. Much of the urban sprawl of Bournemouth has spread across the terraces left by the Stour. In the east of the Bournemouth conurbation the steep slopes of Pokesdown Hill separate the higher terraces on which Pokesdown and Boscombe were built from the much lower ones on which Iford and Tuckton are found. On the opposite side of the Stour, Bournemouth International Airport flourishes on one of the largest spreads of the Stour’s gravels, and the commanding heights of St. Catherine’s Hill brooding over Christchurch represent the highest level of terraces in the lower Stour and Avon valleys. A glimpse at the cliffs on either side of Branksome Chine will reveal Pleistocene gravels at the top of the cliff, overlying the Branksome Sand below.

These Pleistocene gravels in the valleys of Dorset’s rivers have been and continue to be of considerable economic value, being worked as an important source of building aggregates. Much valuable agricultural land remains in cultivation on the terraces, particularly in the Frome valley between Woodsford and Moreton.
During the warmer phases or interglacials in the Pleistocene, sea levels were higher than at present, since melting ice was returned to the oceans. These higher sea levels are often marked by so-called raised beaches, two of which are found quite close to Portland Bill.
On the western side, the raised beach is approximately fifty feet above present sea level, and it can be seen capping the cliffs to the north of Pulpit Rock. It has been dated as 210,000 years BP (before the present). On the eastern side the raised beach is about fifteen feet lower and is dated at about 125,000 years BP. Beneath this eastern raised beach angular and broken limestone fragments are found, possibly providing evidence of a colder period before the interglacial.

After the ice sheets disappeared from northern Britain, global sea levels began to rise, with important repercussions along the coast of Dorset. About 10,000 years ago, the Chalk ridge between the Isle of Purbeck and the Isle of Wight was still more or less continuous, but the rising sea had eroded softer rocks to the south and began to attack the ridge itself, breaking through the existing river-cut gaps in it and eventually eroding away the softer rocks to the north to form what is now Poole Bay, although it has been suggested that Christchurch Bay to the east may have been formed somewhat later. Water then began to flood into the lower courses of the Stour, Avon, Frome and Piddle rivers and Poole Harbour and Christchurch Harbour began to take shape. With rising sea levels in post-glacial Holocene times much of the Dorset coast began gradually to assume its present day outline. At the beginning of this period, much of Lyme Bay was still dry land, covered with aprons of debris brought down by streams and rivers from the north. Rising sea levels began to sweep up this debris and push it landwards, thus initiating the formation of Chesil Beach, the fourteen-mile-long shingle bank that today runs from Chiswell to West Bay. During this period further material was added to Chesil Beach from West Dorset and possibly East Devon, although this supply of shingle has now largely ceased as a result of the building of the Cobb at Lyme Regis, and the harbour works at West Bay. During this post-glacial time the coast has been modified by erosion and the effect of landslides, particularly in West Dorset, the Isle of Portland, and parts of the Purbeck coast. Landslide activity has also been important inland, particularly around Shaftesbury, where it may date back to late glacial times.

We do not know exactly when man appeared in Dorset. Terrace river gravels have yielded artefacts, such as flint and chert hand-axes. Mesolithic (Middle Stone Age) settlements are known to have existed in Portland, and also on Cranborne Chase and in the river valleys to the south, but it is not known whether these were only temporary hunting encampments. It was not until the Neolithic Period (New Stone Age) that permanent farming settlements were established.

   Raised beach on the east side of Portland Bill. It lies above a rock cliff and is c 35 feet above the level of the present storm beach


Mendip Mary said...

Bloody hell.
Triassic!!!! too much White Lightening, too little proofing.
Time for you to take a long lie down in Sweden methinks

BRIAN JOHN said...

Oh dear -- sorry to have upset you, dearest Mary. Not concentrating. Should have been "Tertiary" -- now corrected! Back to the garden.......

TonyH said...

"Mendip Mary" reminds me of that wonderful song to a siren:-

Celia, my Celia......
Who is She?

on the old 78's my dear Papa would play on his wind - up gramophone.

Come to think of it, lamented Myris, no longer a regular contributor, was a bit of a wind - up merchant. After all, he and his friend really acted as the geologist catalysts for all this Rhosyfelin rhubarb..... Talking of which, time for dinner.

TonyH said...

Brian, am I correct in thinking you surmise that it may well have been the Anglian ice sheet that affected the area south of the Severn, possibly as far south as parts of Salisbury Plain? If so, could it also have affected large parts of Somerset, even into north Dorset?

I think you have argued that traces of Anglian glaciation may lie well underneath present surface landforms.

BRIAN JOHN said...

Yes, all the signs are that the Anglian glaciation was the most extensive one in southern Britain. The dating of sediments seems pretty secure in Somerset, Devon and Cornwall. And yes, in the Somerset Levels the glacial materials are deeply buried beneath later sediments. Glaciological modelling suggests that ice at its greatest extent could have reached Dorset, but as far as I can see there are no traces of glaciation there.

AG said...

Any rock climber who's climbed anywhere between Swanage and Portland Bill, can attest that up to 20ft of the uppermost rock of most climbs consist of a layer of extremely loose brecciated bedrock! So much so that the guidebooks recommend the installation of a stake and 20ft of rope at the top of a climb before beginning an ascent.

My own opinion is that this brecciated bedrock layer represents the top of the permafrost and the base of the active permafrost layer! In agreement with research into brecciated bedrock by Sussex University! It is most probable that study and measurement of the thickness of this layer would provide important information about average air and ground temperatures during the last glacial period!