[Illustration: Fig. 35.--Structure section showing rocks representing three geologic eras separated by millions of years of time. Length of section 12 miles, vertical scale much exaggerated. At the bottom are Archeozoic (Precambric) rocks and resting upon them on the left are early Paleozoic strata 1,500 feet thick. A glacial lake deposit of late Cenozoic age lies on the Archeozoic rock toward the right. It is evident that the Paleozoic strata formerly extended much farther eastward. (By the author as published in a New York State Museum Bulletin.)]
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.