CHAPTER IV. 
THE GREAT DENUDATION. 
The great lesson of the terraces is erosion. — Its amount stated. — The amount though very great is not 
abnormal. — Erosion and deposition are complementary processes. — Methods of erosion hy reces- 
sion of cliffs. — Examination of the evidences of the erosion. — Argument from the stratification. 
Argument from the displacements. — Argument from the drainage system. — Base levels of erosion. 
Before leaving the terraces we may with advantage pause to contem- 
plate the great lesson which they lay open to us. The subject of the lesson 
is Erosion. The geologist, seeing that around a considerable part of the 
periphery of the Grand Canon district the Eocene and Mesozoic strata 
suddenly terminate in great cliffs facing the Carboniferous platform, would 
at once conclude that these strata formerly reached beyond their present 
boundaries. But how far f The answer may be proposed at once. They 
extended over the entire Grand Canon district and reached into central 
Arizona, where they ended along the shore of an ancient mainland, from 
which their materials were in part derived. The distance of that shore-line 
from the summit of the Pink Cliffs is from 130 to 180 miles, and the width 
of the denuded district is from 120 to 140 miles. From the base of the 
Vermilion Cliffs the distance is from 26 to 30 miles less. The area of maxi- 
mum denudation is from 13,000 to 16,000 square miles, and the average 
^thickness of the strata removed from it was about 10,000 feet. 
The general reader will no doubt feel a strong aversion to such pro- 
digious figures, and even the geologist may hesitate. In order that the reader 
may not be obliged to carry a heavy burden of prejudice as he follows the 
various steps of the argument, it is well to anticipate some part of the dis- 
cussion and thus relieve him of a great part of the load at the outset ; for 
it can be shown that the figures, while they are certainly very large, are in 
no respect abnormal and in only one respect are they at all unusual. 
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