The District of Columbia, Its Rocks and Their Geologic History (Classic Reprint)

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Excerpt from The District of Columbia, Its Rocks and Their Geologic HistoryIn Pleistocene time, when the glaciers on the northern part of the continent were melting, the Potomac Basin was subjected to recurrent ¿oods. Then as the region gradually rose, the shifting waters of the old rivers carved terraces in the rocks, especially in the soft Coastal Plain beds, and deposited on the terraces new sheets of gravel, sand, and clay. The main part of the city is built on these terraces.The earliest life of which any evidence has been found in the District of Columbia existed about 100 million years ago during early Cretaceous time. Animal life of that epoch included fresh-water mollusks, turtles, crocodiles, and huge dinosaurs, among the plants were ferns, sequoia. Water-lily, and grape. Fossil remains from later deposits include various mollusks, shark teeth, and leaves of the oak, elm, sumac, holly, and blueberry.Of the mineral resources of the District of Columbia, stone has had a leading part. Gneiss and granite quarried near Rock Creek and on the banks of the Potomac above Georgetown were used in early buildings and still are employed to some degree. The extensive deposits of gravel, sand, and clay of the Coastal Plain have long been used in the manufacture of brick and terra-cotta tile and in road building. Also present in or near the District are iron ore and diatomaceous earth in the Coastal Plain area and manganese, mica, feldspar, and gold in the Piedmont area, but attempts made at various times to develop these have not been very successful, generally because of relatively low quantity or quality.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.