. Elementary physical geography . mit of navigable waters, for ships cannot pass overthem; and since the cities were- so placed in order that theymight have the advantage of ocean traffic, and still be as farinland as possible, theywere usually located atthe head of such cities as Phila-delphia, Baltimore, Wash-ington, and others, aresituated just on the sea-ward side of this fall line,and small falls and rapidsare found almost withinthe city limits. As a stream deepensits channel, it may actu-ally form waterfalls as aresult of its work. Theriver is able to removesoft rocks mor
. Elementary physical geography . mit of navigable waters, for ships cannot pass overthem; and since the cities were- so placed in order that theymight have the advantage of ocean traffic, and still be as farinland as possible, theywere usually located atthe head of such cities as Phila-delphia, Baltimore, Wash-ington, and others, aresituated just on the sea-ward side of this fall line,and small falls and rapidsare found almost withinthe city limits. As a stream deepensits channel, it may actu-ally form waterfalls as aresult of its work. Theriver is able to removesoft rocks more rapidlythan hard ones, and ifthe stream channel iscrossed by layers of dif-ferent hardness, the differ-ence in rate of cutting inthe two kinds of rock will produce a rapid, or even a water-fall (Figs. 162,183, and 165). The hard layer tends to standup above the soft one, and thus there is a steep descent in thestream valley. As soon as the stream has cut down to the linewhere its power of deepening ceases, the waterfalls disap-. Fig. 165. Small waterfalls in a gorge near Ithaca,, where the water flows over nearlyhorizontal rocks of varying hardness. 298 PHYSICAL GEOGRAPHY.
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