Often the earth movements to which the uplift or depression was due involved tilting or folding of the earlier strata, so that the strata are now nonparallel as well as unconformable.  In Figure 201, for example, the record includes deposition, uplift, and tilting of a; erosion, depression, the deposit of b; and finally the uplift which has brought the rocks to open air and permitted the dissection by which the unconformity is revealed.  From this section infer that during early Silurian times the area was sea, and thick sea muds were laid upon it.  These were later altered to hard slates by pressure and upfolded into mountains.  During the later Silurian and the Devonian the area was land and suffered vast denudation.  In the Carboniferous period it was lowered beneath the sea and received a cover of limestone.

The age of mountains.  It is largely by means of unconformities that we read the history of mountain making and other deformations and movements of the crust.  In Figure 203, for example, the deformation which upfolded the range of mountains took place after the deposit of the series of strata a of which the mountains are composed, and before the deposit of the stratified rocks, which rest unconformably on a and have not shared their uplift.

Most great mountain ranges, like the Sierra Nevada and the Alps, mark lines of weakness along which the earth’s crust has yielded again and again during the long ages of geological time.  The strata deposited at various times about their flanks have been infolded by later crumplings with the original mountain mass, and have been repeatedly crushed, inverted, faulted, intruded with igneous rocks, and denuded.  The structure of great mountain ranges thus becomes exceedingly complex and difficult to read.  A comparatively simple case of repeated uplift is shown in Figure 204.  In the section of a portion of the Alps shown in Figure 179 a far more complicated history may be deciphered.

UNCONFORMITIES in the Colorado canyon, Arizona.  How geological history may be read in unconformities is further illustrated in Figures 207 and 208.  The dark crystalline rocks a at the bottom of the canyon are among the most ancient known, and are overlain unconformably by a mass of tilted coarse marine sandstones b, whose total thickness is not seen in the diagram and measures twelve thousand feet perpendicularly to the dip.  Both a and b rise to a common level nn and upon them rest the horizontal sea-laid strata c, in which the upper portion of the canyon has been cut.

Note that the crystalline rocks a have been crumpled and crushed.  Comparing their structure with that of folded mountains, what do you infer as to their relief after their deformation?  To which surface were they first worn down, mm’ or nm?  Describe and account for the surface mm’.  How does it differ from the surface of the crystalline rocks seen in the Torridonian Mountains, and why?  This surface mm’ is one of the oldest land surfaces of which any vestige remains.