TYLOR QITATERNARY GRAYELS. 89 



5 inches (c), and near another fallen piece, — the whole enveloped in 

 a mass of purple clay {d), which may be either Plastic or London clay 

 rearranged, and which often contains pieces of rolled chalk, hke what 

 is called Till or Bonlder-clay. The situation is near M, Plate YIII., 

 Erith ; and the height of h, fig. 21, is about 65 feet above the Ord- 

 nance datum-hue. Large pieces of the Woolwich shell-bed, with 

 masses of purple clay, occur in the ordinary covering bed (e) near 

 I, at a height of 55 feet. This covering-bed loses its pebbles at J, 

 and at L M N" assumes the character of a purple clay enclosing large 

 unrolled fragments of the Eocene beds and small pieces of chalk. 

 The Woolwich shell-bed, with the pebble-bed, is visible in situ 

 at the south-west corner of the ballast-pit at Erith Station, at a 

 height of 60 feet above the chalk, the whole series of Thanet sands 

 being exposed there below it. It is probable that the escarpment of 

 Thanet sands and chalk is only a little to the west of M. It is evi- 

 dent that this particular mass (b, fig. 21), must have been, when in 

 situ, at least 110 feet above the Ordnance datum-line; for part of the 

 pebble-bed is still attached to it, and that pebble-bed is 60 feet above 

 the basement-bed of the Thanet sands, which sands would be, if in 

 situ, 50 feet above the datum-line at the point M, according to my cal- 

 culation. The positions of the top and bottom of the Thanet sands 

 can be determined, as they are visible in situ between J and I, and 

 also along the face of the chalk -pit between G and E. 



The difficulty is, to understand what kind of wash of water re- 

 moved this incoherent mass of Thanet sands and deposited it 

 again (without breaking it up into sand) 35 feet lower down, upon 

 a mass of the Woolwich shell-bed (c), which must have descended 

 45 feet and changed places with h. 



As the Thanet sand has been removed from the large area of chalk 

 to the west, the mass of sand (fig. 21) may have been derived from 

 a distance. 



The Thanet sands before their denudation dipped towards the 

 north, from the Kentish hills to Erith, at a slope of 1 in 100 on 

 an average of ten miles. It may have been owing to the exist- 

 ence of a rainy period, and to high land to the west, that water could 

 have moved the Thanet sands. Eastward, towards the low ground near 

 the Thames, on the opposite side of the river, at Grays, I have only seen 

 one small mass of Thanet sand imbedded in the covering gravel (e)near 

 E, Plate YII. ; but to the east of Grays the chalk soon dips eastward *, 

 and therefore falls below the level of the river ; so that there was no 

 high land to give transporting-power and velocity to the water that 

 fell upon it. The nearest English deposit in physical character, and 

 perhaps in age, to that represented in fig. 21, is at Cromer. It is 

 also probable that without the high background near Cromer such a 

 deposit as that in the Cromer cliffs could not have been formed. 

 The great mass of chalk at Trimmingham is an instance of an un- 

 broken rock having been detached and moved down to a lower level 

 in the Quaternary period, and enveloped in clay of a dififerent forma- 

 tion. The stratified fossiHferous gravel on the Norfolk coast has 

 been tranquilly deposited in front of, or near, the great slipped masses 



* And northward. 



