Forest physiography; physiography of the United States and principles of soils in relation to forestry . leysformed upon them are likewise narrow for the most part; in easternNew York they are thick and have weathered into broad valley lowlands. Glacial Effects Reference to the map, 51ateIV, will show that the northernpart of the great AppalachianSystem was glaciated but thatthe southern and central por-tions were not covered withice. It follows that the inter-pretation of the topography anddrainage of the latter districtsis much easier than in thoseregions where glaciation has in-terfered wit
Forest physiography; physiography of the United States and principles of soils in relation to forestry . leysformed upon them are likewise narrow for the most part; in easternNew York they are thick and have weathered into broad valley lowlands. Glacial Effects Reference to the map, 51ateIV, will show that the northernpart of the great AppalachianSystem was glaciated but thatthe southern and central por-tions were not covered withice. It follows that the inter-pretation of the topography anddrainage of the latter districtsis much easier than in thoseregions where glaciation has in-terfered with the normal de-velopment of the landscape orhas partially buried the surfaceunderneath a cover of glacialtill. One of the most importanteffects of glaciation was thechanging of river courses eitherby the bodily diversion ofstreams, the ice occupying the valleys long enough to enable a newcourse to be cut by the river in a different situation, or by the block-ing of the preglacial channels with glacial drift, causing streams to bedeflected into adjacent valleys. One of the most striking instances of. #New Martinsville ,g( Fig. 240. — Probable preglacial drainage of westernPennsylvania, the limit of glaciation is shown bythe broken crossed line. (Modified from Leverett.) 6oo FOREST PHYSIOGRAPHY glacial diversion is that of the Allegheny, which formerly flowed north-ward into Lake Erie northwest of Pittsburg, Fig. 240. The continentalice sheet displaced the river and kept it to a southern course so longand so modified the ancient valleys by drift deposits that with the re-treat of the ice the streams had established themselves in new channelsdraining in the opposite direction and forming a system tributary to theMississippi.^ Many other tributaries of the Ohio suffered similar changes of posi-tion and direction of flow through glacial action. The Ohio itself wasin large part diverted from its preglacial channel. The variation in widthof the present channel of the Ohio is the re
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectforestsandforestry
