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

by William Herbert Hobbs

Lastly, it has been shown, as a result of the use of precise leveling methods, that not one but several hinge lines of movement lie within the region, and that the separate sections into which they divide the area are each in turn characterized by increased up-cant as we proceed to the northward ([Fig. 373, E]. and [Fig. 374]).

Fig. 375.—Series of idealistic diagrams to indicate the nature of the quick recovery of the crust by uplift in blocks unloaded of the ice in succession. A further and slower uptilt, added after the completion of the first movement, is brought out in the last diagram (b´).

The beaches of Lake Maumee, the earliest of the series of lakes within the Huron-Erie lobe and within the extreme southern portion of the Great Lakes area, show only the slightest possible northerly uptilt, and the well-marked hinge line disclosed in the Whittlesey beach is evidence that the elastic recoil, as it were, from the weight of the mantling glacier did not begin until after the draining of Lake Whittlesey. The determination by Taylor that there is a similar initial hinge line in the Warren beach—that this strand begins its uptilt some fifteen miles farther northeast than does the Whittlesey beach—is one of the greatest importance in obtaining a correct idea of the recent uplift; for it shows that the draining of Lake Whittlesey was followed by a period of quick uplift and seismic activity, that the stage of Lake Warren was one of comparative stability of the land, and, lastly, that the draining of Lake Warren was followed by a second period of rapid uplift and earthquake disturbance. The strongly marked hinge lines, additional to the initial one indicated for the Algonquin beaches in the profiles by Goldthwait from the west shore of Lake Michigan, when considered in the light of this northeasterly migration of the still earlier hinge line in the southern district, are best explained through the assumption of a succession of quick recoveries of the crust by uplift, separated by periods of relative stability, and brought on by the removal in turn of the ice burden from successive blocks of the shell which are separated by the several hinge lines ([Fig. 375]).

The elaborate study of erosion in the outlet of Lake Agassiz had indicated identical interruptions in the up-canting process for that basin.

Future consequences of the continued uptilt within the lake region.—One of the most distinguished of American geologists, Dr. G. K. Gilbert, in order to determine whether the uptilt revealed by canted beach lines is still in progress, carried out an elaborate study upon the gauge records preserved at the various gauging stations about the Great Lakes. Upon the basis of these studies, he concluded that the uplift continues, that the axes of equal uplift (isobases) take their course about fifteen degrees north of west, so that the lines of greatest uptilt should be perpendicular to this direction, or fifteen degrees east of north. He further believed that the basin was undergoing an up-cant in the simple manner of a trap door, the hinge of which lay to the southward of Chicago, and the study of the gauge records led him to believe that “the rate of change is such that the two ends of a line one hundred miles long and lying in a south-southwest direction are relatively displaced four tenths of a foot in one hundred years.”

Gilbert’s prophecy of a future outlet of the Great Lakes to the Mississippi.—The natural rock sill, over which the waters of Lake Chicago once flowed to the Mississippi, is to-day but eight feet above the common mean level of Lakes Michigan and Huron, and if the tilting of the lake region were to continue upon Gilbert’s assumption of a canting plane with the hinge of the movement to the south of Chicago, a time must come when the “Chicago outlet” will again come into use and the lakes once more drain to the Mississippi and the Gulf. Upon the basis of his measurements, Gilbert ventured the prophecy that the first high-water discharge into the Mississippi should occur in from five hundred to six hundred years, and for continuous discharge in fifteen hundred years. In twenty-five hundred years Niagara Falls should at low water stages be dry from this cause, and in thirty-five hundred years it should have become extinct.

This prophecy, emanating from a high scientific authority and relating to changes of such profound economic and commercial importance, has been often quoted and has taken a firm hold upon the popular imagination. Obviously, it depends upon the now exploded theory that the lake basin has been canted as a plane and that the axis of uptilt lies somewhere to the southward of the lake region, or, in any event, to the southward of the present Port Huron outlet. We know to-day that instead of being uniformly distributed over the entire lake region, the uptilting goes on at a much higher rate within the northern areas, and that since the early stage of Lake Whittlesey the hinge line of uplift has been steadily migrating northward with the retreat of the ice and is now well to the northward of the present outlet. There is, therefore, no known uptilt of the district which separates the present from the former Chicago outlet, and there is no apparent natural cause which should result in the reoccupation of the old outlet to the Mississippi. The prophecy must be regarded as one that has been outgrown with the progress of science.

Geological evidences of continued uplift.—It has recently been claimed, on the basis of a reëxamination of Gilbert’s study of the lake gauge records, that his methods are open to serious criticism and that in reality the figures afford no evidence of continued uplift of the region. However this may be, there are not lacking geological evidences which do not admit of doubt, and these are in a striking way confirmatory of the latest conclusions upon the manner of the recent uplift.