A HISTORY OF HERTFORDSHIRE represented in our county must therefore be considered as open to con- troversy and liable to modification from future research. The Quaternary era is usually divided into two periods, Pleistocene or Post-Pliocene, and Recent, the Pleistocene being equivalent to the Glacial period, and being divided into Lower, Middle, and Upper Glacial, and the Recent period being divided into Prehistoric and His- toric. 'The oldest Pleistocene gravels in Hertfordshire and the south of England generally are however of pre-Glacial age ; the Till or Lower Glacial boulder-clay is not represented here ; and later in the Pleisto- cene period arctic conditions did not prevail uninterruptedly. Our two chief beds of gravel Professor T. McKenny Hughes long ago distinguished as ' Gravels of the Upper Plain ' and ' Gravels of the Lower Plain,' the former being the older of the two, and having been deposited by the sea which levelled the county into a plain of which we now see the remnants in the highest ground of the area of the London Clay. 1 These older gravels have been investigated by several other geo- logists, and especially by the late Sir Joseph Prestwich, 2 who has given to the greater part of them the name ' Westleton Shingle,' separating under the term 'Southern Drift' the gravel which caps our most southern London Clay hills and also occurs south of the Thames, this being considered of earlier formation than the pebble-gravel of Westleton and the eastern counties generally. The largest patch we have of this oldest shingle-gravel spreads over Stanmore Heath from Little Bushey to Bentley Priory at a height of 400 to 450 feet, and there are smaller patches on the hill between Pinner and Watford, and east of Stanmore on Elstree and Brockley hills, nowhere less than 380 nor more than 450 feet in height. The great ice-sheet of Norway and Britain, approaching from the north-east, does not appear to have extended farther to the south than these hills, but it is more probable that this was due to the melting of the ice than that the hills, or the range or plateau of which they then formed a part, created a barrier against its further progress. According to the views of Professor Hughes they are the remnants of an extensive plain which then existed, having been formed into hills by subsequent erosion of valleys on the north and on the south. A little to the north of these hills are others capped by true Westle- ton Shingle. All these are Tertiary hills, either forming a part of the London Basin, in which case the shingle rests directly on the London Clay, or being outliers of Reading Beds with or without London Clay. We have no Westleton Shingle lying directly on the Chalk, which seems to show that the erosion of the Tertiaries from the surface of the Chalk had not taken place when this marine pebble-gravel was deposited. Mr. 1 ' On the Two Plains of Hertfordshire and their Gravels,' Quart. Journ. Geol. Sac., vol. xxiv. p. 283 (1868). 8 In three papers on the Westleton Beds read before the Geological Society, Quart. Journ. Geol. Sac., vol. xlvi. pp. 84-119, 120-154, and 155-181 (1890). 18
