Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/183

Rh 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, like what is called Till or Bonlder-clay. The situation is near M, Plate VIII., Erith; and the height of (b), fig. 21, is about 65 feet above the Ordnance 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 evident 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 calculation. 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 removed 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 existence 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 VII.; 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 unbroken rock having been detached and moved down to a lower level in the Quaternary period, and enveloped in clay of a different formation. The stratified fossiliferous gravel on the Norfolk coast has been tranquilly deposited in front of, or near, the great slipped masses