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Geology and hydrogeology ... …

Report No. SRL/FP/004.1 (10/05/2006)

Other inclusions and impurities in the Clay may also reduce its hydraulic permeability. Indeed, the uppermost layers of the Kimmeridge Clay are likely to be sandy and fissured, as at Nuneham Courtenay13.

(b)

Palaeo channels

The Kimmeridge Clay lying at the base of the Northmoor Floodplain Terrace river terrace is susceptible to erosion due to the movement of water. The Kimmeridge Clay is bounded on its upper side by an unconformity, which was caused by erosion of the overlying geological layers. The Upper Thames and its other main tributaries, the Evenlode and the Windrush, are flanked by oolitic gravel terraces formed during the Pleistocene, when rivers were more capable of denuding the Cotswolds than they are today. The highest terrace (Hanborough), possibly dating from the Anglican glacial, is composed in part of Triassic pebbles derived glacially, termed the Northern drift. A layer of chalky Boulder clay ice from the East Midlands reached the northern edge of the Cotswolds and its meltwaters provided the flinty outwash gravels down the valleys of the Cherwell and through the Evenlode Gorge to construct the Wolvercote Terrace. A thick boulderclay lies on the valley floor at Sugworth, just to the north of Radley, and this author believes that the same boulder clay was found in the North of Abingdon during excavations for the extension of his house. Although not formally identified at the time, the stones had all the hallmarks of glacial moraine origin. All of these glacial episodes provided source-water for a far larger river system than is demonstrated by the Thames today. Flow would have been variable in magnitude and location, dominated not just by glacial melting, at the end of the Pleistocene ice-age, but also by the variable impermeability of the underlying permafrost layer as it melted.

All of the above lead to the development of unconformities both in large scale and in smaller channels. The channels, excavated in the surface of the Kimmeridge Clay are the subject of our concern. These channels, once containing relatively impervious clay, have been filled with other deposits, mainly gravel and Lower Greensand; occasionally with silts and clays; and sometimes peaty material, which develops in many mountainous areas in

the vicinity of

glaciers.

and they are

known to

These palaeo-channels exist in the Kimmeridge

are generally not Clay at Radley.

impervious to water flow The presence of palaeo

channels in the Kimmeridge Clay would make u n l e s s t h e s e c h a n n e l s w e r e g r o u t e d o u t a n d it unsuitable for use r e p l a c e d b y g o o d as a bunding sealant, q u a l i t y f o s s i l - f r e e c l a y a n d p h o t o g r a p h i c e v i d e n c e 4 t o s u p p o r t t h e were constructed, the palaeo channels were of leaching. A number of local people have the clay bunds, which have the appearance during the installation. There is eye-witness contention that, when the bunds for Lakes H/I left in situ. If so, they would now be a source commented on the apparently dark regions in

E x c a v a t i o n o f a p a l a e o c h a n n e l u n d e r L o n g m e a d L a k e ( L of palaeo channels. 2 ) s h o w e d 1 4 d o w n t o , a t l e a s t , 1 5 m e t r e s w i t h n o s i g n o f a n y c l a y 1 5 .

that it extended

13 Corser C E, The sand and gravel resources of the country around Abingdon, Oxfordshire. Description of parts of 1:25000 sheets SU

49, 59 and SP 40, 50. Mineral Assessment Report 38. Institute of Geological Sciences, National Environment Research Council. HMSO, London (1978)

14 Eeles R M G, pers comm (Dr Eeles spent 7 years excavating the base of this lake for archaeological remains.)

15 apart from a linear band of Kimmeridge Clay running east-to-west, which was mostly full of large siltstone nodules and appeared not

to be in situ, as it was undermined by sands and gravels.

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© SAVE RADLEY LAKES 2006

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