Page:Quarterly Journal of the Geological Society of London, vol. 31.djvu/84

38 38 J. PKESTWICH ON THE QUATEENARr PHENOMENA south to north, and of which the fissures discovered in digging the Yerne moat are an example. These fissures (/, fig. 5), which are from 1 to 10 feet wide, are generally open at bottom, while the top has been blocked up with debris of the upper strata, and, in this case, of the beds overlying them also. At Portland Bill blocks of the fissile Purbeck beds have been let down to a greater or lesser depth until stopped by the narrowness of the passage : the shingle bed, e> has fallen upon the rock-debris ; and the loam bed, d has followed on the shingle, forming vertical seams which, when the shingle came to be quarried, were left in ridges over the rectilinear fissures, as shown in the preceding section (fig. 5). None of this angular rubble and loam exists in the centre of the island ; but at the northern extremity of Portland there is a remark- able mass of it, though under very different conditions from that at the Bill. The high escarpment which ends abruptly above Chesilton is subtended at its base by a low cliff, which overhangs the southern extremity of the Chesil Bank for a short distance. This cliff, which rises to the height of 60 feet, is composed entirely of angular debris with sand, derived from the Purbeck beds and Portland Stone and Sands, spread out in great lenticular masses, and interstratified with Fig. 6. — Old land-debris, with large boulders and seams of Loess^ with land shells (at s). Cliff, Chesilton. K K K K. Kimmeridge Clay. See also AD in fig. 8, p. 47. irregular beds of loam, 1 to 4 feet thick, in places having the character of loess (fig. 6). The upper 6 to 10 feet consists almost entirely of broken Portland flint. Scattered through the mass of debris are large angular blocks of Portland Stone and Portland flint or chert, often several tons in weight*. The rough bedding is in general slightly 10x6x2,8x7x2, 3 X 2^ x H feet. The last is of Portland flint, the others of Portland Stone. Mr. Fisher has since found in it a small block of Sarsen stone (? from the Portland drift).
 * The following is the measurement in feet of some of these blocks : — 9 X 8£ X 3£,