﻿I860.] 



FISHER DENUDATION. 



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deposited, is not permitted to accumulate, so that a slower movement 

 of the water is effectual daily to clear out the little valleys. These 

 valleys in the surface of the mud are not formed by wavelets, which, 

 at low water, begin to cut a miniature straight cliff at the margin of 

 the mud. 



These arguments lead the author to infer that the land must have 

 been elevated by a sudden movement sufficient to have caused a 

 rush of water from the raised portions to seek a lower level, — either 

 the land being raised high and dry at once, or the sea-bottom raised, 

 though still remaining beneath water. Such an elevation might be 

 repeated again and again with intervals of submergence ; and many 

 of the phenomena connected with sunk forests, and other Pleistocene 

 phenomena, seem to show that such conditions have really obtained 

 in places at considerable distances apart. 



The author stated that, in his opinion, escarpments, such as are 

 so common among the secondary and tertiary strata, are rarely old 

 cliffs, and he thinks that their rounded forms are due to such agency 

 as he has described. In forming this opinion, he relies upon the 

 following arguments : — 



Cliffs in the softer beds run in approximately straight lines, or 

 sweeping curves, while escarpments abound in nooks and combs 

 running up into the hill-face (according to his view, gullies), down 

 which the water has poured where it happens to have broken through 

 the crest. Again, a line of cliff usually cuts off stratum after stratum, 

 while the line of an escarpment usually follows the course of one 

 stratum, and is entirely determined by the intersection of an approxi- 

 mately constant level with its undulating surface, that level rising 

 slightly where the stratum is harder, and sinking where it is softer. 

 Again, in an escarpment, like those of the Chalk, the mouths of the 

 valleys run down into the plain below ; but in a cliff many valleys 

 are seen to be cut off at a level considerably above the beach, — a 

 condition which would be clearly marked in a hill-side which had 

 formerly been a cliff. The author cannot remember to have ever 

 seen such a termination to a valley in the face of a Chalk or other 

 escarpment. 



Proofs of torrential action in Chalk-districts were referred to, — 

 for instance, the perfect drainage-system of the dry valleys of Salis- 

 bury Plain and other extensive tracts of Chalk-downs, the loose 

 flints filling the bottoms of such valleys, and the immense blocks of 

 tertiary pudding-stone and " Druid-sandstone," scattered along the 

 bottoms of Chalk-valleys, as in the Portisham and Bridehead valleys 

 near Weymouth, and the Marlborough " Wethers " in Wiltshire. 

 Mr. Fisher also thinks that he sees evidence of the friction of a 

 great body of water rushing down a hill side in the manner in which 

 vertical or nearly vertical strata are usually bent over at their ex- 

 posed edges. 



In estimating the denuding power of such a cause as that sug- 

 gested, it is evident that the effects would be least of all on water- 

 sheds, great at the crests of valley-sides, and greatest of all in their 

 bottoms, and that, where the velocity was diminished from any 



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