. Elementary physical geography;. runnen (nothing but fountains) of the Alps arenames that suggest the character of these regions. Sometimes the stream has little to do with making thecliffs over which it plunges; in other cases, the river itselfhas made the falls. If a stream flows over the edge of ahard layer that rests on a softer material, the latter will bemore quickly removed; moreover, as the softer layer isworn away, the height of the fall becomes greater and thewater acquires an increased cutting power because of itsgreater fall; a cataract therefore results. In this manner the falls
. Elementary physical geography;. runnen (nothing but fountains) of the Alps arenames that suggest the character of these regions. Sometimes the stream has little to do with making thecliffs over which it plunges; in other cases, the river itselfhas made the falls. If a stream flows over the edge of ahard layer that rests on a softer material, the latter will bemore quickly removed; moreover, as the softer layer isworn away, the height of the fall becomes greater and thewater acquires an increased cutting power because of itsgreater fall; a cataract therefore results. In this manner the falls of Niagara River were , an upper layer of hard limestone surmounts sev-eral layers of softer rock. The upper layer offers consid-erable resistance to the water; the lower rock is easily cut 126 PHYSICAL GEOGRAPHY away. Hence the falls are increasing rather than decreas-ing in height; but the upper layer, however, is being under-mined and the falls are receding upstream at the rate ofabout two and one-half feet a ^-^T^fiygg A SECTION OF A WATERFALL As Niagara River flows toward Lake Ontario, it encounters a layer of hard Niagaralimestone thatcomes to the sur-face midway be-tween the twolakes. When theriver reaches theedge of this hardstratum, it pitchesa depth of the are softshales, sandstones,and a layer of hardlimestone. The fall-ing water beatsthese away, clearing a deep pool under the cliff of Niagara edge of the latter breaks off, the underlying strata wear back-ward, and the whole front of the falls recedes. At the point where the angle in the ledge is formed, the recession since1875 has been more than two hundred feet; at the American Fall, since1842, it has been very slight. It is a question of time only until theCanadian Fall will recede to a line between Dufferin and Sister this has taken place the American Fall will have nearly or quitedisappeared. Had the conditions of a hard stratum at the
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