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 



* The following is the measurement in feet of some of these blocks : — 9 X 8£ X 3£, 

 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). 



