108 J. V, ELSDEN — POST-TEETIAET 



escarpment. (See Fig. 2.) Generally speaking the deposits them- 

 selves testify to the truth of this theory, for the coarser gravels are 

 found nearest the ridge, where the current would be strongest, and 

 the most distant deposits, as at Hertford, where the current 

 would be weakest, generally take the form of brick-earth. 



In the Upper Glacial period the submergence still went on, and 

 the strong current was replaced by a more open sea. The bottom 

 of this sea now became covered with a thick deposit of ice-trans- 

 ported clay, the Boulder-clay, which was brought down from the 

 north on icebergs and dropped in masses over the Middle Glacial 

 beds, capping the highest hills, and occupying the deepest valleys, 

 except where recent denudation has removed it. 



At length, in the Post-Glacial period, the land began to rise, 

 and every part in turn, as a receding shore-line, was subjected 

 to the action of the waves. Thus the surface of the Boulder-clay 

 was eroded and reconstructed, causing a clayey loam to accumulate, 

 These denudation gravels are often found tilling hollows in the 

 Boulder-clay, although most of them have been swept away by 

 subsequent erosion. 



Mr. S. V. Wood's vieios. — In their paper on the Later Tertiary 

 Geology of East Anglia,* Messrs. Wood and Harmer agree with the 

 views just given on the gradual submergence of the land. They find 

 reason, however, to infer that the North of England became at the 

 same time enveloped in a great ice-sheet, which may have been 

 more than 1,000 feet thick. A branch of this gigantic glacier ad- 

 vanced until it reached the borders of Hertfordshire, which by this 

 time had sunk beneath the ocean. The sand and gravel of the 

 Middle Glacial was produced by powerful currents washing out 

 the moraine profonde of this glacier, and distributing it over the 

 sea bottom. This would account for the limited extension of these 

 gravels, which could not be deposited over those parts covered by ice. 



By the time of the Upper Glacial period the ice began to recede. 

 As recession went on, the moraine matter, no longer washed out 

 and distributed as gravel over the sea-bottom, was left behind as 

 unstratified glacial clay. Some of it was carried off at the bottom 

 of icebergs, as they broke off, and was dropped over the Middle 

 Glacial gravels. 



Whichever of the views just given may be the nearest in truth, 

 in each of them the borders of our county formed an important 

 feature in the physical geography of the period, being in one case 

 the termination of a great glacier, in the other case providing a 

 barrier, which caused a strong current in the glacial sea. 



Origin of Post- Glacial deposits. — The origin of the denudation 

 gravels has already been explained, and of the fluviatile Post- 

 glacial gravels something has before been given in our ' Trans- 

 actions.' f It is unnecessary, then, to do more here than recall the 

 fact that they owe their origin to existing rivers, and that their 

 varying level, in connexion with the level of present rivers, has 



* 'Quart. Journ. Geol. Soc.,' vol. xxxiii, p. 113. See also vol. iivi, p. 102. 

 t 'Trans. Watford Nat. Hist. Soc.,' Vol. I, p. 198. 



