242 Mr. De la Beche o;j the Excavation of Valleys. 



It seems to me that aqueous excavations are of two kinds : 

 1. Ihose produced by vast and violent causes not now in ac- 

 tion; and, 2. Those resulting from the continuous and gradual 

 operation of lakes, rivers, and other agents that have been 

 termed meteoric : the latter series of causes operating upon 

 valleys that most frequently owe their prior existence to the 

 former series, and both offering very distinct appearances. 

 Excavations of the second kind, or those produced by actual 

 streams, present cliffs, gorges, and ravines; while the first are 

 marked by grand and extensively rounded outlines, and by 

 valleys of a breadth and magnitude which would seem only 

 referable to a voluminous mass of moving waters. 



I shall endeavour to illustrate my opinions by the following 

 examples. 



I. — Valleys of Excavation in Dorset and Devon. 



Valleys of the first class, which have been usually termed 

 valleys of denudation, are very common in districts where 

 rocks are not far removed from an horizontal position ; these, 

 to take examples from our own country, are very abundant 

 in Dorsetshire and the east of Devon. In these valleys, the 

 former continuity of the strata on either side is most apparent, 

 and neither elevations nor depressions could have caused them : 

 they are exclusively due to the excavation of the materials 

 by which their sides were connected. The question then arises, 

 what has excavated them ? At the bottoms of each of these 

 valleys we find a small stream, the natui'al drain of the land. 

 Could these streams have cut out such valleys as they now flow 

 through ? If thei'e be any true relation between cause and 

 effect, they could not. Fig. 1. (Plate II.) represents a general 

 section of the valleys of Lyme Regis and Charmouth. The 

 summits of the hills are chiefly composed, as has already been 

 noticed by Professor Buckland, of angular flint and chert, the 

 remains of the former superincumbent strata of chalk and 

 green-sand, that have been partially dissolved in place. Be- 

 neath this is green-sand, with an unequal upper surface, i"e- 

 sulting from the causes that produced the gravel ; still lower is 

 the lias in which the spacious valleys of Lyme and Charmouth 

 have been principally scooped out: in the bottom of each val- 

 ley is a little stream, which I have necessarily represented in 

 the section on a scale much too large. If I had confined my- 

 self strictly to proportions, it would have been invisible; yet 

 to such insignificant streamlets, and the rain-waters which 

 acted in conjunction with them, the advocates for the excava- 

 tion of valleys by actual causes would refer the whole. The 

 most remarkable of these valleys is that of the Char at Char- 

 mouth, 



