Whatever be the connection between the deposition and
119
the subsequent upheaval, _the element of great depth of
accumulation seems a necessary condition and must evidently enter
as a factor into the Physical Processes involved_. The mountain
range can only arise where the geosyncline is deeply filled by
long ages of sedimentation.
Dana's description of the events attending mountain building is
impressive:
"A mountain range of the common type, like that to which the
Appalachians belong, is made out of the sedimentary formations of
a long preceding era; beds that were laid down conformably, and
in succession, until they had reached the needed thickness; beds
spreading over a region tens of thousands of square miles in
area. The region over which sedimentary formations were in
progress in order to make, finally, the Appalachian range,
reached from New York to Alabama, and had a breadth of 100 to 200
miles, and the pile of horizontal beds along the middle was
40,000 feet in depth. The pile for the Wahsatch Mountains was
60,000 feet thick, according to King. The beds for the
Appalachians were not laid down in a deep ocean, but in shallow
waters, where a gradual subsidence was in progress; and they at
last, when ready for the genesis, lay in a trough 40,000 feet
deep, filling the trough to the brim. It thus appears that epochs
of mountain-making have occurred only after long intervals of
quiet in the history of a continent."[1]
[1] Dana, _Manual of Geology_, third edition, p. 794
120
On the western side of North America the work of
mountain-building was, indeed, on the grandest scale. For long
ages and through a succession of geological epochs, sedimentation
had proceeded so that the accumulations of Palaeozoic and
Mesozoic times had collected in the geosyncline formed by their
own ever increasing weight. The site of the future Laramide range
was in late Cretaceous times occupied by some 50,000 feet of
sedimentary deposits; but the limit had apparently been attained,
and at this time the Laramide range, as well as its southerly
continuation into the United States, the Rockies, had their
beginning. Chamberlin and Salisbury[1] estimate that the height
of the mountains developed in the Laramide range at this time was
20,000 feet, and that, owing to the further elevation which has
since taken place, from 32,000 to 35,000 feet would be their
present height if erosion had not reduced them. Thus on either
side of the American continent we have the same forces at work,
throwing up mountain ridges where the sediments had formerly been
shed into the ocean.
These great events are of a rhythmic character; the crust, as it
were, pulsating under the combined influences of sedimentation
and denudation. The first involves downward movements under a
gathering load, and ultimately a reversal of the movement to one
of upheaval; the second factor, which throughout has been in
[1] Chamberlin and Salisbury, _Geology_, 1906, iii., 163.