DEPOSITS OF HERTFORDSniRE. 107 



II. Mode of Formation. 



Prof. T. McK. Hughes' views* — The high ground near Hertford 

 Heath, Brickeudon, Bayford, Essendon, and Hatfield Park, north- 

 west of Bramfield, and north of St. Albans, forms part of a great 

 plain, extending as far as the eye can reach. This plain Prof. 

 Hughes considers was formed by the denudation of a sea, which 

 also deposited upon it a pebble-gravel, the Gravel of the Tipper 

 Plain. During the emergence of that plain from the sea, a great 

 valley was scooped out of it, the bottom of which forms another 

 plain, upon which stand Bayfordbury, Hertingfordbury, Bengeo, 

 Bramfield, Cole Grreen, Welwyn, and the lower part of Hatfield 

 Park. Then followed a second submergence beneath the sea ; old 

 valley-deposits were re-sorted and Boulder-clay deposited under, 

 in, and on them. On the final emergence from the sea, the smaller 

 valleys of the Rib, Beane, Mimram, and Lea were excavated out 

 of the lower or valley plain. (See Fig. 1.) 



Mr. TV. H. Penning' s views. — In a paper on the Physical Geology 

 of East Anglia during the Glacial Period, f it is maintained by Mr. 

 Penning that during the Lower Glacial period the land was sub- 

 merged gradiially to a depth of not less than 400 feet. During 

 this submergence the advancing shore -line gave rise to the pebbly 

 sands, which form the base of the whole glacial series, | and indicate 

 shore conditions and the first setting in of the great glacial sub- 

 sidence. Arctic conditions of climate then began to prevail, and 

 patches of clay were dropped here and there by icebergs. 



By the time of the Middle Glacial period all the land around 

 here was below water except the chalk escarpment in the north 

 of the county, which, as a long narrow ridge, stood well above 

 the sea and formed a barrier opposing itself to the strong current 

 sweeping round from the North Sea to the Atlantic. By this 

 current was brought down the material of the Middle Glacial 

 sands and gravels, derived from the rocks of the north and east 

 coast and from the chalk barrier itself. Icebergs occasionally 

 brought down heavy loads of boulder-clay, which, dropping heavily 

 on the gravels, distorted them and became intercalated with them. § 

 In the mean time the submergence went on until the lower parts 

 of the chalk escarpment had sunk beneath the sea. Then, the 

 waters having admission to a larger area, the strong current was 

 weakened, and its power of transporting gravels lessened, until 

 at last, as submergence went on, it was entirely lost. Hence 

 the reason why, as was shown before, the Middle Glacial gravels 

 are found running not quite up to the lower levels of the chalk 



• ' Quart. Journ. Geol. Soc.,' vol. xxiv, p. 287. 



t lb. vol. xxxii, p. 191. 



X "Wood and Harmer, ' Outlines of the Geology of the Upper Tertiaries in 

 East Anglia,' p. 16. 



^ This explains the occurrence at Bishop's Stortford of a number of bones of 

 Pliosaurus. These were transplanted by icebergs in a large mass of Secondary 

 rock from the north. 



