Geology: The Science of the Earth's Crust

by William J. Miller

The Ordovician period closed with a great mountain-making disturbance in eastern North America, and at the same time all, or nearly all, of the continent was land. Throughout most of the Cambrian and Ordovician periods, the strata accumulated to a thickness of thousands of feet in the marine waters which spread over the eastern border of New York, the sites of the Green Mountains of Vermont, the Berkshire Hills of Massachusetts, and southward at least as far as Virginia, over the area of the Piedmont Plateau. At, or toward the close of the Ordovician period, a great compressive force in the earth’s crust was brought to bear upon the mass of strata and they were tilted, highly folded, and raised above sea level into a great mountain range known to geologists as the Taconic Range. It is quite the rule throughout this region of Taconic disturbance to find the strata either on edge or making high angles with the plane of the horizon. Many of the folds were actually overturned, and in some cases notable thrust faults developed, that is, the upper strata broke across and great masses were shoved over each other. These facts all go to show that the mountain-making compressive force applied to the region was of rather an extreme type. Since the origin of the Taconic Range a tremendous amount of erosion has taken place, so that literally only the roots of the range are now exposed in the Green Mountains, Berkshire Hills, Highlands-of-the-Hudson, and the northern Piedmont Plateau.

How do we know that the Taconic disturbance took place toward the close of the Ordovician period? By way of answer to this question two facts need to be considered. First, relatively late (or young) Ordovician strata are involved with the folds, thus proving that the folds formed after those late Ordovician sediments were deposited. Second, undisturbed strata formed during the middle of the next (Silurian) period, rest upon the eroded edges of the folds, which proves that the folds must have developed well before middle Silurian time because the only time they were subjected to erosion must have been during early Silurian time.

Fig. 36.—Structure section showing profile and underground relations of the rocks across part of the Highlands-of-the-Hudson region in southeastern New York. Length of section, sixteen miles. The rocks are mostly of Prepaleozoic Age, but with belts of highly infolded early Paleozoic strata toward the middle right. (After Berkey, New York Museum Report.)

Mention should also be made of the profound metamorphism (alteration) of the Cambrian and Ordovician strata along the main axis of the range, where the intense compression, aided by heat and moisture, caused the deeply buried portions of the strata to become plastic, and hence they became more or less foliated (cleavable) and crystallized into various metamorphic rock types, the limestone having changed to marble, the shale to slate or schist, and the sandstone to quartzite. Thus we explain the rocks of the extensive marble quarries of Vermont and western Massachusetts, the slate quarries of central eastern New York, and the Berkshire schist of the Berkshire Hills of Massachusetts.

One of the grandest and most significant of all the profound geological processes is the birth and history of a great folded mountain range. Since the Taconic Range affords us such an excellent example of a large-scale, well-understood folded range of great antiquity we may do well to consider it in the light of certain other broad relationships. The great compressive force which folded and upraised the Taconic Mountains did not accomplish its work rapidly in the ordinary human history sense of the word. The force was slowly and irresistibly applied, and the strata well below the surface were gradually bulged or folded, or fractured where near the surface, the length of time required for the operation having been, at the very least calculation, some hundreds of thousands of years, and more than likely a million years or more. Such a length of time is, however, so short compared with all known earth history, that we are accustomed to refer to the formation of such a mountain range as simply an event of geological history.

Even before such a range attains its maximum height a very considerable amount of erosion has already taken place. When the first fold appears above sea level, erosion begins its work and continues with increasing vigor as the mountain masses get higher and higher. Thus we have warfare between two great natural processes—the building up and the tearing down. After a time the building-up process wanes and then ceases, while the tearing-down process (erosion) continues either until the whole range has been completely worn down or until some rejuvenating force causes a renewed uplift. Here is an example of one of the remarkable procedures of nature. After millions of years of work causing the deposition of thousands of feet of strata, piled layer upon layer on the sea floor, a force of lateral pressure is brought to bear and a mountain range is literally born out of the sea. No sooner is the range well formed than the destructive processes (erosion) unceasingly set to work to destroy this marvelous work. But the sediments derived from the wear of the range are carried into the nearest ocean again to accumulate and, perchance, after long ages, to be raised into another range; and so the process may be often repeated. From this we learn that the mountain ranges of the earth are by no means all of the same age. The original Adirondacks were formed long before the Taconics, which originated millions of years before the Appalachians, these latter having been folded up long before the Sierras. The Rockies, followed by the Coast Ranges, are each younger than the Sierras as regards their original folding and uplift. Among foreign countries special mention should be made of the British Isles, where Ordovician strata thousands of feet thick were, late in the period, notably folded and upraised, the crustal disturbance having been accompanied by great intrusions of molten rocks and vast outpourings of lavas, so that this region ranks among the greatest of the ancient volcanic areas of Europe.