474 Or the Agricultural Geology of England and Wales. 



constitute the upper erratics. The clay may be seen in some of 

 the fresher railway cuttings near Wolverton. Dr. Mitchell has 

 described it on the Castle Hill, six miles south of Bedford ; and 

 on the line of the Birmingham Railway, near Leighton Buzzard, 

 resting on the lower green sand, and overlaid by gravel. He did 

 not note the clay in Herts, but found gravel about Puckeridge, 

 Hare Street, and Newnham, near Baldock, abounding with frag- 

 ments of secondary and other transported rocks. At Ware Mill, 

 also at Wade's Mill, he observed a few such fragments in gravel.* 

 In tracing the erratic blocks down the western side of England 

 it was observed that the granitic blocks begin to decline in quan- 

 tity and size south of Bridgenorth, passing off into coarse gravel, 

 which again passes into the fine gravel and silt of the vale of 

 Gloucester. 



The erratic gravel of the south midland counties is a thinner 

 deposit than that of the north ; and among its foreign detritus, 

 fragments, sufficiently large to merit the name of boulders, are 

 less frequent. This circumstance, which has induced some to 

 regard it as belonging to a different epoch, appears a natural re- 

 sult of formation during different portions of a protracted period 

 of depression and elevation, both proceeding from the north ; the 

 depression greatest in the north part of the island and not complete 

 in the southern, till the extreme glacial climate had commenced 

 its retreat northwards ; so that boulder clay was no longer a litto- 

 ral deposit, and large erratic blocks were no longer transported 

 to be embedded in the upper erratics. 



Erratic Gravel of the Southern Counties — absence of Boulder 

 Clay. — We have treated hitherto only of deposits north of the 

 Thames. Those south of that river deviate still more than those 

 of the south-midland counties from the northern type. South of 

 the Thames not a trace of boulder clay has yet been found ; but 

 the chalk and the eocene tertiaries of Hants, Dorset, and parts of 

 the adjacent counties of Kent and Sussex, are covered with a 

 peculiar flint gravel, seldom more than 30 feet thick, and 

 generally much less, which occurs at all heights, from 600 feet 

 on the chalk hills to the sea level. It caps Headon-Hiil, 

 in the Isle of Wight, and the high grounds about Osborne 

 House. It is found on the chalk near Romsey, and on all 

 the table-lands of the eocene tertiaries : the flints of which 

 it is composed are but slightly waterworn, and are embedded in 

 a ferruginous, sandy, sometimes clayey base. They appear to 

 diminish in size with every stage of descent; but, except along the 

 river courses, are scarcely more rolled at the lower than at higher 

 levels. This is a remarkable fact, very difficult of explanation ; 



* Quart. Journ. Geol. Soc, vol. iii. p. 4. 



