Earth History of a Portion of the Pacific Northwest

Abstract

Of some parts of the land areas of the earth the geological history is readily decipherable, and a long series of physical events can be demonstrated, typified by such changes as submergence below or emergence above the sea, vulcanism, uplift and resulting deep dissection and eventual complete erosional planation, mountain building with development of very complex structure in the crust, and broad down-bowing or down-faulting of districts to form basins in which bodies of sediments often thousands of feet thick accumulate. Such episodes constitute the physical history of the earth. But in large areas the bedrock of the country, from which the history must mainly be read, is covered by alluvium, or lava flows have spread over it to the extent of thousands of square miles, or glacial detritus has masked the underlying rocks and structures. The geological history of such concealed parts of the crust was until quite recently almost entirely unrecoverable, and though great advances have been made in drilling and geophysical methods during the past decade or two, the accessible record of physical events in such areas is even now a partial rather than a complete one. Examples of districts in which the historical records can be made out with facility are the Coast Ranges of California and the Grand Canyon of the Colorado River; in striking contrast are the deeply alluviated floor of the San Joaquin Valley, the wide prairie plains of the interior of the continent, and parts of the northeast quarter of North America deeply encumbered with glacial waste.

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