[Illustration: Fig. 202. Carboniferous Limestone resting unconformably on Early Silurian Slates, Yorkshire, England] 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 we 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. [Illustration: Fig. 203. Diagram Illustrating how the Age of Mountains is determined] =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. [Illustration: Fig. 204. Section of Mountain Range showing repeated Uplifts _a_, strata whose folding formed a mountain range; on, baseleveled surface produced by long denudation of the mountains; _b_, tilted strata resting unconformably on _a_; _c_, horizontal strata parted from _b_ by the unconformity _u´u´_. The first uplift of the range preceded the period of time when _b_ was deposited. The and uplift, to which the present mountains owe their height, was later than this period but earlier than the period when strata _c_ were laid] 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. [Illustration: Fig. 205. Unconformity showing Buried Valleys