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.
[Illustration:
FIG. 368.—The notched rock headland of Boyer Bluff between Green Bay
and Lake Michigan (after Goldthwait).]
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.
[Illustration:
FIG. 369.—View of Mackinac Island from the direction of St. Ignace.
The irregular central portion is the only part of the island that was
not submerged in Lake Algonquin. The terrace at its base is the old
shore line of Lake Algonquin, and the lower terrace the strand of Lake
Nipissing (after a photograph by Taylor).]
[Illustration:
FIG. 370.—The “Sugar Loaf”, a stack near the shore of Lake Algonquin,
as it is seen from the observatory upon Mackinac Island (after a
photograph by Taylor).]
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.
[Illustration:
FIG. 371.—View from the observatory upon Mackinac Island across the
“Short Target Range” toward the Fort. Beach ridges appear in succession
within the cleared space (after a photograph by Rossiter).]
[Illustration: FIG. 372.—Notched stack of the Nipissing Great Lakes at
St. Ignace (after a photograph by Taylor).]
[Illustration:
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.
[Illustration:
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); _b^1_, 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.
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).
[Illustration:
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.
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.
[Illustration:
FIG. 376.—Portion of the Inner Sandusky Bay, to afford a comparison of
the shore line of 1820 with that of to-day (after Moseley).]
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.