Forest physiography; physiography of the United States and principles of soils in relation to forestry . is clearlyshown in Fig. 284, and proves that the valleys were highways of moreactive glacial motion. The most important facts pointing to complexity of origin relate tothe hanging valleys tributary to the Finger Lakes. On the steepenedslopes of the main valley sides a series of buried gorges has been found. • Williams, Tarr, and Kindle, Watkins Glen-Catatonk Folio U. S. Geol. Surv. No. 169,field ed., 1909, p. 124. 7IO FOREST PHYSIOGRAPHY The gorges are occupied by drift deposits of the last
Forest physiography; physiography of the United States and principles of soils in relation to forestry . is clearlyshown in Fig. 284, and proves that the valleys were highways of moreactive glacial motion. The most important facts pointing to complexity of origin relate tothe hanging valleys tributary to the Finger Lakes. On the steepenedslopes of the main valley sides a series of buried gorges has been found. • Williams, Tarr, and Kindle, Watkins Glen-Catatonk Folio U. S. Geol. Surv. No. 169,field ed., 1909, p. 124. 7IO FOREST PHYSIOGRAPHY The gorges are occupied by drift deposits of the last (Wisconsin) ice inva-sion, which indicates that they were formed before that invasion.^ From the known facts it is concluded that before the glacial periodthere was a system of mature drainage with main valleys along the axesof Cayuga and Seneca lakes and with tributaries entering them atgrade. With the overspreading of the region by glacial ice there wasbegun a process of exceptional deepening in the main valleys because theyserved as lines of most rapid glacial flow. At the end of the first ice. UD EZ] B ^ [3 MB ORDOVICIAN SILURIAN DEVONIAN MISSISSIPPIAN Fig. 285. — Map of portion of New York, i, Ordovician; 2-5, Silurian; 6-13, Devonian; 14, Mississipian. invasion the valleys had been broadened and deepened, the amount ofthe deepening being about 500 feet. Lakes may have formed in theseoverdeepened valleys after the ice had been melted away. At any ratethe discordance of level between tributary and master valleys was pro-nounced and the tributaries began to cut down their valleys to the levelof the main valleys, making gorges of notable breadth and depth beforethe second glacial invasion filled them with drift, reexcavated anddeepened the main valleys, increased the discordance between tributariesand master streams, and so altered the topography in detail that thepostglacial streams do not flow everywhere in the interglacial a postglacial stream enters
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