Page:Quarterly Journal of the Geological Society of London, vol. 31.djvu/88

42 42 J. PRESTWICH ON THE QUATERNARY PHENOMENA of the fissile beds of the lower Purbecks, presents a bare rock-surface. There is no trace of anymore recent Secondary or of any Tertiary strata anywhere in the island ; nevertheless the mammaliferous drift of the Admiralty Quarries contains well-worn pebbles of chert from the Upper Greensand, and of iron sandstone and Sarsen-stone, together with small blocks of the latter from the Tertiary strata, and chalk-flints from the hills between Up way and Dorchester, a distance of from eight to ten miles to the north of Portland, from which it is separated by a low plain of older strata, where none of the few scattered drift-beds has any resemblance to the Portland drift. This latter stands alone ; and although at a greater distance from the Tertiary strata in situ, it contains nevertheless a much larger proportion of their debris than do any of the nearer and lower-level drift-beds. These Tertiary and Greensand materials could only have been transported to their present position by a floating iceberg or by a running stream. The former is not probable, as the transported rock-specimens are all waterworn and mostly well rounded, and they are associated with mammalian remains and a silt which has the ordinary characters of a freshwater loess, although, as is common with the loess itself, no shells have been found in it. I can only conclude, therefore, that this deposit has been formed by direct transport by a stream running southward from the Greensand and Tertiary area and passing over Portland ; and this, necessarily, could only have taken place when the intervening district was bridged over by strata since removed. It follows that the denudation of the plain of Weymouth and the deposition of the several drift-beds found thereon are of more recent date than the mammiliferous drift of Portland (see Sections 1 and 2, PI. I.). If the dip of the Portland and Purbeck beds at Portland be prolonged northward, they would reach an elevation of about 1000 feet at Broadway, and of about 1500 feet at the Eidgeway, and thus interpose a high ridge between Portland and the newer strata from which the pebbles of the Portland drift have been de- rived; consequently either the crest of the anticlinal had then been removed, and a high level plain of the Jurassic strata extended from Portland to the Greensand and Tertiary area, or else at that time the north end of Portland had not been upheaved, and a con- tinuous plain of Portland and Purbeck extended to the Bill of Portland from the point where those formations were brought into contact with the newer strata by the great Eidgeway fault. Looking to the facts that the Portland drift is at a height of 400 feet, or nearly that of the supplying Greensand and Tertiary strata — that the Portland plateau i3 prolonged northward to 100 feet higher, and is then abruptly truncated — and that the drift-bed, although rich in Tertiary sandstone, does not contain a fragment of the Coral Eag, Forest Marble, or other rocks which, under the first-named condi- tions, must have formed the surface of the plain over which the stream would have passed, it is, I think, more probable that the second condition, or that of a gradually sloping plain of Portland and Purbecks extending from the Bill of Portland to the Greensand