Page:VCH Hertfordshire 1.djvu/50

 A HISTORY OF HERTFORDSHIRE At the inlier half a mile north-east of Northaw the Reading Beds are cut through as well as the London Clay, and the Chalk is laid bare, showing an anticlinal axis, or axis of elevation. The Chalk hill on which Wind- sor Castle is situated is an inlier on the same line of flexure, but that is some distance from our county. The Eocene beds of Hertfordshire form part of the north-western margin of the London Tertiary Basin, usually designated ' The London Basin ' only, but it is not strictly speaking a basin. It is a shallow trough running nearly east and west, and tilted up slightly towards the west, thus giving it the form of a wedge with the apex on the west. It may be inferred from the lines of flexure which pass through the county that the slight crumpling of the strata which took place after the de- position of these beds, affecting them as well as the Chalk beneath them, was due to lateral pressure exerted from the north-west or the south-east, which might either be caused by shrinkage of the earth from its loss of internal heat, or by volcanic activity, or by both these actions combined. This shrinkage is continually going on, and has been in progress ever since the earth commenced to be formed into a sphere of molten matter from its original incandescent nebulous state. It is the chief initial cause of volcanic outbursts, and we know that such outbursts occurred in the British Isles on the close of the Eocene period, that is in Oligocene and Miocene times. It is not improbable therefore that these flexures were caused by pressure from the north-west during the period when volcanoes were pouring out lavas and throwing out ashes upon the Chalk and older rocks of the north-east of Ireland and the west coast and western islands of Scotland. How soon after the close of the Lower Eocene period Hertford- shire was upheaved from beneath the sea we do not know, for what remains of the London Clay may be but a small fragment of the strata which have been deposited in our area and removed by denudation. The proximity of outliers of the Lower Bagshot Beds, as on Harrow Hill, indicates that the southern portion of the county, if not the whole, continued beneath the sea until at least the commencement of Middle Eocene times, but it may have risen before the end of the Eocene epoch, and have been dry land while the fluvio-marine (Oligocene) series of southern Hampshire was in course of formation, continuing to be a land-surface during Miocene and Pliocene times. In that case its surface would then have become greatly diversified by sub-aerial denudation, under perhaps a tropical rainfall ; but it was shortly to be subjected to the levelling action of a great sheet of ice. The fossils of the London Clay indicate a tropical climate, and the climate continued tropical or sub-tropical during Middle and Upper Eocene, Oligocene, and Miocene times. It then became cooler, and during the long interval which elapsed between the close of the Miocene and the commencement of the Pliocene period it reached the temperate stage, the molluscan fauna of the earliest Crag deposits being similar to that at present inhabiting the Mediterranean. Britain then stood high 16