178 LYELL'S ELEMENTS OF GEOLOGY. 



Eocene Strata London Clay. 



high, of coralline crag, running in a direction N. E. and S. W., 

 against which the red crag abuts with its horizontal layers, but 

 this cliff occasionally overhangs. The rock composing it is 

 drilled everywhere by Pholades belonging to the period of the 

 red crag. The cliff may have been caused by submarine denu- 

 dation, in a shallow sea ; and had the red crag been equally 

 solid, it would probably have presented many similar perpendicu- 

 lar cliffs ; for beds, ten or twelve feet thick, of loam or sand, in 

 this formation, are often seen to be unconformable to older beds, 

 which have been in part cut away. Similar excavations are now 

 made, even on a larger scale, by the sea, in the great sandbanks 

 off Yarmouth, in part of which Captain Hewett, R. N., found, in 

 1836, a broad channel, sixty-five feet deep, where there had been 

 only a depth of four feet in 1822. This remarkable change was 

 ascertained during two hydrographical surveys, in the years 

 above mentioned, and shows how denudation, amounting to sixty 

 feet in vertical depth-, can take place under water in the course 

 of fourteen years. The new channel thus formed, serves now 

 (1838) for the entrance.of ships into Yarmouth Roads. 



Eocene formations in England London Clay. In the sec- 

 tion already given of the tertiary strata of Suffolk (p. 175.), it 

 will be seen that the crag rests on a formation called the London 

 clay, which there consists of alternating beds of blue and brown 

 clay, with many nodules of calcareous stone, used for Roman 

 cement. This formation is well seen in the neighbouring cliffs 

 of Harwich, where the nodules contain many marine shells, and 

 sometimes the bones of turtles. The relative position of the 

 chalk, London clay, and crag, between the coast of Essex and 

 the interior, may be understood by reference to the annexed dia- 



Fig. 136. 



Crag. London clay. Chalk. 



gram. The London clay has been so named, because it occurs 

 in the neighbourhood of the metropolis, in a trough or basin of 

 the chalk. (See section, p. 182.) We know, by numerous 

 borings made for water, that the chalk exists everywhere below, 

 after we have penetrated through clay and sand to the depth of 

 from 200 to 600 feet ; and, if we proceed to the south of Lon- 

 don, we find the chalk rising up to the surface and forming the 

 Surrey hills ; while if we proceed northwards, into Hertford- 

 shire, or, westward, by the Thames, into Oxfordshire, we again 

 meet with the same chalk. 



