Page:Quarterly Journal of the Geological Society of London, vol. 31.djvu/89

IN THE ISLE OF PORTLAND AND AROUND WEYMOUTH. 43 and Tertiary area, obtained at the period of the Portland Drift (see Section 3, PL I.). The disturbance which raised the Weymouth district and elevated the north of Portland, is the central anticlinal of the Jurassic series (A in Map, PL I.), which runs east and west and brings up the Forest Marble between Broadway and Buckland Eipers ; for I find that on prolonging the angle of rise of the Portland beds northward, and that of the Purbeck beds (where they crop out on the south of the great Eidgeway fault) southward, the two planes meet exactly over the ridge of that line. The great Eidgeway fault is, I believe, of older date. If I am right in this intepretation, then it is probable that the anticlinal bringing up the strata below the Chalk at Purbeck, and likewise the great upheaval at the back of the Isle of Wight, both of which are on the same line of disturbance as the Broadway anti- clinal, are also of the same age ; and we thus obtain a marked in- stance of elevation and denudation during the Quaternary period. The great difference of level between the mammaliferous drift and the raised beach might lead to the supposition that they were of diffe- rent ages. Still, as the former is not a case of capping horizontally an isolated hill, but of forming part of a sloping surface continuous from the northern to the southern end of Portland — and as that sur- face, for the reasons before mentioned, had its inclination given it* subsequently to the deposition of the drift, and when consequently its level above the sea may not have exceeded from 100 to 150 feet, or about 50 or 100 feet above that of the old beach, while the old beach itself (like the drift-bed) contains materials derived from the same Tertiary and Greensand area, — it is probable that the drift-bed and the raised beach were contemporaneous. In fact, while in the upper part of the Admiralty Quarries the drift-bed attains a height of 400 feet, at the angle of the road near the new church it is only 346 feet high, still dipping in the direction of the Bill. On the other hand, the Old Beach, which at the Bill is only 24 feet above the present beach, rises gradually northward until near the Sand- holes, where it attains a height of 36 feet. It is probable, there- fore, that the old stream emptied itself a short distance off the present Southwell shore into a small bay, of which the land passing from the Sand-holes in the direction of the lower lighthouse formed the western horn. The Portland raised beach is by far the most interesting one in the south of England, whether for its extent, its thickness, its large exposure, or its general conditions. In the first place, the materials of which it is formed are partly of local, but still more are of distant origin. Although it contains pebbles of Portland Stone and chert, it is essentially a chalk-flint shingle with a considerable pro- portion of pebbles of chert from the Greensand. It also contains subangular brown -coated flints, derived apparently from an old gravel, with a few which may be referred to Tertiary strata, together with some angular fragments of chalk-flint showing no wear. With these are a number of pebbles of red sandstone and of light-coloured and