TRACES OP AN EARLY ARID CLIMATE. 
227 
tlie river found its base-level soon after the uplifting ceased, and the canon 
slowly widened by the recession of the cliffs. In this stage of the develop- 
ment an arid climate reigned throughout the district. Its effect is apparent 
chiefly in two ways; 1st, in the paucity of lateral tributaries and in the 
meagerness of small details in the land sculpture, and 2d, in the sharpness 
and abruptness given to all the cliffs, valleys, and mesa profiles. 
(1.) Allusion has frequently been made in previous chapters to the 
first mentioned effect of an arid climate. The only tributaries which 
remained during this period were the large and more powerful ones which 
had their sources far away in the highlands, around the margin of the water- 
shed. Within the inner platform of the district no streams took their rise. 
The large tributaries continued to sink their lateral gorges in unison with 
the corrasion of the main river, but no new chasms were generated. On 
the contrary, some of the lateral tributaries, which for a time valiantly 
sustained a doubtful struggle for existence, at last succumbed and dried up, 
leaving their troirghs opening into the main gorge far up near the summit 
of the canon wall. 
(2.) The sharpness and abruptness of profiles which characterizes the 
plateau scenery is not of recent origin, but dates back no doubt to the 
beginning of the Pliocene. An arid climate is an important factor in pro- 
ducing this effect. In such a climate weathering proceeds slowly. If the 
conditions are such as to produce a high efficiency in the agencies which 
transport the ddbris, the rocks will he left comparatively naked; little soil 
and talus will be formed, and little will be left. The attack upon the edges 
of the horizontal strata will thus be facilitated and the profiles will be deter- 
mined chiefly by undermining. Such profiles are invariably cliffs. 
We come now to the final upheaval which has brought the region to 
its present condition. The Colorado River, after remaining without corra- 
sion at the level of the esplanade of the Canon during the greater part of 
Pliocene time, at length resumed the operation of sinking its channel. A 
new paroxysm of upheaval set in; the faults increased their displacement; 
the volcanic vents reopened. This time the upheaval was greater than 
before, amounting probably from 3,000 to 4,000 feet. The narrow, inner 
gorge at the Toroweap was swiftly cut, and it is in this respect a type of 
