
STRUCTURE AND SURFACE FEATURES 405 
ceases, and may be replaced by movement in the opposite direction— 
a general bulging-up or elevation of the area may take place. Should 
such ensue, then the buried plain of erosion will again rise out of the 
sea, and may even attain a height of many thousand feet above that 
level. In that case we should speak of the newly formed plateau as a 
plateau of accumulation. A section across it would show that the upper 
portion of the elevated area consisted of a great thickness of approxi- 
mately horizontal strata resting upon and concealing the old plain of 
erosion. 
Although Tectonic mountains tend to be gradually ground down to 
their base-level, it is seldom that the cycle of erosion is quite completed. 
Long before the mountains have entirely vanished, renewed crustal 
deformation may take place, and the much-denuded area be either 
re-elevated or submerged, according as the earth-movement is up or 
down. In the former case we get a plateau of erosion, the surface of 
which may be more or less irregular—ribbed and knotted with the 
straggling cores and stumps of the ancient mountains. In the latter 
case, the sorely-denuded mountain-land, carried down below sea-level, 
becomes in time covered with sediments, underneath which the lower 
lying parts of the plain of erosion may eventually become very deeply 
buried. Should the cores and stumps of the old mountains remain 
above sea-level as islets, they will, of course, escape burial, only to be 
subject, however, to continuous erosion. But should they be submerged, 
then they also will in time become partially or entirely concealed under 
gradually accumulating sediments. At a later period, should the sunken 
area be re-elevated to a very considerable height, we shall have a plateau 
of accumulation, consisting of approximately horizontal strata resting 
upon the irregular surface of the old plain of erosion. The horizontal 
strata will naturally attain their greatest thickness upon the lowest-lying 
portions of that plain, and will thin away as they approach the stumps 
of the degraded mountains—the summits of which may even peer above 
the surface of the plateau, as so many islets in a far-stretching sea. 
2. SUBSEQUENT OR RELICT MOUNTAINS.—These have 
not been constructed or built as mountains, but are merely 
remaining portions or fragments of a formerly more exten- 
sive elevated area. They have been carved out of an old 
tableland and shaped into mountains by the gradual removal 
of masses by which they were at one time surrounded. 
The form assumed by Relict mountains depends mainly 
upon the nature and arrangement of the materials out of 
which they have been carved. A plateau of accumulation, for 
example, tends to be cut up into a series of pyramidal or 
tabular mountains, and crested or flat-topped ridges, separat- 
ing the various valleys from each other. And as the latter 
are deepened and widened, the massive segments of the old 
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