Iowa County democrat. [volume] (Mineral Point, Wis.) 1877-1938, 1888-07-20
THE GREAT LAKES.

Geological History of Their Formation and
Change in Their Level
The Forum for June contains an article
by c h. Gilbert on Changes Of Level of
the Great Lakes" Promising thatIake
are destroyed by rivers, by the double
process of filling them up with sediment
and OF deepening the channel of escape,
and that, on the other hand, lakes are
formed chisfy by the upheaval of the
earth's surface and by its erosion by the
movements of glaciers, the author de
votes the greater part of his article to the
geologic history of the formation of
the great lakes Their beds, with the
possible exception of Erie, were secured
out of the solid rock by the great glaciers
OF the ice age, but accumulation of de-
brlS In many cases increase their depth
and influence the direction of their out
flow. The removal of this debris in the
case of the Erie, for instance would re
duce 1t greatly perhaps eRtirely drain it
and would make it flow into Ontario and
Hamilton. The increase OF heat caused the
edge of the great glacier to retract from
the southern border al Fort Wayne, where
the water generated upon it flowed into
the Wabash, and separated Ecie and
Ontario into two lakes. But as long as
the ice remained in the ST. lawrence val
ley the waters found their escape into
the Mohawk at Rome. One of the
beaches formed by the action of the
waves has its crest marked by the ridge
road from Lewiston to Sodus. At this
time Lake Erie was only one-third as long
as now, and Its surface was only QUE fifth
as great, Toledo, Cleveland, and the Bass
islands being far inland. When Lake
Ontario forced e. new outlet through the
st. Lawrence valley its lever was drawn
down five hundred feet aud its area re-
duced so per cent, becoming 80 per cent.
smaller than at present. The rising of
the depressed land to the northwest gave
the lakes their present dimensions, and
the oscillation then received q check. "a
further fitting of the land to the extent
of three inches in each mile would send a
great river from Chicago to the Missis-
sipi, reverse tho current in the Detroit,
stop Niagara falls, and rob the upper St.
Lawrence of seven heights of its water"
Whether, contrary 20 what appears to be
law against abs JiUte stability, the OscIll-
alton has entirely ceased or not, it Is
now so slow as TO prevent the detection
of any movement, the present level of
the water having remained nearly con
slant for many centuries.
Other changes are, however, in progress
from the actions of the rivers. The S.
Clair is feebiy securing its channel and
building its delta; the Niagara is eating lis
way back to Lake Erie; the st. Marie
Detroit, and s.. Lawrence are deepening
their channels. "All these processes ale
too slow to affect our hopes and fears con.
corning che immediate future, and for
our posterity IL the year 10000 we have
no solicitude. The men who shall watch
the draining of Lake Erie-or who
perchance, shall find il worth their while
to prevent twill as far surpass US il
powers and resources as we sGrpass the
men who watched the lakes creation
For all practical purposes our inland seas
are permanent and their basins stable
The only notifications that affect oul
economy are those wrought by the waves
upon their coasts" The changes in the
water levels thai are noticed are due tC
the variations il the rainfall upon the
lakes themselves and upon the land drain
ed by the streams that pour into them
The amount Of rainfall varies from year
to year and from one season to another
in the same year, and the level of the
water ocillates around an average position
that remains fixed. Thus during the
period from 1860 to 1887, the mean an
nual range of Lake Erie was Is feet, and
the extreme range for the same period
80 feet. In 1888 Lake Erie rose Is inches
above its ordinary high stage, and in 1915
1t fell 3 3 feet below Its usual plane. "Ihe
present low water Is the sequel of last
summers drought" the lake region hav
ing received only about % inches of rain
f2l! instead of It's usual 80 inches. This
should cau5e fall of the lakes of about
Il inches, half from direct loss and halt
from the loss of the streams that EN- Dty
into them. But the average fall of all the
lakes was about inches, owing probably
TO the GOODNESS of the autumn, which re.
tarded evaporation. In Lake Superior
the fall was inches below the average
low stage, the lowest point reached III
seventeen years except Oh two occasions
In Erie it was two inches below the aver
age.
TFese variations relate to the entire
surfaces of the lake. A part of one may
be depressed and another part raised by
gale, especially in the case of Lake Erie
on account of its shallowness. A gale
has been known to raise the level of one
of its ends seven or eight feet and to de-
press the other to an equal amount. Diff
ferences in atmospheric pressure also ef-
feet different parts of the same lake. The
rapid change of air pressures, as in the
case of tornadoes, sometimes causes
rhythmic undulations as high as the larg
est created by the wind. There are
also tides which are as regular as on the
ocean, but the highest spring tide rises
only about three inches, while the aver
age height is probably not more than an
inch.