Fig. 44.—Sketch of a mountain range along Skolat Creek, Alaska, showing Tertiary lava beds resting upon deeply eroded tilted limestones and lavas of late Paleozoic (Carboniferous) Age. The present topography has been produced by erosion since the Tertiary lavas flowed out. (After U. S. Geological Survey.)
Especially in Alabama and Texas the Cretaceous system is remarkable for its richness in chalk deposits. In Alabama a widespread formation of late Cretaceous Age, about 1,000 feet thick, contains much nearly pure white chalk, and in Texas a similarly constituted formation of early middle Cretaceous Age is from 1,000 to 5,000 feet thick. These chalk deposits consist almost wholly of carbonate of lime shells or very tiny single-celled animals which accumulated under exceptionally clear sea water which spread over those parts of Alabama and Texas where the chalk now occurs. Here again we have a bit of evidence supporting the fact of very long geologic time. Think of how long it must have taken for the tiny (even microscopic) shells to form a widespread layer of chalk nearly a mile thick!
The close of the Cretaceous period, or what is the same, the close of the Mesozoic era, was marked by some of the grandest crustal disturbances in the known history of the earth. In fact, it is not known that the western hemisphere was ever affected by more profound and widespread mountain-making disturbances than those which took place toward the close of the Mesozoic era, and continued into the succeeding Tertiary period. These disturbances were of three kinds: folding of strata, volcanic activity, and renewed uplift of old mountains without folding of the rocks. Greatest of all was the “Rocky Mountain Revolution,” during which the thick strata, which accumulated during the Paleozoic and Mesozoic eras over the site of the Rockies, yielded to vigorous deformation when they were more or less folded and dislocated from Alaska to Central America. This was in truth the birth of the Rocky Mountains, although their existing altitude and configuration have, to a very considerable degree, resulted from later uplift and erosion. In the northern United States and southern Canada the Rocky Mountain strata, up to over 40,000 feet thick, were most severely folded and fractured, forming a range which quite certainly was fully 20,000 feet high. In this district a great thrust fault, hundreds of miles long, developed, and rocks as old as the Proterozoic were shoved at least seven miles, and probably as much as twenty miles, westward, over Cretaceous and other rocks much later than the Proterozoic. At the same time the Andes Mountains throughout South America were notably upraised and the rocks folded.
The second type of physical disturbance was volcanic activity which took place on a tremendous scale, and which appears to have started as a direct accompaniment of the Rocky Mountain Revolution. This igneous activity took place not only in the Rocky Mountains but also westward to and in the Sierra-Cascade Range, as well as in the mountains of western British Columbia and Alaska. This activity continued well into the succeeding Cenozoic era, and it is more fully considered in the next chapter.
The third type of crustal disturbance took place on a large scale when the Appalachian Mountains, which had been almost wholly planed away by erosion during Mesozoic time, were reelevated from 1,000 to 3,000 feet by an uplifting force not accompanied by folding. All or nearly all of New York and New England, as well as much of southeastern Canada, were similarly upraised at the same time. This notable uplift of so much of eastern North America is a matter of great importance because the major relief features of that area have been produced by erosion or dissection of the upraised surface since late Mesozoic or early Cenozoic time. In view of the fact that this work of erosion took place almost wholly during the Cenozoic era, it will be discussed in the next chapter.
In conclusion, brief mention may be made of the kind of climate of the Mesozoic era. As shown by the character and distribution of fossil plants and animals, the Mesozoic climate was in general mild and rather uniform over the earth, but with some distinction of climatic zones. Such distinction of climatic zones is unknown for the Paleozoic era, while it was notably less than at present.