GEOLOGY 



only evidence in our county of such a mild interval is the presence of 

 Ostrea edulis where the land must have been submerged at least 300 

 feet, for Stevenage now stands higher than that above sea-level. 



The land rose and the cold increased ; the snow-line gradually 

 extended southward from the northern islands of Scotland to the south 

 of Ireland and South Wales ; and glaciers descended from the snow- 

 fields and ploughed up the land at least as far south as the Chalk of the 

 eastern counties. The debris was deposited in the depths of the valleys 

 and on the slopes of the hills, and even up to the top of the escarpment 

 of the Chalk, as on Reed Hill near Royston, but none is to be seen on 

 the higher part of the escarpment towards the west, which would then 

 be an island in the Glacial sea. This is but one of many views which 

 are held as to the conditions under which the ' great chalky boulder- 

 clay ' was deposited, and it seems to be the most likely, but it has been 

 well said : ' Where, as is too often the case with Glacial deposits, there 

 is room for much diversity of opinion, geologists fully avail themselves of 

 it. Hence it is best to picture the Glacial period in a general way, 

 and to admit that glaciers and ice-sheets, icebergs and coast-ice, have all 

 had their share in the production of the phenomena, although we cannot 

 always localize their action. 1 



The Upper Glacial boulder-clay (Middle Glacial of S. V. Wood) 

 is generally known as the ' great chalky boulder-clay,' owing to the 

 numerous boulders of chalk which it contains. It is usually a rather 

 dark bluish-grey calcareous clay, containing chalk in all forms ground 

 up with it, as small pellets or pebbles, and in all gradations of size up to 

 very large masses, most of the larger chalk boulders being so hard as to 

 have preserved, with the protection afforded by the clay in which they 

 are imbedded, the scratches and grooves made by contact with harder 

 rocks whilst they were being carried along imbedded in ice, this being 

 the meaning of the somewhat misleading term 'ice-grooved rocks.' 

 Imbedded in the boulder-clay are also many chalk-flints ; boulders from 

 various formations, chiefly of rocks of Jurassic age, but also of much 

 older and more distant strata, such as Carboniferous Limestone, deeply 

 ice-grooved ; pebbles of quartz and small boulders of granite derived 

 from formations still more distant both in time and space ; and fossils 

 derived chiefly from the Lias and Oxford Clay. No fossils contempora- 

 neous with its formation have been found in it. 



Boulder-clay is spread over the greater part of north-east Hert- 

 fordshire as a continuous bed except where it has been cut through by 

 the rivers ; it covers most of the higher ground in the centre of the 

 county where the rivers have cut more deeply into it than on the east ; 

 and the most south-westerly patch is at Bricket Wood between St. 

 Albans and Watford. Sections of it may be seen there and at Little 

 Berkhamsted, Bayford, Hertford Heath, Buntingford and several other 

 places. 



1 H. B. Woodward, The Geology of England and Wales, and ed. p. 486 (1887). 



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