Sunday, 5 June 2011

Book review - Exe Valley

Hugh Prudden of the Somerset Geology Group has reviewed this new book as follows:-
The Quaternary of the Exe Valley and adjoining areas: Field guide.
Basell, L.S., Brown, A.G. and Toms, P.S. (eds).  2011. 
Quaternary Research Association.  
The Guide was produced to accompany the Field Meeting to the Exe Valley and adjoining areas 4-7 April 2011.The recently published field guide is an important contribution to our knowledge of the Quaternary of Somerset.  The following chapters are of particular interest for Somerset readers: -
The archaeological context of the Exe Valley and adjoining areas. (Basell, L. and Brown, T.) This chapter summarises the context, chronology, typology and raw materials of artefacts, and draws together the present state of knowledge.
Quaternary fluvial sequences and landscape evolution in Devon and Somerset (R. Westaway). This is a complicated chapter based on shoreline flats, river terraces and karstic features plus magmatic underplating.  Figure 3.3 shows there to have been an estimated post-Ipswichian uplift of 11-12m around Burroughbridge rising to 13-14m around Weston-super–Mare and the Mendips.
The Haldon Gravels Although in Devon this is a tightly argued assessment of the gravels on Haldon and is highly relevant to the wide spread of early Tertiary gravels on the Blackdown Plateau.
The Devil’s Punchbowl (S. Harrison).Visitors to Exmoor will be aware of the cirque-like feature above Winsford. It has been suggested that this was indeed the site of a small cirque but some of us wondered if non-glacial, as opposed to periglacial, processes could account for the features.  However, Stephen Harrison has researched the landforms and sediments in more detail and has added more supportive detail to his original suggestion that the evidence points to a cirque glacier.  Read the account and visit the site to see what you think. It may be relevant that recent research has suggested that the surface bedrock of Lundy in the Bristol Channel became exposed following deglaciation between 40-30 ka.  The presence of the nearby Irish Sea Ice Stream at a relatively recent date might explain increased snowfall and what seem to be fresh glacial landforms. Chard Junction Quarry and the Axe Valley gravels (Basell et al.) There are very few workable gravel resources in Somerset compared with Dorset except to the south of Chard in the Axe Valley; the Jurassic limestones are rapidly worn down and the clays are a non-starter.  However, there are considerably quantities of durable Cretaceous chert and flint outcropping on the sides of the Axe Valley together with a smaller contribution of Tertiary gravels from the Blackdown Plateau. These gravels are unlike the terrace staircases in most of our rivers; they from a stacked sequence resulting from erodible bedrock, high rates of sediment availability and periglacial transport.  Palaeoliths have recently been found in situ and optical age estimates give an intrinsic measure of reliability and a weighted mean age of 326+/- 22 ka.  The stacked gravels include periglacial structures at several levels; at the time of the field visit an excavated horizontal surface showed narrow vertical fissures filled with fine material as part of polygonal structures; it was like walking on the tundra!.
The Middle Pleistocene Deposits and Archaeology at Broom. (Horsfield et al.) The Broom sites are some 3km SW of the Chard Junction pits and were worked in the 1930s. C.E. Bean made valuable records and collections of Lower Palaeolithic hand axes at the time. This chapter summarises the stratigraphy, sedimentology and age of the deposits. Of particular interest is the Broom Member (Broom Gravel Member and Broom Sand and Silt Bed); the Bean archive suggests the association of the majority of artefacts with land surfaces contemporary with the Broom Member.  Recent pollen samples indicate a regional vegetation of boreal forest, with tree stands interspersed with wide expanses of open country and ericaceous heath.
The Doniford gravels (Basell, et al.) This is perhaps the most useful exposure for the general geologist as it has open access and is well-exposed.  The cliff is subject to erosion but this maintains a clear face.  It lies just to the east of Watchet harbour.  We owe much to the Wedlakes for their collections and observations together with those of C. Norman.  OSL samples do form a stratigraphically consistent time-series spanning from 65+/- 5ka to 25 =/- 3ka. These gravels are associated with periods of hillslope recession, mass wasting and periglacial solifluction which contributed to the gravels we see today.  It was a time of shallow braided rivers choked with rock debris.  The widespread angular unsorted head on lower valley sides is evidence of mass wasting; come the next glacial phase doubtless the head would be on the move again.
The geological setting and landform evolution of the Exe Valley and adjoining areas. (Brown, et al.)
This is a broad brush overview of the structure, topography, bedrock geology and superficial deposits.  It is thought–provoking.
Get your copy today from:
Quaternary Research Association.

Members £15: non-members £20.
Please note that on November 3rd Professor Tony Brown and/or Dr. Laura Basell will be talking to the Bath Geological Society about this new research.

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