Fig. 372.—Notched stack of the Nipissing Great Lakes at St. Ignace (after a photograph by Taylor).
Fig. 373.—Series of diagrams to illustrate the evolution of ideas concerning the uplift of the lake region since the ice age. A, simple northerly up-canting (Gilbert): B, northerly acceleration of the up-canting (Spencer and Upham); C, northerly “feathering out” of beaches (Spencer and Upham); D, hinge, line of up-canting found within the lake region (Leverett); E, multiple and northwardly migrating hinge lines of up-canting (Hobbs).
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.
Fig. 374.—Map of the Great Lakes region to show isobases and hinge lines of uptilt. a, isobase of the Chicago outlet; b, main hinge line of the Lake Whittlesey beach (Leverett); b1, hinge line of the Lake Warren beach (Taylor); c, isobase of the Port Huron outlet; d, main hinge line of highest Algonquin beach (Goldthwait); e, f, g, h, additional hinge lines of Algonquin beaches in Door County peninsula (Hobbs); l, isobase of the Lake Superior outlet for the Algonquin beaches (Leverett): m, isobase of the same outlet for the Nipissing beaches (Leverett).
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.