Should you expect the lateral valleys of a rift valley at the time of its formation to enter it as hanging valleys or at a common level?

Block mountains.  Dislocations take place on so grand a scale that by the upheaval of blocks of the earth’s crust or the down-faulting of the blocks about one which is relatively stationary, mountains known as block mountains are produced.  A tilted crust block may present a steep slope on the side upheaved and a more gentle descent on the side depressed.

The basin ranges.  The plateaus of the United States bounded by the Rocky Mouirtains on the east, and on the west by the ranges which front the Pacific, have been profoundly fractured and faulted.  The system of great fissures by which they are broken extends north and south, and the long, narrow, tilted crust blocks intercepted between the fissures give rise to the numerous north-south ranges of the region.  Some of the tilted blocks, as those of southern Oregon, are as yet but moderately carved by erosion, and shallow lakes lie on the waste that has been washed into the depressions between them.  We may therefore conclude that their displacement is somewhat recent.  Others, as those of Nevada, are so old that they have been deeply dissected; their original form has been destroyed by erosion, and the intermontane depressions are occupied by wide plains of waste.

Dislocations and river valleys.  Before geologists had proved that rivers can by their own unaided efforts cut deep canyons, it was common to consider any narrow gorge as a gaping fissure of the crust.  This crude view has long since been set aside.  A map of the plateaus of northern Arizona shows how independent of the immense faults of the region is the course of the Colorado River.  In the Alps the tunnels on the Saint Gotthard railway pass six times beneath the gorge of the Reuss, but at no point do the rocks show the slightest trace of a fault.

Rate of dislocation.  So far as human experience goes, the earth movements which we have just studied, some of which have produced deep-sunk valleys and lofty mountain ranges, and faults whose throw is to be measured in thousands of feet, are slow and gradual.  They are not accomplished by a single paroxysmal effort, but by slow creep and a series of slight slips continued for vast lengths of time.

In the Aspen mining district in Colorado faulting is now going on at a comparatively rapid rate.  Although no sudden slips take place, the creep of the rock along certain planes of faulting gradually bends out of shape the square-set timbers in horizontal drifts and has closed some vertical shafts by shifting the upper portion across the lower.  Along one of the faults of this region it is estimated that there has been a movement of at least four hundred feet since the Glacial epoch.  More conspicuous are the instances of active faulting by means of sudden slips.  In 1891 there occurred along an old fault plane in Japan a slip which produced an earth rent traced for fifty miles (Fig. 192).  The country on one side was depressed in places twenty feet below that on the other, and also shifted as much as thirteen feet horizontally in the direction of the fault line.