THE UPTILT OF THE LAND AT THE CLOSE OF THE ICE AGE

The response of the earth’s shell to its ice mantle.—There is now good reason to believe that the earth’s outer shell makes a response by oscillations of level due to the loading by ice, on the one hand, and to the removal of this burden upon the other. We know, at least, that both in northern Europe and in North America areas which have undergone depression during and elevation after the ice age, correspond closely to the regions which were ice covered. Wherever in these regions there was high relief before the advent of the ice, river valleys were drowned at the land margins and were also gouged out into troughs through erosion by the outlet tongues upon the margin of the ice sheet. Such furrowed and half-submerged valleys have a characteristic U-shaped section, so that their walls rise precipitously from the sea. From their typical occurrence in Scandinavian countries the name fjord has been applied to them.

It is now no less clear that the removal of the ice blanket brought from the earth a relatively quick response in uplift, which began before the ice front had retired across the present international boundary of the United States, and that this uplift continued until the final disappearance of the ice. A far slower elevation of a somewhat different nature has continued, even to the present day.

It is obvious that at the time of their formation all shore lines referable to the work of waves must have been horizontal, and hence any variations from a perfect level which they reveal to-day must indicate that a tilting movement of the ground has occurred since the waters departed from their basins. We have thus provided for us in the positions of these ancient water planes, particularly because of their wide extent, a complete record the refinement of which is not easily overstated. Interpreting this record, we find that it was the uptilt of the land to the northward which brought the glacial lake history to an end and inaugurated the present system of St. Lawrence drainage. The outlet of the Nipissing Great Lakes is to-day more than a hundred feet above the level of the outlet at Port Huron, where the upper lakes are now discharging their waters, and this difference in level can only be ascribed to an upward tilting of the land since the latest of the glacial lake stages.

The abandoned strands as they appear to-day.—The traveler by steamer upon the upper lakes, as he comes within view of each rocky headland, may note how the profile against the horizon is notched by a series of steps or terraces (Fig. 368), and if he has followed the discussion in previous chapters, he will suspect that these terraces mark the now abandoned shore lines which have come to their present position through a series of uplifts of the ground accompanied by earthquake shocks. As his steamer skirts the shore he may chance to note a cave within the rock cliff which represents the now elevated sea-arch of an ancient shore.

Disembarking from the steamer and traveling inland at any point where the shores are high, the traveler is certain to come upon still more convincing proofs of the ancient strands; perhaps in a storm beach of the unmistakable “shingle”, half buried though it may be under dunes of newly drifted sand, or possibly at higher levels the highway has been cut through a shingle barrier as fresh and unmistakable as though formed upon the present shore. Sometimes it is the rock cliff and terrace, at other times barrier ridges of shingle, or, again, it is the sloping cliff and terrace cut in the drift deposits; but of whatever sort, if studied with proper regard to the topography of the district, the evidence is clear and unmistakable.

The records of uplift about Mackinac Island.—Nowhere are the records of the recent uplift of the lake region more easily read than about Mackinac Island in the straits connecting Lake Michigan with Lake Huron. Approaching the island by steamer from St. Ignace, its profile upon the horizon is worthy of remark (Fig. 369). From a central crest broken by minor irregularities and bounded on all sides by a cliff, the island profile slopes gently away to a still lower cliff, below which is another terrace.

When we have reached the island and have climbed to the summit, we there find the surface which is characteristic of erosion by running water, whereas at lower levels are found the forms carved or molded by the action of waves. This central “island”, superimposed upon the larger island, is all that rose above Lake Algonquin, the earliest of the glacial lakes in this northern district; and as we look out from the observatory upon the summit, it is easy to call up a picture of the country when the lake stood at the base of this highest cliff. To the northward one sees the “Sugar Loaf” rise out of a sea of foliage, as it formerly did from the waters of Lake Algonquin (Fig. 370). It is a huge stack near the former island shore. If we turn now to the southward and direct our gaze toward the Fort, we encounter a veritable succession of beach ridges formed of shingle and ranged like a series of waves within the cleared space of the “Short Target Range” (Fig. 371). These ridges mark each a stage within a series of successive uplifts which have brought the island to its present height.

If now we descend from our position and visit the “battlefield”, we find there a great ridge of level crest, behind which the British force was stationed in its defense of the island in 1812. Near by in the woods is Pulpit Rock, a strikingly perfect stack of the Nipissing Lake. Across the straits at St. Ignace is an even finer example of the notched stack (Fig. 372). Other less prominent beaches, but all later than the Nipissing Lakes, intervene between this level and the present shore to mark the stages in the continued uplift of the land.

The present inclinations of the uplifted strands.—It is not enough that we should have recognized the marks of former shores now at considerable elevations above the existing lakes; if we are to know the nature of the uplift, we must prepare accurate maps based upon measurements by precise leveling at many localities. Such methods are, however, of comparatively recent application in this field; and, as in the investigation of so many other problems, the earlier observations were largely of the nature of reconnaissances with the elevation of beaches estimated by comparatively crude methods only. The evolution of ideas concerning the uptilt has, therefore, been a gradual one.

It was early observed that the beaches corresponding to a given lake stage were higher to the northward and northeastward, and the natural conclusion from this was that the earth’s crust had here been canted like a trap door (Fig. 373, A). As we are to see, this but half-correct assumption has led to a striking prophecy relating to future changes within the lake region which we now know to be without warrant in the facts. Later it was learned that the uptilt of the lake beaches is much accelerated to the northward (Fig. 373, B), and that new beaches make their appearance from beneath others as we proceed in this direction—there is a “feathering out” of beaches to the northward (Fig. 373, C).

The hinge lines of uptilt.—Still later in the study of the region, it was learned that the axis or fulcrum about which the region has been uptilted, instead of lying to the southward of the lake district, as had been assumed by Gilbert, lay within the region and about halfway up the basin of Lake Michigan (Fig. 373, D, and Fig. 374). Similarly, in the uptilt which followed the ice retreat in northern Europe a definite hinge line of movement has been discovered.

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).

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.

If our conclusions have been correct, the several lake basins should now be behaving in different ways as regards the changes upon their shores. If it is true that the lines of greatest uptilt run north-northeasterly, there should be, speaking broadly, a “spilling over” of waters upon the south-southwesterly shores and a laying bare of the north-northeasterly shore terraces of the basins. This should, however, be true only of basins whose outlets are to the northeastward of the existing main hinge line of uptilt. Lake Huron, having its outlet at the southern margin of its basin, should not have its waters encroaching upon the southern shore, for the simple reason that any continued uptilt of the basin can only have the effect of pouring more water through the outlet. Lake Michigan and Saginaw Bay, which are arms of the Huron basin, ought, however, to become flooded upon their southern shores, were it not that the hinge line of uptilt to-day lies to the northward of the outlet at Port Huron, and, further, that the two connecting channels still have their beds lower than the sill of the outlet channel. Now the evidence goes to show that no encroachment of waters is occurring upon the Chicago shore of Lake Michigan, and although the shores of Saginaw Bay are so excessively flat as to reveal slight changes of level by large migrations of the strand, yet the ancient meander posts fixed by the early surveys are still found near the water’s edge.

Drowning of southwestern shores of Lakes Superior and Erie.—Within the basins occupied by Lakes Superior and Erie, a wholly different condition is found. In each case the outlet is found to the northeastward (Fig. 374, p. 345), and the northwesterly trend of the isobases from these outlets is responsible for a continued elevation from uptilt of the outlets with reference to the western and southern shores. In consequence, the waters are encroaching upon these shores, and rivers which there enter the lake are drowned at their mouths, with the formation of estuaries. Upon Lake Superior these changes are very marked near Duluth and particularly in the St. Louis River, within which, since the early treaty with the Indians, certain rapids have disappeared and submerged trunks of trees are now found in the channel of the river. As far east as Ontonagon essentially the same conditions are found.

Upon the shores within the Porcupine Mountain district, the waters are clearly rising. Here old cedar trees may be seen, in some cases dead but still upright and standing in from six to eight inches of water a number of feet out from the present shore, while others near the shore, but upon the land and still living, are washed by the waves, and losing their lower bark in consequence. An old road along the shore has had to be abandoned because of the encroaching water.

Upon the opposite or northeastern shore of the lake, on the other hand, the land is everywhere rising out of the water, and the waves are now building storm beaches well out upon the wave-cut terrace. Here the streams, instead of forming estuaries by drowning, drop down in rapids to the level of the lake.

At the southwestern margin of Lake Erie there is everywhere evidence of a rapid encroachment by the water. In the caves of South Bass Island stalactites, which must obviously have formed above the lake level, are now permanently submerged. It is, however, about Sandusky Bay upon the southwest shore that the most striking observations have been made. Moseley has collected historical records of the killing of forest trees through a submergence which was the result of an advance of the water upon the shores. It seems to be proven from his studies that the water is now rising in Sandusky Bay at a rate of about 2.14 feet per century. In Fig. 376 there is a comparison of the shores of the inner bay separated by an interval of about ninety years.

Reading References for Chapter XXIV

Uptilt in basin of Lake Agassiz:—

Warren Upham. The Glacial Lake Agassiz, Mon. 25, U. S. Geol. Surv., pp. 474-522.

Uptilt in Laurentian Basin:—

G. K. Gilbert. Recent Earth Movement in the Great Lakes Region, 18th Ann. Rept. U. S. Geol. Surv., 1898, Pt. ii, pp. 595-647.

J. W. Spencer. Deformation of the Algonquin Beach, etc., Am. Jour. Sci. (3), vol. 41, 1891, pp. 14-16.

F. B. Taylor. The Highest Old Shore Line of Mackinac Island, Am. Jour. Sci. (3), vol. 43, 1892, pp. 210-218.

A. C. Lawson. Sketch of the Coastal Topography of the North Side of Lake Superior, with reference to the abandoned strands, etc., 20th Ann. Rept. Geol. and Nat. Hist. Surv. Minn., 1893, pp. 181-289, pls. 7-12.

J. B. Woodworth. Ancient Water Levels of the Champlain and Hudson Valleys, Bull. 84, N.Y. State Mus., 1905, pp. 265, pls. 28.

E. L. Moseley. Formation of Sandusky Bay and Cedar Point, Proc. Ohio State Acad. Sci., vol. 4, 1905, Pt. v, pp. 179-238.

F. E. Wright. Rept. Geol. Surv. Mich. for 1903, 1905, p. 37.

J. W. Goldthwait. The Abandoned Shore Lines of Eastern Wisconsin, Bull. 17, Wis. Geol. and Nat. Hist. Surv., 1907, pp. 134, pls. 37; A Reconstruction of Water Planes of the Extinct Glacial Lakes in the Lake Michigan Basin, Jour. Geol., vol. 16, 1908, pp. 459-476; Isobases of the Algonquin and Iroquois Beaches and their Significance, Bull. Geol. Soc. Am., vol. 21, 1910, pp. 227-248, pl. 5; An Instrumental Survey of the Shore Lines of the Extinct Lakes Algonquin and Nipissing in Southwestern Ontario, Mem. 10, Dept. of Mines, Canada, 1910, pp. 57, pls. 4.

William H. Hobbs. The Late Glacial and Post-glacial Uplift of the Michigan Basin, Pub. 5, Mich. Geol. and Biol. Surv., 1911, pp. 68, pls. 2.

Lawrence Martin. [Post-glacial Modifications in and Around the Great Lakes], Mon. 52, U. S. Geol. Surv., 1911, pp. 455-459.

Uptilt in northern Europe:—

G. de Geer. Quaternary Changes of Level in Scandinavia, Bull. Geol. Soc. Am., vol. 3, 1892, pp. 65-68, pl. 2.

H. Munthe. Studies in the Late Quaternary History of Southern Sweden, paper No. 25, Livret Guide, Cong. GÉol. Intern., 1910, pp. 96, many plates and maps.