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Monday 7 December 2009

Clay-with-flints and Chalky till

Back to the UK. Been pondering on the ancient deposits that sit on the chalk in Eastern England. The map shows that in Hertfordshire there is a south-western zone with "clay-with-flints" (brown) on the interfluves (implying that these deposits are very old, and are indeed older than many of the river valleys) and a north-eastern zone (light blue) with Chalky Till. This is a greyish colour (see the photo above) and contains chalk, flint, and erratics from the north. It has all the characteristics of a typical till laid down by a wet-based glacier -- supposedly in the Anglian Glaciation. Does the distribution of this till actually give us the extent of the maximum glaciation in this area?

OK -- on to the "clay-with-flints" (which occurs quite frequently on Salisbury Plain and on the higher Chalk Downs. Here is the Wikipedia entry:

Clay-with-Flints was the name given by W. Whitaker in 1861 to a peculiar deposit of stiff red, brown or yellow clay containing unworn whole flints as well as angular shattered fragments, also with a variable admixture of rounded flint, quartz, quartzite
and other pebbles. It occurs in sheets or patches of various sizes over a large area
on the north to Sussex on the south, and from Kent on the east to Devon
on the west. It almost always lies on the surface of the Upper Chalk, but in Dorset
it passes on to the Middle and Lower Chalk, and in Devon it is found on the Chert-Beds of the Selbornian group (A. J. Jukes-Browne, The Clay-with-Flints, its Origin and Distribution, Q.J.G.S., vol. lxii., 1906, p. 132).

Many geologists have supposed, and some still hold, that the Clay-with-Flints is the residue left by the slow solution and disintegration of the Chalk by the processes of weathering; on the other hand, it has long been known that the deposit very frequently contains materials foreign to the Chalk, derived either from the Tertiary rocks or from overlying drift. There is evidence against the view that the deposit is mainly a Chalk residue, This shows that many patches of the Clay-with-Flints lie upon the same plane and may be directly associated with Reading Beds. He concludes that the material of the Clay-with-Flints has been chiefly and almost entirely derived from Eocene
clay, with addition of some flints from the Chalk; that its presence is an indication of the previous existence of Lower Eocene Beds on the same site and nearly, at the same relative level, and, consequently, that comparatively little Chalk has been removed from beneath it. Finally, I think that the tracts of Clay-with-Flints have been much more extensive than they are now. Clay-with-flint is a nutrient rich substance unlike chalk.(bc. cit. p. 159).

It is noteworthy that the Clay-with-Flints is developed over an area which is just beyond the limits of the ice sheets of the Glacial epoch, and the peculiar conditions of late Pliocene and Pleistocene times; involving heavy rain, snow
and frost, may have had much to do with the mingling of the Tertiary and Chalky material. Besides the occurrence in surface patches, Clay-with-Flints is very commonly to be observed descending in pipes often to a considerable depth into the Chalk; here, if anywhere, the residual chalk portion of the deposit should be found, and it is surmised that a thin layer of very dark clay with darkly stained flints, which appears in contact with the sides and bottom of the pipe, may represent all there is of insoluble residue.

A somewhat similar deposit, a congbomirat de silex or argile a silex, occurs at the Paris basin, in the neighborhood of Chartres, Thimerais and Sancerrois.

So what are the relationships of these two deposits? Both are highly variable -- and the Clay-with-Flints has -- in some instances -- been interpreted as a glacial deposit because it contains so much erratic material. Mostly, however, it is interpreted as a residue or remnant of younger (Tertiary) rocks that once capped the Chalk and which have subsequently been eroded away.

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