. Annual Reports of the Department of the Interior for the Fiscal Year Ended June 30, 1900--Twenty-First Annual Report of the United States Geological Society. nderlying gneissand schist in an upthrown block, and began to develop above it a new plain of ero-sion. Above this obstruction a lake has been formed either by subsequent elevationof the block or by the formation of a dam of glacial drift after a migration of thedivide between the youthful Pomperaug and its main eastern tributary and theupthrown schist block. Mature (present) stage. The main eastern tributary in the lower reach of theri
. Annual Reports of the Department of the Interior for the Fiscal Year Ended June 30, 1900--Twenty-First Annual Report of the United States Geological Society. nderlying gneissand schist in an upthrown block, and began to develop above it a new plain of ero-sion. Above this obstruction a lake has been formed either by subsequent elevationof the block or by the formation of a dam of glacial drift after a migration of thedivide between the youthful Pomperaug and its main eastern tributary and theupthrown schist block. Mature (present) stage. The main eastern tributary in the lower reach of theriver liaving accjuired an atlvantage in declivity of its headwaters, has pushed back itsdivide along the sandstone of the eastern valley so as completely to circumscribe thearea of basalt, capture the headwaters of the river, and partially drain the it has been compelled to cross the basalt (as elsewhere) the river has chosenits course over downthrown blocks lined by upthrown blocks (graben), in doingwhich it has discovered the upper surface of basalt in the bottom of its channel atPomperaug village, and thus formed a new plain of HuBBs] EROSION HISTORY OF POMPERAUO RIVER. 155 boeii mainly responsible for this are: (1) The Cretaceous plain of ero-sion, whose former drainage and whose gentle southeastern slopeconditioned the direction of the initial drainage of the cycle; (2) thedownthrow of the orographic blocks of the valley below the surround-ing walls of crystallines, which, in a rough wa}, has in the subsequenterosion determined the size of the basin; (3) the presence of soft sand-stones, which fixed the general position of the trunk stream, as thecanal-like trenches did its more exact direction; (i) the upthrown blockof schist which projected into the basin from its western wall and broughtabout a new temporary base-level and ultimately the reversal of the drain-age, and (5) the altitudes of the basalt block in the canyon in Pomperaugvillage, of the
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