. Elementary physical geography . Fig. Falls, Niagara. the process of this retreat, the gorge has been cut to a depthof from 200 to 300 feet, with a width of from 200 to 400 yards,while the fall itself is now about 160 feet in height. Carefulsurveys made many years apart, show that the retreat of thewaterfall toward Lake Erie is rather rapid, on the aver-age being not far from five feet a year. If this averagehas been maintained throughout the entire history of Niag- 296 PHYSICAL GEOGRAPHY. ara, the time occupied in cutting the gorge from Queenstownto the base of the falls, is som
. Elementary physical geography . Fig. Falls, Niagara. the process of this retreat, the gorge has been cut to a depthof from 200 to 300 feet, with a width of from 200 to 400 yards,while the fall itself is now about 160 feet in height. Carefulsurveys made many years apart, show that the retreat of thewaterfall toward Lake Erie is rather rapid, on the aver-age being not far from five feet a year. If this averagehas been maintained throughout the entire history of Niag- 296 PHYSICAL GEOGRAPHY. ara, the time occupied in cutting the gorge from Queenstownto the base of the falls, is somewhere between 7000 and10,000 years. The falls of St. Anthony, in the Mississippivalley, are of the same origin, and have had nearly thesame history; and the same is true of a vast number ofwaterfalls in the northern states of the Union. Any other obstacle in the way of a stream will transformit into a waterfall, such for instance as the folding of moun. Fig. 164. Yosemite Falls. tains, or the passage of a lava flow across a stream valley, orany one of several similar accidents. When rocks break,and move on one side of the crack, as is done when faultsoccur, the movement increases the slope of the stream nearthe fault line. Thus between the plains bordering theeastern coast of the United States, and the hilly region justinland from these, there is a line of movement, on the land-ward side of which the country has been raised; and thisline has determined the existence of a large number of smallfalls and rapids. Because of this it has been called the fall DELTAS, FLOOBPLAINS, WATERFALLS, ETC. 297 line; and this small geological accident has been largely-responsible for the location of several of the great cities alongthe Atlantic coast. The falls and rapids mark the approxi-mate limit 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
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