. Elements of geology : a text-book for colleges and for the general reader. Geology. 22 AQUEOUS AGENCIES. rate of these two processes. They would continue, however, to cut deeper and deeper, until they finally reached their base-level. Then they would cut no deeper, but sweep from side to side, widening their channels. Meanwhile, rain-wash would continue to cut down their divides. Thus wide channels and low divides, or rounded and sweeping curves, are very characteristic of old topography, while deep and narrow gorges and canons are a sign of recent elevation, and therefore comparatively new
. Elements of geology : a text-book for colleges and for the general reader. Geology. 22 AQUEOUS AGENCIES. rate of these two processes. They would continue, however, to cut deeper and deeper, until they finally reached their base-level. Then they would cut no deeper, but sweep from side to side, widening their channels. Meanwhile, rain-wash would continue to cut down their divides. Thus wide channels and low divides, or rounded and sweeping curves, are very characteristic of old topography, while deep and narrow gorges and canons are a sign of recent elevation, and therefore comparatively new topography. Moreover, successive movements are each faithfully re- corded. Thus Fig. 13, which is a section across the Grand Canon of the Colorado, shows the following events: 1. The plateau region was raised and the river cut down 3,000 feet, and reached its base-level. 2. The river sweeping from side to side, and the crumbling of the cliffs, gradually widened the cation to its width in the upper part, b b. 3. Another rise occurred, and the river again cut 3,000 feet more, and made the inner gorge, a a. This second rise is so recent that the river has not yet reached its base-level. On the other hand, suppose a continent, by sinking, carries its river-beds below their base-level: then the decreasing slope will check the velocity of the current, and the rivers will immediately begin to build up again by sedimentation until they again reach their base-level. For example, in the Mississippi Kiver the following events are recorded : 1. A higher condition of land, during which it reached its base-level, and formed the broad trough r" r" r". 2. A subsidence of land and a Fig. 13.—Ideal Section across Grand Cailon (after Dutton): a a, inner gorge; b b, outer caflon Fig. 14.—Generalized Section across the Mississippi River: r" r" r"'. old bed; r1 r', second bed; r, present bed; d' d', old deposits; d d, present deposits. building up by sedimen
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