BASE- LEVELS OF EEOSION. 
77 
If it is depressed it becomes immediately an area of deposition. If it is 
elevated new energy is imparted to tbe agents and machinery of erosion. 
The declivities of the streams are increased, giving an excess of transport- 
ing power which sweeps the channels clear of ddbris; corrasion begins; 
new topographical features are literally carved out of the land in high relief; 
long rapid slopes or cliffs are generated and vigorously attacked by the de- 
stroying agents; and the degradation of the country proceeds with energy. 
It is not necessary that a base-level of erosion should lie at extremely 
low altitudes. Thus a large interior basin drained by a trunk river, across 
the lower portions of which a barrier is slowly rising, is a case in point. 
For a time the river is tasked to cut down its barrier as I’apidly as it rises. 
This occasions slackwater in the courses above the barrier and stops cor- 
rasion, producing temporarily a local base-level. Another case is the Great 
Basin of Nevada. It has no outlet, because its streams sink in the sand or 
evaporate from salinas. Its valley bottoms are rather below base-level than 
above it. The general result of causes tending to bring a region to an 
approximate base-level of erosion is the obliteration of its inequalities. 
During tbe progress of the great denudation of the Grand Canon dis- 
trict the indications are abundant that its interior spaces have occupied for 
a time the relation of an approximate base-level of erosion. Throughout 
almost the entire stretch of Tertiary and Quaternary time the region has 
been rising, and in the aggregate the elevation has become immense, vary- 
ing from 11,000 to 18,000 feet in different portions. But it seems that the 
movement has not been at a uniform rate. It appears to have proceeded 
through alternations of activity and repose. Whether we can point to 
more than one period of quiescence may be somewhat doubtful, but we 
can point decisively to one. It occurred probably in late Miocene or early 
Pliocene time, and while it prevailed the great Carboniferous platform 
was denuded of most of its inequalities, and was planed down to a very 
flat expanse. Since that period the relation has been destroyed by a gen- 
eral upheaval of the entire region several thousands of feet. The indica- 
tions of this will appear when we come to the study of the interior spaces 
of the Grand Canon district and of the Grand Canon itself. To this study 
we now proceed. 
