158 GROVE KARL GILBERT— DAVIS [MkM0IB8 [ ^^ 



The young Niagara, cascading over the north-facing escarpment of the limestone-capped 

 upland, at once proceeded to cut back its gorge and continued to work valiantly at this task 

 until the gorge was some miles in length. In the pre-Gilbertian studies of the age of Niagara 

 Falls, it had been tacitly assumed, as has been pointed out above, that the river had been of 

 constant volume and that the lengthening of the gorge by the recession of the Falls had gone 

 on at a constant rate; under these assumptions the age of the Falls could be determined by 

 dividing the length of the gorge by the retreat of the Falls in a single year. Gilbert's method 

 was more circumspect. He had in 1886 searched out every factor that is involved in the erosion 

 of the gorge, and then and later he scrutinized every factor to learn how far it may have varied. 

 The stratified rocks of the Niagara upland were found to be remarkably constant through the 

 whole length of the gorge, except that at the Whirlpool the river had found a previously eroded 

 and drift-filled valley, concerning which speculation had been active before Gilbert's time. The 

 recession of the Falls during the reexcavation of this valley must have been rapid. The height 

 of the Falls has long been decreasing, not only because of the gentle southward dip of the capping 

 limestones and the southward rise of the rapids gives the retreating Falls a smaller and smaller 

 measure, but also because at an early stage of their history Lake Iroquois was, by reason of the 

 small amount of land uplift in the northeast then accomplished, for a time lower near its western 

 end than Ontario is now ; hence at that time the river north of the escarpment deepened its 

 valley with respect to the lowered level of the lake into which it ran, and the Falls plunged 

 down into the deepened val'ey. The subsequent invasion of the deepened valley by the waters 

 of the present Ontario, as the uplift of the land in the northeast caused the lake waters to 

 rise toward the southwest, has already been told. A variation in the height of the Falls being 

 recognized, it was inferred that their recession would have been more rapid when the plunge 

 pool beneath them was excavated in weak strata than when it was excavated at least in part 

 in more resistant strata. 



VARIATION IN THE VOLUME OF NIAGARA 



But the chief cause of variation in the recession of the Falls was found in a variation of 

 river volume. So long as the ice sheet lingered over much of the lowland north of Lake 

 Ontario the drainage of the upper lakes could find no cross-country outlet to Ontario and 

 the St. Lawrence Valley, even though the lowland was at that time, in consequence of the small 

 uplift of the northeastern area then accomplished, lower than the Detroit outlet. But when the 

 ice margin withdrew sufficiently, the lower stand of the lowland permitted the upper lakes 

 to short circuit their discharge across it by the so-called Algonquin River or Trent outlet, 

 running from Georgian Bay of Lake Huron to Lake Ontario, as told above; and thereupon 

 Niagara River, receiving only the drainage of the Erie basin, must have been greatly reduced, 

 and the recession of the Falls must have been correspondingly slow. With the farther retreat 

 of the ice, a second and a more northern short circuit, known as the Nipissing outlet, was opened 

 at a still lower level in the then attitude of the land, and thus the reduction of the Niagara to 

 small volume was prolonged. Whether an intermediate epoch of roundabout lake discharge 

 by Detroit temporarily gave Niagara its full volume was not then deter m ined. But eventually 

 the continued rise of the land in the northeast raised even the northern short circuit to a higher 

 level than the Detroit channel; then the roundation discharge was resumed, Niagara regained 

 its full volume, and the recession of the Falls went on at a livelier rate again. And so the 

 recession has continued ever since. Although the southern of the two short-circuit discharges 

 had been found and its significance recognized by another observer before Gilbert came upon 

 it, he traced both short circuits along parts of their length, and briefly described the features 

 by which they are shown to be abandoned river channels; and he eventually made it clear 

 that while the volume of Niagara River was diminished, its gorge was cut to a less width 

 and less depth than before; but that after its full volume was restored, the gorge was again 

 cut to a full measure of width and depth. Thus the narrows of the gorge where two great rail- 

 road bridges cross it above the Whirlpool are accounted for as the work of the diminished river; 

 while the wider uppermost part of the gorge and the deep pool there evacuated are the work of 

 the increased river in its tnundering plunge of a full-volume cataract. Where can a more strik- 



