334 PKOF. J. PRESTWICH ON THE RAISED 



that it was not one uniform uplift, "but that the rise, though on the 

 whole continuous, was sometimes slow, and at other times com- 

 paratively rapid, the maximum uplift and force being displayed in 

 the final stage x. This last addition to the Head filled up all 

 previous hollows, and levelled the surface flush with the summit of 

 the cliff, though no doubt the uneven underlie of this top bed in 

 Chalk districts has been increased by the percolation of surface- 

 waters. The force with which this last bed has been propelled 

 downwards has often driven back a portion of the debris upon itself 

 —a reversal particularly well shown at Portland Bill {antea^ p. 277), 

 and at Chesilton, where a tongue of Kimmeridge Clay, 20 to 30 feet 

 high, is squeezed up from the bed beneath by the impact of the 

 rubble on the soft clay.^ 



The well-marked alternation of fine and coarse materials is 

 confined to areas where the substrata consist of soft alternating 

 with hard beds, such as the Chalk- with-flints. Where, as in Devon 

 and Cornwall, the substrata consist entirely of hard rocks, no such 

 divisions are at first sight apparent. Nevertheless, traces of these 

 divisional planes generally exist in the form of thin, intercalated 

 seams of smaller rubble or of sand and loam. At Palmouth, as 

 before mentioned, there are five beds of impalpable sand inter- 

 calated in the Head. In the sketches drawn by De la Beche this 

 form of structure is sufficiently apparent ^ ; Mr. Ussher also speaks 

 of the Head presenting in several instances a stratified appearance 

 Avith a seaward inclination. In some places, however, the Head 

 has the appearance of a homogeneous deposit without divisions. In 

 no case is there any appearance of a break in continuity between 

 the successive layers. 



There are instances where the force of the currents and their 

 frequent renewal have been sufficient to carry the debris over the 

 comparatively flat plain at the foot of the hills, as on the coast of 

 Sussex between Brighton and Pagham, and, in a less degree, between 

 Eastbourne and Pevensey. The long trail of flint-rubble in many 

 of the dry Chalk valleys may be ascribed to the same cause. These 

 trails rarely contain any organic remains in their upper course, though 

 there is a notable exception at Green Street Green ; but where they 

 shoot out into the main river-valley, and the velocity of the current 

 has been checked, as at Faversham, XIpchurch, &c., mammalian 

 remains are of common occurrence. 



Where a cavitj- or terrace has been traversed by this drift in its 

 descent from the higher ground, it has filled up the one or lodged 

 on the other, as on the eastern slope of the Malverns (p. 315), and 

 on the Chalk plain at Chiltern (PL VII. fig. 4). Minor instances 

 have been cited at Cbilworth and at Sandling. A similar cause has 

 led to the masking of many Caves, where the entrance is on the 

 slope of hills, as at Brixham, or in the face of a bluff with higher 

 ground above. The great stream of Tertiary sandstone-blocks 

 (sarsenstone) in the Chalk valleys near Marlborough, and in several 



1 See Quart. Journ. Gaol. Soc. vol. xxxi. (1875) p. 38, fig. 6. 

 - 'The Geological Observer,' 1st ed. pp. 526-27. 



