Peneplain Sentence Examples
In the nature and position of the upland rocks - mainly crystalline schists and gneisses, excessively complicated and disordered in mass, and also internally deformed - there is found abundant proof that the peneplain is a degraded mountain region.
By a re-elevation of a peneplain the rivers of an old land surface may be restored to youthful activity, and resume their shaping action, deepening the old valleys and initiating new ones, starting afresh the whole course of the geographical cycle.
To the east and north-east of the Bitterroot Mountains is a considerable basin or peneplain dissected by short ranges having a northwest and south-west trend.
The height and massiveness of the mountains decrease to the south-west, where the piedmont belt sweeps westward around them in western Georgia and eastern Alabama Some of the residual mountains hereabouts are reduced to a mere skeleton or framework by the retrogressive penetration of widening valleys between wasting spurs; the very type of vanishing forms, Certain districts within the mountains, apparently consisting of less resistant crystalline rocks, have been reduced to basin-like peneplains in the same time that served only to grade the slopes and subdue the summits of the neighboring mountains of more resistant rocks; the best example of this kind is the Asheville peneplain in North Carolina, measuring about 40 by 20 m.
The piedmont belt may be described as a maturely dissected peneplain over much of its extent; it is indeed one of the best examples of that class of forms. Its uplands are of fairly accordant altitude, which gradually decreases from 500 to 1000 ft.Advertisement
The erosion of the region must have been far advanced, perhaps practically completed, in very ancient times, for the even surface of the peneplain is overlapped by fossiliferous marine strata of early geological date (Cambrian); and this shows that a depression of the region beneath an ancient sea took place after a long existence as dry land.
The northern section of the Great Plains, north of latitude 44, including eastern Montana, north-eastern Wyoming and most of the Dakotas, is a moderately dissected peneplain, one of the best examples of its class.
The peneplain is no longer in the cycle of erosion that witnessed its production; it appears to have suffered a regional elevation, for the riversthe upper Missouri and its branchesno longer flow on the surface of the plain, but in well graded, maturely opened valleys, several hundred feet below the general level.
The two sections are also unlike in that residual eminences still here and there surmount the peneplain of the northern section, while the fluviatile plain of the central section completely buried the pre-existent relief.
The summits of some of the big mountains, such as the Cascades, appear to be remnants of a peneplain developed in post-Miocene time.Advertisement
There is also in the southern inland region an interior plateau, once probably a peneplain, but now elevated and greatly dissected by river valleys, which extends north-westward for Soo m.
A land may thus be characterized by its position in the " geographical cycle," or cycle of erosion, as young, mature or old, the last term being reached when the base-level of erosion is attained, and the land, however varied its relief may have been in youth or maturity, is reduced to a nearly uniform surface or peneplain.
This upland is an uplifted peneplain of subaerial denudation,' now so far advanced in a " second " cycle of weathering and so thoroughly dissected that to an untrained eye it appears to be only a country of hills confusedly arranged.
The more resistant rocks, even though dissected by Tertiary erosion, retain in their summit tiplands an indication of the widespread peneplain of Cretaceous tinie, now standing at the altitude given to it by the Tertiary upwarping and post-Tertiary uplift; and the most resistant rocks surmount the Cretaceous peneplain as unconsumed monadnocks of the Mesozoic cycle.
In this province, therefore, we find a part of one of those ancient mountain regions, initiated by crustal deformation, but reduced by long continued erosion to a peneplain of modern relief, with occasional surmounting monadnocks of moderate height not completely consumed during the peneplanation of the rest of the surface.Advertisement