S. S. Warren — Bubble JDri/t, Portslade, Sussex. 303 



4. Surface soil 1 ft. 



3. Dark-coloured stony clay descending into pipes . 6 in. to 3 ft. or more. 



2. Contorted chalky loam, a large proportion of its mass being 

 composed of flints, often broken, said to yield mammalian 

 remains at the base 10 ft. to 12 ft. 



1. Light-coloured sand, red in the upper part, with layers of well- 

 rolled flint pebbles, and many concretionary nodules, which 

 ai-e sometimes tabular. Marine Mollusca and Balanidse fairly 

 abundant : Mytilus edulis in the pebbly layers, Natica, etc., 

 in the sand seen to 9 ft. 



No. 1 is the well-known Raised Beach, in which the shells, 

 particularly M. edulis, have frequently their colour preserved. 



No. 2 is locally known as " Coombe Eock." It is the " Rubble 

 Drift" of Prestwich (1892), and the "Elephant Bed" of Mantell 

 (1822). 



No. 3 much resembles the Trail (Fisher, 1866) of the valleys 

 of the Thames, Lea, etc., but could not safely be correlated with that 

 deposit. It has more probably been formed in place by solution 

 of the calcareous matter out of No. 2. 



The Rubble Drift here presents rather the appearance of ice 

 having been concerned in its formation ; although, as the Raised 

 Beach is undisturbed, this could not have been in strong force. 

 The contortions may very possibly have been produced by the 

 melting of intercalated blocks of ice. I am by no means sure, 

 however, that a moving mass of semi-fluid mud and stones would 

 not assume contortions similar to those presented in this exposure, 

 without the aid of ice, in a manner analogous to the knotty twists 

 and tortuous waving lines so frequently seen in vitreous (and, of 

 course, devitrified) lava-flows. 



On Sir Joseph Prestwich's theory (Q.J.G.S., vol. xlviii, p. 271) 

 of this drift having been formed by submarine currents produced 

 by the rising of the land after a short dip in the sea, these con- 

 tortions are inexplicable. And, in addition, there is no direct 

 evidence of such a submergence ; indeed, the absence of marine 

 shells is strong negative evidence against it. Neither is there any 

 particular a priori probability of such an event. 



That of Mr. Clement Reid (Q.J.G.S., vol. xliii, p. 364) I am in- 

 clined to accept as the best explanation yet suggested. There is no 

 indication of glaciation, in the full sense of the term, south of the 

 Thames. But the ground must have been deeply frozen during the 

 winter. The summer thaw and rains would cause extensive floods ; 

 for, the chalk being frozen, and therefore impermeable, all the rain 

 and water from the melted snow would sweep over the surface of 

 the ground, and carry along large quantities of chalk rubble and 

 flints. Many of the flints would become broken and battered, but 

 not much water- worn, which is the appearance they actually present 

 in the Rubble Drift. 



This section is admirably interpreted by this theory. For whether 

 the contortions were produced by the melting of contained blocks of 

 ice or masses of frozen snow, or whether by the internal movements 

 of a semi-fluid mass of mud and stones, or were referable to both 



