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 



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