OCR Text |
Show 282 in contour than in the recess into which the Oimarroncito opens oat on leaving its picturesque mountain- hemmed valley. Here, in consequence of the rapid pitch of the valley, the terraces have a more marked inclination with the stream than is observed on the Cimarron, and where they present several distinct levels, often isolated, forming low drift-covered mesas or wide slopes rising upon the flank of Urac Mountain. Besides these more strictly flu viatile formations, in places the borders of the basin are marked by low benches of considerable extent, and which are distinguishable from those occurring immediately along the water- courses by their more level surfaces. Between the Bayado and Cimarron, as also between the streams to the north of the latter, these benches form several levels connected with the main slope flanking the Tertiary plateau, and which are usually distinctly defined by the abrupt descent facing the plain, as shown in the sketch of the Baldy range and that of the Tanaja Mountains. Their tops are covered by a thin sheet of fine drift- material, or the soft Cretaceous shales constitute the subsoil. At other places, limited tracts of barrens are encountered, where deposits of loose sand have accumulated, which the winds mold into ever- varying miniature downs, and whose vegetation, if not peculiar, is distinguished by the prevalence of cacti and those plants which thrive in a sterile soil. The latter system of terraces probably are more properly referable to an earlier time, when the drainage of the basin was effected; while the modifications resulting in their present conformation is as probably due to the combined erosive action of the Canadian and its numerous affluents, when their volume of waters, issuing from the vast reservoirs in the mountains, was far greater than at the present time. The nature of the loose materials covering the terraces in the mouths of the valleys plainly indicates the source of their derivation in the mountains to the west, and which were brought down at a period antecedent to the time when the streams had deepened their beds to their present level. The coarser materials and bowlders ( the latter generally of small size) largely consist of granitic and gneissic rocks, with a variety of igneous and metamorphic material, whose parent ledges, from which they were torn during the Glacial epoch, are met with in the mountains a few miles distant. But these plains accumulations are insiguificant compared with the vast deposits which fill some of the park- like valleys in the heart of the mountains, and which rise high upon the environing declivities. Besides this comparatively thin sheet of drift- material, the plain is here and there occupied by low buttes and mesas of limited extent, remnants of the Cretaceous shales, the summits of which are ofteu covered by a sort of concrete, consisting of coarse gravel cemented with lime. One of these outcrops occurs on the summit of the elevation near Mr. Arms's, forming a coping to the little mesa upward of three feet in thickness, its presence serving to protect the soft underlying shales from atmospheric erosion. Similar deposits are met on the lower course of the Poiiil, a tributary of the Cimarron, where the outcrop appears iu the low bluff- banks, at a level possibly not much higher than the exposure on the Canadian just mentioned. It is evident that this concrete deposit is of quite modern origin, but its relations to the drift and late Tertiary deposits are not so clear. It would, however, appear to antedate the drift, the modified strata of which its comparatively limited extent recalls, from the fact of its occurrence on the tops of the outlying remnants of the Cretaceous in the midst of the basin; while it is undoubtedly of much more recent origin than the sandstones at the base of the Lignitic formation, from which it widely differs in the char- |