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The Mesozoic peneplain has been upwarped by later crustal movements and has suffered profound erosion, but the remnants of it which remain on the upland of southern New England and the even summits of the Allegheny ridges suffice to prove that it once existed.
The western sea stretched from the Canadian Rockies over the Great Basin and at least as far south as the Grand Canyon of the Colorado in Arizona. Between these mediterraneans lay a great central land which included the pre-Cambrian U-shaped area of the Laurentian peneplain, and probably extended southward to the latitude of New Orleans.
THE MESOZOIC PENEPLAIN. The immense thickness of the Mesozoic formations conveys to our minds some idea of the vast length of time involved in the slow progress of its successive ages. The same lesson is taught as plainly by the amount of denudation which the lands suffered during the era.
The rocks have melted into soil, as the snow-banks in spring melt into water. The rocks that remain are like fragments of snow or ice that have so far withstood the weather. Geologists tell us that the great Appalachian chain has been in the course of the ages reduced almost to a base level or peneplain, and then reelevated and its hills and mountains carved out anew.
Dating from the same time are the valleys of the Hudson, the Susquehanna, the Delaware, the Potomac, and the Shenandoah. In Pennsylvania and the states lying to the south the Mesozoic peneplain lies along the summits of the mountain ridges.
In the great plains of North America two of the divisions, that is, the Atlantic coastal plain of the southeast and the peneplain of the northwest, owe their present form to the forces of erosion. The other two, that is, the prairies and the high plains, still bear the impress of the original processes of deposition and have been modified to only a slight extent by erosion.
The whole land area at the time of flooding was virtually a peneplain. A rise of a few hundred feet in the ocean level was all that was needed to drown most of the land." "I see. Yes, it's possible that life could have developed here under those conditions. A peneplain topography argues permanence for hundreds of millions of years." "You have studied geology?" Alexander asked curiously.
Along the Atlantic coast the Mesozoic peneplain may be traced shoreward to where it disappears from view beneath an unconformable cover of early Tertiary marine strata. The beginning of the Tertiary was therefore marked by a subsidence.
Even surfaces developed on rocks of discordant structure, such as the Piedmont shows, are produced by long denudation, and we may consider the Piedmont as a peneplain formed by the wearing down of mountain ranges, and recently uplifted. THE LAURENTIAN PENEPLAIN. This is the name given to a denuded surface on very ancient rocks which extends from the Arctic Ocean to the St.
A few more of these geologic years and they will be reduced to a peneplain, only their stumps left. This seems to hold truer of mountains that are wrinkles in the earth's crust squeezed up and crumpled stratified rock, such as most of the great mountain-systems are than of mountains of erosion like the Catskills, or of upheaval like the Adirondacks.
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