The Eastern Upland includes the land between the Connecticut Valley and the Atlantic Ocean. At present, it has the general form of a broad rolling highland with ridges and valleys that have a north-south trend. Close inspection shows that the rocks in the ridges are different from those in the long valleys. Also the layering of the materials ranges from a vertical attitude, as at Ware and Brimfield, to undulating and almost horizontal positions, as at Spencer and Worcester.
Through vertical and horizontal beds alike run those reinforcing sheets—some tabular and vertical, called dikes; others also tabular but horizontal, called sills; and some are just huge, irregular masses without visible bottoms, called stocks and batholiths. Some of them, composed of uniform, small, light-colored minerals, are granite; others are made entirely of large minerals over an inch across and are called pegmatite; a few, with cuneiform intergrowths of a dark mineral in a light one, arranged like Arabic writing, are known as graphic granite.
Pl. 5. Intrusive and extrusive igneous rocks.
a. Columnar lava rests upon red sandstone in the cliffs at Greenfield.
b. Fissures were filled with liquid rock that became solid and bonded wall to wall at the Windsor Dam.
Every one of these masses flowed into the rocks along fractures and other zones of weakness, crystallizing as they lost their heat and solvents to the hot springs of that ancient time. They are all invaders, or intrusives, which inserted themselves into the older beds. Whether they were squeezed into the fissures by the pressures that crumpled the original beds into their upturned positions, or whether they, like the liquid in a hydraulic press, transferred pressure from a deep reservoir to the walls of the fissures and so pushed the beds into their distorted forms, is unknown. Two features are clear; the distortion of the beds and intrusion of the liquid bodies were almost simultaneous, and the hot springs associated with them were still active at the dawn of Triassic time. These profound disturbances transformed the land into a series of elevated, wave-like folds, and rains promptly began to tear away at the summits of the newly raised mountains. From them was carved a serrate and rugged landscape, part of which was later buried beneath the Triassic fill.
Coal Swamps in Massachusetts and Rhode Island
Among the strata of the Eastern Upland which were folded, intruded, baked hard, and stewed in hot spring water, one group stands pre-eminent. It forms a broad band starting north of Worcester and reaching to Providence and beyond. Nearly everywhere it carries coaly material or impressions of plants which are now extinct, but which flourished in the Coal Age or Carboniferous period. Some of the coal seams were mined in the Providence basin, but they had been so heated by intrusive granite that they are partly graphitic and proved difficult to burn. The great extent of some of the coal seams suggests a panorama of immense swamps, and of land so flat that, for long periods, streams brought no sediment, and the trees and water-loving plants furnished the only fill. At other times sluggish rivers, flowing from the northwest, laid thick layers of sandy mud over the surface of the bogs. The alternating muds and coal seams are thousands of feet thick, and they record the story of a basin which sank as fast as it filled—a depression which was never built high enough to be a well drained plain, yet never subsided sufficiently to be inundated by the sea. The Carboniferous peat bogs and mud flats may have extended westward almost to the Connecticut Valley; and farther to the northwest they were bounded by a chain of rolling hills.