﻿362 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



heavy argillaceous loam, in parts used as brick-earth. The highly 

 fractured condition of many of the flints and their admixture with 

 sand, loam, and chalk-debris produce here and there excellent samples 

 of the so-called " Combe Rock," which under that name is now much 

 used as ballast for the railroads. It is here a rusty-coloured mass, 

 owing to the flints having been mixed up with ferruginous detritus 

 of other rocks chiefly tertiary ; whilst at Brighton, the chalk itself 

 being the chief matrix, the "Combe Rock" is mostly of a whitish 

 tint. Many of these flints, so entirely fresh-fractured and unrolled, 

 were, I believe, suddenly translated into this recess from the north 

 side of the anticlinal before spoken of, the line of which is only about 

 one mile to the south of this copious accumulation, in which the 

 railroad-termini of the trains to London, Brighton, and East Bourn 

 are established. The high inclination to the north of a mass of 

 chalk with flints, in the southern part of the "Cliff" promontory at 

 Southerham on the left bank of the stream, and which ought, if un- 

 disturbed, to have been inclined to the south, is at hand to explain, 

 that many of the angular flints were most probably added to the 

 accumulation in this manner from the reversed rocks. Their present 

 stained and party-coloured character is doubtless due to their having 

 been long imbedded in a mixture of ferruginous sand or clay, proba- 

 bly derived from the breaking-up of the red and ochreous beds of the 

 tertiary Plastic-clay. 



I would here beg the reader to remember, that the position of an 

 underlying flint-drift and an overlying heavy clay or loam, which rises 

 to heights of about 40 feet above the Ouse, is exactly analogous to 

 what I shall have hereafter to dwell upon at other places where the 

 remains of fossil mammalia have been found in the lower detritus. 

 Dr. Mantell authorizes me to say that fossil bones have been recently 

 found in this debris of Lewes. He has also formerly stated that the 

 low grounds of the flat expanses of the Valley of the Ouse were oc- 

 cupied by the sea at no very remote period, and have passed from 

 salt-marshes into desiccated lands ; so that here, as elsewhere, we have 

 the clearest distinction between ancient Drift and modern Alluvium. 



In passing across the Lewes anticlinal to the south, the chalk, on both 

 banks of the transverse-flowing Ouse, is seen to dip gently to the south, 

 near Rodmill and Itford. Thence to Piddinghoe, the summits of the 

 chalk-hillocks in situ, which flank the stream, have been largely eroded 

 and their surfaces and cavities have been filled with the angular flint- 

 drift, mixed more and more with reddish loam as you advance to the 

 south, or into the zone where the older tertiary deposits once covered 

 the Chalk. The whole of the sides and summits of the Chalk-hills 

 then present an appearance wholly unknown in the districts of pure 

 Chalk nearer the escarpments, and where no anticlinal disturbance has 

 acted ; for the red loam and flint-detritus are very frequently at the 

 surface. This great feature of the Drift will now be considered. 



Drift on the Southern Slope of the South Downs. — When the 

 southern slopes of the South Downs are surveyed from Beachy Head 

 on the east to Portsdown Hill and the Forest of Bere on the west, 

 their surface is found to be prodigiously eroded. With the exception 



