Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Fig. 385.—Bird’s-eye view of the captured Canadian Fall at Wintergreen Flats, showing the section of the river bed above the cliff and the blocks of fallen Niagara limestone strewn over the abandoned channel below (after Gilbert).

The Whirlpool Basin excavated from the St. Davids Gorge.—It has already been pointed out that a rock channel now filled with glacial deposits extends from the Whirlpool Basin to the edge of the escarpment at St. Davids ([Fig. 389], [p. 363]). In plan this buried gorge has a trumpet form, being more than two miles wide at its mouth and narrowing to the width of the upper gorge before it has reached the Whirlpool. Near the Whirlpool it has been in part excavated by Bowman Creek, thus revealing walls that are well glaciated. Different opinions have been expressed concerning the origin of this channel, one being that it is the course either of a preglacial river or one incised between consecutive glacial invasions; and another that it is a cataract gorge drilled out between glacial invasions after the manner of the later Niagara gorge. In either case its contours have been much modified by the later glacier or glaciers, whose work of planing, polishing, and widening is revealed in the exposed surfaces; and it is not improbable that a cataract has receded along the course of an earlier river valley.

As we shall see, there are facts which point rather clearly to an earlier cataract which ended its life immediately above the present Whirlpool. When the later Niagara cataract had receded to near the upper end of the Cove section, or near the present Whirlpool, the falling water must have been separated from this older channel and its filling of till deposits by only a thin wall of rock, and this must have been constantly weakened as its thickness was further reduced.

When this weakened dam at last gave way, it must have produced a debacle grand in the extreme. It is hardly to be conceived that the “washout” of the ancient channel to form the Whirlpool Basin could have occupied more than a small fraction of a day, though it is highly probable that the broken rock partition below the Whirlpool was not immediately removed entire. The mandible-like termination of the Eddy Basin immediately above the Whirlpool has led Taylor to believe that the cataract quickly reëstablished itself at this point upon the last site of the extinct St. Davids cataract. If reduced in power for a short interval, as a result of the obstructions still remaining in the lately broken dam below the Whirlpool, the remarkable narrowing of the gorge at this point would be sufficiently accounted for.

Being compelled to turn through more than a right angle after it enters the Whirlpool Basin, the swift current of the Niagara River is forced to double upon itself against the opposite bank and dive below the incoming current before emerging into the Cove section below the Whirlpool ([Fig. 386]).

Fig. 386.—Map of the Whirlpool Basin, showing the rock side walls like those of the Niagara Gorge, and the drift bank which forms the northwest wall (after Gilbert).

In tearing out the loose deposits which had filled this part of the buried St. Davids Gorge, many bowlders of great size were left which slid down the slope and in time produced an armor about the looser deposits beneath, so as to protect them and prevent continued excavation. Thus it is found that the submerged northwestern wall of the basin is sheathed with bowlders large enough to retain their positions and so stop a natural process of placer outwashing upon a gigantic scale ([Fig. 386]).

The shaping of the Lewiston Escarpment.—To understand the formation of the Lewiston Escarpment cut in the hard Niagara limestone, it is necessary to consider the geology of a much larger area—that of the Great Lakes region as a whole. To the north of the Lakes in Canada is found a most ancient continent which was in existence when all the area to the southward lay below the waters of the ocean. In a period still very many times as long ago as the events we have under discussion, there were laid down off the shore of this oldland a series of unconsolidated deposits which, hardened in the course of time, and elevated, are now represented by the shales, sandstone, and limestone which we find, one above the other, in the Niagara gorge in the order in which they were laid down upon the ocean floor. The formations represented in the gorge are but a part of the entire series, for other higher members are represented by rocks about Lake Erie and even farther to the southward. These strata, having been formed upon an outward sloping sea floor, had a small initial dip to the southward, and this has been probably increased by subsequent uptilt, including the latest which we have so recently had under discussion. At the present time the beds dip southward by an angle of less than four degrees, or about thirty-five feet in each mile.