NIAGARA FALLS A CLOCK OF RECENT GEOLOGICAL TIME
=Features in and about the Niagara gorge.=—A striking example of those
permanent alterations of drainage which have resulted from the presence
of the late continental glacier in North America is to be found in the
Niagara gorge between Lakes Erie and Ontario. With the aid of borings
many of the now buried channels of the region have been followed out,
and in a later paragraph we shall refer to some of the stronger lines
of the earlier drainage system. Before undertaking the study of Niagara
history, it is essential that one become somewhat familiar with the
present topography in and about the Niagara gorge.
Below the present cataract the river flows through a deep gorge for
about seven miles before issuing at the Lewiston Escarpment (Fig. 381,
p. 355). This gorge has been cut in beds of rock sediments which dip
at a gentle angle southward toward Lake Erie. The capping of the rock
series is a compact and relatively resistant limestone which is known
as the Niagara limestone, beneath which there are alternating beds
of shale with thinner limestone and sandstone. The plain formed by
the upper surface of the limestone capping terminates in the Lewiston
Escarpment, which is transverse to the direction of the gorge and seven
miles distant below the Falls. The depth of the gorge varies markedly,
the above-water portion being represented at the upper end by the
height of the cataract, one hundred and sixty-five feet, while at its
lower end near Lewiston it is twice that amount. Halfway down the gorge
a sharp turn is made at an angle of more than ninety degrees, and the
upstream arm is extended to form a basin which contains the famous
whirlpool. This visible extension of the upper gorge is continued in a
buried channel, the St. Davids Gorge, which extends to the escarpment,
broadening as it does so in the form of a trumpet. The materials which
fill this earlier channel are notably coarse glacial deposits (Fig.
389).
[Illustration:
FIG. 377.—Ideal cross section of the Niagara gorge to show the
marginal terrace.]
Directly above the whirlpool the Niagara gorge is first contracted, but
almost immediately swells out into the form of a sausage, which under
the name of the Eddy Basin extends to the constricted channel occupied
by the Whirlpool Rapids. This Gorge of the Whirlpool Rapids extends to
and a little above the railroad bridges, where it again suddenly widens
and deepens and with surprisingly uniform cross section now continues
as far as the cataract. This uppermost section is known as the Upper
Great Gorge. About a mile below the whirlpool is that remarkable
projection into the gorge from the Canadian wall which is known as
Wintergreen Flats, below which and nearer the river are Fosters Flats.
Almost throughout its entire length the Niagara gorge is bordered on
either side by a narrow and gently incurving terrace eroded below the
general level of the plain and meeting the gorge in a sharp angle (Fig.
377).
The features immediately about the cataract show that the Falls are
to-day in a condition which, so far as we know, has occurred but once
before in their entire history—the waters of the river are divided
unequally by an island, and for this reason, as we shall see, the
cataract enters over the _side wall_ of the gorge instead of at its
_end_ (Fig. 381), although the turning of the channel from this cause
is combined with a bend of the river.
[Illustration:
FIG. 378.—View of the bed of the Niagara River above the cataract,
where water has been drained off in installing a power plant. Some
separated blocks of limestone are still in place (after J. W. Spencer).]
=The drilling of the gorge.=—There appear to be two important
processes which are responsible for the recession of the Falls, the
rate of which is determined largely by the resistance of the limestone
capping and the tenacity of the looser shale beneath it. One of the
eroding processes operates from below and undermines the cap until
the unsupported cornice falls in blocks to the bottom of the gorge;
the other makes its attack directly from above, selecting for the
purpose the lines of jointing of the rock which it widens by solution
and corrasion until the included blocks are in so far separated that
they are torn out and go over the brink of the Falls (Fig. 378). This
process of overhead attack in the powerful currents just above a
cataract is even better illustrated by the Falls of St. Anthony near
Minneapolis, which have had a similar history of recession to that of
the Niagara Falls (Fig. 379).
[Illustration: FIG. 379.—Falls of St. Anthony, looking westward from
Hennepin Island in 1851 (after N. H. Winchell, daguerreotype by Hessler
of Chicago).]
The blocks of the capping limestone at Niagara Falls are to some
extent fixed in size by the joint planes present in them, and as they
fall to the bottom of the gorge, they promote or retard the further
recession of the Falls according as they can or cannot be moved about
by the churning currents beneath the cataract. Of the retarding effect
there is an illustration in the accumulation of the blocks below the
American and the intermediate Luna Falls (plate 23 A), which the weaker
currents upon the American side find too heavy to handle.
[Illustration:
FIG. 380.—Ideal section to show the nature of the drilling process
beneath the cataract.]
[Illustration:
FIG. 381.—Plan and section of the Niagara gorge, showing how in each
section the depth is proportional to the width, except in the lowest
section where subsequent river action of the normal type has modified
the bed of the channel (plan after Taylor and section after Gilbert).]
The Canadian Fall, with its much greater power, is an example of the
promotion of recession through the churning about of the blocks at the
base of the cataract. We have here to do with a churn drill which bores
its way into the bottom of the gorge with increasing radius of rotary
motion with each increase in volume of the falling water. Under this
rotary churning the soft shales are torn out near the bottom and in
succession the harder layers above until the capping is reached (Fig.
380). The conditions appear now to be such that the effective work is
largely concentrated, as it usually has been, near the middle of the
channel; and so the gorge recedes with a margin of the earlier river
bed remaining as a terrace on either side and extending to the former
river bank (Fig. 377).
As must have been noted, one peculiarity of the operation of the churn
drill beneath the cataract is that the depth of the gorge will bear
a direct proportion to its width, and if the volume of water has
varied during the process of recession, these changes in volume will be
registered in the width and also in the depth of that section of the
gorge which was drilled at the time—the cross section of the gorge at
any place is proportional to the volume of the water falling in the
cataract which produced it, modified, however, by the competency to
handle the joint blocks of definite size (Fig. 381).
[Illustration:
FIG. 382.—Comparison of a sketch of the Canadian Fall made with the
aid of a camera lucida in 1827 with a photograph taken from the same
view point in 1895 (after Gilbert).]
=The present rate of recession.=—There are various sketches, more or
less accurate, made in the early part of the nineteenth century, and
from the later period there are daguerreotypes, photographs, and maps,
which refer especially to the Canadian Fall; and which, taken together,
render possible a comparison of the earlier with the later brinks. By
comparing the earliest with the recent, views it is seen at a glance
that the Falls are receding, and at a quite appreciable rate (Fig.
382). A careful comparison of the maps made in 1842, 1875, 1886, 1890,
and 1905 of the brink of the Canadian Fall (Fig. 383) indicates that
for the period covered the rate of recession has been about five feet
per year, and similar studies made of the American Fall show that it
has been receding at the rate of only three inches per year, or one
twentieth the rate of the recession of the Canadian Fall.
[Illustration:
FIG. 383.—Map to show the recession of the brink of the Canadian Fall,
based upon maps of different dates (after Gilbert).]
=Future extinction of the American Fall.=—It is because of this many
times more rapid recession of the Canadian Fall that the Niagara
cataract, instead of lying athwart the gorge, enters it from its side.
The Canadian Fall is thus in reality swinging about the American, and
the time can already be roughly estimated when this more effective
drilling tool will have brought about a capture, so to speak, of the
American Fall through the cutting off of its water supply. It will then
be drained and left literally “high and dry”, an enduring witness to
the geological effect of an island in making an unequal division of the
waters for the work of two cataracts.
As already pointed out, the inefficiency of the American Fall as an
eroding agent is amply attested by the wall of blocks already appearing
above the water below it. The tourist who a thousand years hence pays
a visit to the Niagara cataract, provided the water flow is allowed to
remain as it has been, will find above this rampart of blocks a bare
cliff in part undermined, and surmounted by a nearly flat table surface
which is cut off from the existing cataract by a higher section of the
gorge (Fig. 384). It is quite likely that this table will furnish the
most satisfactory viewpoint of the future cataract of that date.
[Illustration:
FIG. 384.—Comparison of the present with the future falls.]
=The captured Canadian Fall at Wintergreen Flats.=—What we have
predicted for the future of the present American Fall will be the
better understood from the study of a monument to earlier capture
made long before the upper section of the gorge had been cut or the
whirlpool had come into existence. The tables were then turned, for it
was a fall upon the Canadian side of the gorge that was captured by
one upon the American. The locality is known as Wintergreen Flats, or
sometimes as Fosters Flats; though the first name properly applies to
a higher surface near the brink of the gorge, and Fosters Flats to a
lower plain near the level of the river (see Fig. 381, p. 355). The
peculiar topographic features at this locality are well brought out in
Gilbert’s bird’s-eye view of the locality (Fig. 385); in fact, in some
respects better than they appear to the tourist upon the ground, for
the reason that the abandoned channel and the Flats on the site of the
since undermined island are both heavily forested and so not easy to
include in a single view. For one who has studied the existing cataract
this early monument is full of meaning. Standing, as one may, upon the
very brink of the former cataract, it is easy to call up in imagination
the grandeur of the earlier surroundings and to hear the thunder of the
falling water. A particularly vivid touch is added when, in digging
over the sand about the great blocks of fallen limestone underneath
the brink, one comes upon the shells of an animal still living in the
Niagara River, though only in the continual spray beneath the cataract.
[Illustration:
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).
[Illustration:
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.
[Illustration:
FIG. 387.—Map to show the cuestas which have played so important
a part in fixing the boundaries of the Lake basins, and also the
principal preglacial rivers by which they have been trenched (based
upon a map by Grabau).]
When the elevation of the land in the vicinity of this shore had
caused a recession of the waters, there was formed a coastal plain
on the borders of the oldland like that which is now found upon our
Atlantic border between the Appalachians and the sea (Fig. 272, p.
246). The rivers from the oldland cut their way in narrow trenches
across the newland, and because of the harder limestone formations,
their tributaries gradually became diverted from their earlier courses
until they entered the trunk stream nearly at right angles and produced
the type of drainage network which is called “trellis drainage.” It
is characteristic of this drainage that few tributaries of the second
order will flow up the natural slope of the beds, but on the contrary
these natural slopes are followed in the softer rock nearly at right
angles again to the tributaries of the first order of magnitude (Fig.
387). Thus are produced a series of more or less parallel escarpments
formed in the harder rock and having at their base a lowland which
rises gradually in the direction of the oldland until a new escarpment
is reached in the next lower of the hard formations. Such flat-topped
uplands in series with intermediate lowlands and separated by sharp
escarpments are known as _cuestas_ (see p. 246), and the Lewiston
Escarpment limits that formed in Niagara limestone (Figs. 387 and 388).
[Illustration:
FIG. 388.—Bird’s-eye view of the cuestas south of Lakes Ontario and
Erie (after Gilbert).]
=Episodes of Niagara’s history and their correlation with those of
the Glacial Lakes.=—Of the early episodes of Niagara’s history, our
knowledge is not as perfect as we could desire, but the later events
are fully and trustworthily recorded. The birth of the Falls is to
be dated at the time when the ice front had here first retired into
what is now Canadian territory, thus for the first time allowing the
waters from the Erie basin to discharge over the Lewiston Escarpment
into the basin of the newly formed Lake Iroquois (Fig. 364, p. 334).
Since the level of Lake Iroquois was far above that of the present Lake
Ontario, the new-born cataract was not the equivalent in height of
the escarpment to-day. The Iroquois waters then bathed all the lower
portion of the escarpment, so that the foot of the Fall was upon the
borders of the Lake.
In order to interpret the history of the Niagara gorge, we must
remember that the effective drilling of this gorge was in each stage
dependent mainly upon the volume of water discharged from Lake Erie,
a large discharge being recorded by a channel drilled both wide
and deep, while that produced by the discharge of a smaller volume
was correspondingly narrow and shallow. To-day the gorges of large
cross section have, moreover, a relatively placid surface, whereas
through the constricted sections the water of the river is unable to
pass without first raising its level at the upper end and under the
head thus produced rushing through under an increased velocity. The
best illustration of such a constricted section is the Gorge of the
Whirlpool Rapids.
[Illustration:
FIG. 389.—Sketch map of the greater portion of the Niagara Gorge to
show the changes in cross section in their relations to Niagara history
(based upon a map by Taylor).]
Our reading of the history should begin at the site of the present
cataract, since the records of later events are so much the more
complete and legible, and it should ever be our plan to proceed from
the clearly written pages to those half effaced and illegible.
As we have learned, the most abrupt change in the cross section of the
gorge is found a little above the railroad bridges, where the Upper
Great Gorge is joined to the Gorge of the Whirlpool Rapids (Fig. 389).
In view of the remarkably uniform cross section of the Upper Great
Gorge, there is no reason to doubt that it has been drilled throughout
under essentially the same volume of water, and that its lower limit
marks the position of the former cataract when the waters from the
upper lakes were transferred from the “North Bay Outlet” into the
present or “Port Huron Outlet” and Lake Erie. As the upper limit of the
Gorge of the Whirlpool Rapids thus corresponds to the closing of the
“North Bay Outlet” and the extinction of the Nipissing Great Lakes, so
its lower limit doubtless corresponds to the opening of that outlet and
the termination of the preceding Algonquin stage; for in the stage of
the Nipissing lakes the water of the upper lakes, as we have learned,
reached the ocean through the northern outlet.
Mr. Frank Taylor, who has given much study to the problem of Niagaran
history, believes that the Middle Great Gorge, comprising the Eddy
Basin and the Cove section, represents the gorge drilling which
occurred during the later stage of Lake Algonquin after the “Trent
Outlet” had been closed and the waters of the upper lakes had been
turned into the Erie Basin.
Summarizing, then, the episodes of the lake and the gorge history are
to be correlated as follows:—
GLACIAL LAKE NIAGARA GORGE
Early Lakes Iroquois and Algonquin. Drilling of the gorge from the
Lewiston Escarpment to the Cove
section above the Wintergreen
Flats.
Later Lakes Iroquois and Algonquin Drilling of Middle Great Gorge.
with upper lakes discharging
into Erie basin.
Nipissing Great Lakes with the Drilling of the narrow Gorge of
upper lake waters diverted from the Whirlpool Rapids.
Lake Erie.
Recent St. Lawrence drainage Drilling of Upper Great Gorge to
since the waters of the upper lakes the present cataract.
were discharged into Lake Erie
through occupation of the Port
Huron Outlet.
=Time measures of the Niagara clock.=—In primitive civilizations time
has sometimes been measured by the lapse necessary to accomplish a
certain task, such, for example, as walking the distance between two
points; and the natural clock of Niagara has been of this type. But men
possess differences in strength and speed, and the same man is at some
times more vigorous than at others, and so does not work at a uniform
rate. The cataract of Niagara, charged with the pent-up energy of the
waters of all the Great Lakes, can rush its work as it is clearly
unable to do at times when the greater part of this energy has been
diverted. Units of distance measured along the gorge are therefore too
unreliable for our use, with the unique exception of the stretch from
the railroad bridges to the site of the present cataract, within which
stretch the gorge cross sections are so nearly uniform as to indicate
an approximation to continued application of uniform energy. This
energy we may actually measure in the existing cataract, and so fix
upon a unit of time that can be translated into years.
In order to secure the normal rate of recession of this Upper Great
Gorge, we should add to the volume of water in the Canadian Fall that
now passing over the American; and for the reason that the blocks
which fall from the cataract cornice and are the tools of the drilling
instrument approximate to a definite size fixed by their joint planes,
the effect of this added energy it is not easy to estimate. We may be
sure, however, that the drilling action would be somewhat increased by
the junction of the two Falls, and thus are assured that the average
rate of recession within the Upper Great Gorge has been somewhat in
excess of the five feet per year determined by Gilbert for the present
Canadian Fall. The Upper Great Gorge is about two miles in length,
and its beginning may thus be dated near the dawning of the Christian
Era. The Whirlpool Gorge was cut when the ice vacated the North Bay
Outlet in Canada, and still lay as a broad mantle over all northeastern
Canada. For the earlier gorge and lake stages, the time estimates are
hardly more than guesses, and we need not now concern ourselves with
them.
=The horologe of late glacial time in Scandinavia.=—A glacial
timepiece of somewhat different construction and of greater refinement
has been made use of in Scandinavia to derive the “geochronology
of the last 12,000 years.” Instead of retreating over the land and
impounding the drainage as it did so, the latest continental glacier
of Scandinavia ended below sea level, and as it retired, its great
subglacial river laid down a giant esker known as the Stockholm Os,
which was bordered by a delta and fringed on either side by water-laid
moraines of the block type. These recessional moraines are upon the
average less than 1000 feet apart, and are believed to have each been
formed in a single season. The delta deposits which surround the esker
are of thin-banded clay, and as an additional uppermost band is found
outside every moraine, these bands are also believed to represent each
the delta deposit of a single year. In studies extending over many
years, Baron de Geer, with the aid of a large body of student helpers,
has succeeded in completing a count of moraines and clay layers, and so
in determining the time to be 12,000 years since the ice front of the
latest continental glacier lay across southern Sweden. The fertility
of conception and the thoroughness of execution of this epoch-making
investigation recommend its conclusion to the scientific reader.
READING REFERENCES FOR CHAPTER XXV
G. K. GILBERT. Niagara Falls and their History, Nat. Geogr. Soc. Mon.,
vol. 1, No. 7, 1895, pp. 203-236.
F. B. TAYLOR. Origin of the Gorge of the Whirlpool Rapids at Niagara,
Bull. Geol. Soc. Am., vol. 9, 1898, pp. 59-84.
A. W. GRABAU. Guide to the Geology and Paleontology of Niagara Falls
and Vicinity, Bull. N. Y. State Mus., vol. 9, No. 45, 1901, pp. 1-85,
pls. 1-11.
J. W. SPENCER. The Falls of Niagara, etc. Dept. of Mines, Geol. Surv.
Branch, Canada, 1907, pp. 490, pls. 43.
G. K. GILBERT. Rate of Recession of Niagara Falls, etc. Bull. 306, U.
S. Geol. Surv., 1907, pp. 31, pls. 11.
G. DE GEER. Quaternary Sea Bottoms of Western Sweden. Paper 23, Livret
Guide Cong. Géol. Intern., 1910, pp. 57, pls. 3.