64-i 



DYNAMICAL GEOLOGY. 



Fig. 1078. 



Erosion, Monument Park, Colorado. 



moraine, occurring near Botzen in the Tyrol, in which the result is a region of manv 



hundreds of slender pillars and columns of 

 half consolidated earth, twenty to a hun- 

 dred feet in height, and each capped with 

 a bowlder, — some of the stones two or 

 three feet in diameter. He gives a view 

 of one such scene in his "Principles," 

 chapter xv. 



The positions of the strata have 

 also great modifying influence. 

 This is especially true when they 

 are horizontal. All lateral wear 

 in gorges intersecting horizontal 

 beds, such as takes place with 

 great energy during periods of 

 floods, tends to remove the ex- 

 posed lower layers, and so under- 

 mine those above ; and conse- 

 quently the latter, from time to 

 time, fall, making a vertical or 

 overhanging precipice either side of the stream. The debris made by 

 the fall may be wholly removed by the violent torrent. The excava- 

 tion in such rocks, unless they are very hard, is carried forward with 

 comparative rapidity ; because undermining is facilitated by the bed- 

 ding of the rock, and because gravity acts so promptly in bringing the 

 dislodged masses within reach of the flowing waters. The river portion 

 of the valley may thus be carried far into the heart of the mountains, 

 and terminate in a great amphitheatre ; and although so deep, it will re- 

 main a gorge or canon, owing to the force of the flood waters, until the 

 pitch of the stream has been reduced to a very small amount, and usu- 

 ally until it is less than twelve inches a mile. By such method, gorges 

 with vertical walls hundreds and thousands of feet in elevation, have 

 been excavated by running waters. And when the descent has become 

 gentle, the stream, in these depths, meanders through a ribbon of allu- 

 vial land rich in verdure at one season, and in others mostly flooded. 



Examples of such valleys occur in all elevated regions of horizontal 

 rocks. The upper part of the Mississippi valley, the Rocky Mountain 

 region, and the plateaus of its western slope, the eastern Alps, and 

 eastern Australia, are full of them. The Canon of the Colorado, be- 

 tween the meridians of 111° and 115° W. has, for the greater part of 

 200 miles (as described by Newberry, and since by Powell and others), 

 nearly vertical walls 2,000 to 6,000 feet in height, made of Carbonifer- 

 ous limestone and other Paleozoic rocks, with, in some places, the 

 bottom, and the sides for the lower 500 to 1,000 feet of granite ; and 



