The Laurentian Highland
The Laurentian highland presents a monotonous waste of rough hills,
irregular valleys, picturesque lakes, and crooked rivers. Most of it is
thinly clothed with pine trees and bushes such as the blueberry and
huckleberry. Yet everywhere the ancient rock crops out. No one can travel
there without becoming tiresomely familiar with fine-grained, shattered
schists, coarse granites, and their curiously banded relatives, the
gneisses. This rocky highland stretches from a little north of the St.
Lawrence River to Hudson Bay, around which it laps in the form of a V, and
so is known as the Archaean V or shield.
Everywhere this oldest part of the Western Hemisphere presents
unmistakable signs of great age. The schists by their fine crumpling and
scaly flakes of mineral show that they were formed deep in the bowels of
the earth, for only there could they be subjected to the enormous pressure
needed to transform their minerals into sheets as thin as paper.
schist (shist)n. Any of various medium-grained to
coarse-grained metamorphic rocks composed of laminated, often
flaky parallel layers of chiefly micaceous minerals. |
The coarse granites and gneisses proclaim still more clearly that
they must have originated far down in the depths of the earth; their
huge crystals of mica, quartz, hornblende, feldspar, and other
minerals could never have been formed except under a blanket of rock
which almost prevented the original magmas from cooling. The
thousands or tens of thousands of feet of rock which once overlay
the schists and still more the granites and gneisses must have been
slowly removed by erosion, for there was no other way to get rid of
them. This process must have taken tens of millions of years, and
yet the whole work must have been practically completed a hundred or
perhaps several hundred million years ago. We know this because the
selfsame ancient eroded surface which is exposed in the Laurentian
highland is found dipping down under the oldest known fossiliferous
rocks. Traces of that primitive land surface are found over a large
part of the American continent. Elsewhere they are usually buried
under later strata laid down when the continent sank in part below
sea-level. Only in Laurentia has the land remained steadily above
the reach of the ocean throughout the millions of years.
Today this old, old land might be as rich as many others if climate
had been kind to it. Its soil, to be sure, would in many parts be
sandy because of the large amount of quartz in the rocks. That would
be a small handicap, however, provided the soil were scores of feet
deep like the red soil of the corresponding highland in the Guiana
region of South America. But today the North American Laurentia has
no soil worth mentioning. For some reason not yet understood this
was the part of America where snow accumulated most deeply and where
the largest glaciers were formed during the last great glacial
period. Not once but many times its granite surface was shrouded for
tens of thousands of years in ice a mile or more thick. As the ice
spread outward in almost every direction, it scraped away the soil
and gouged innumerable hollows in the softer parts of the underlying
rock. It left the Laurentian highland a land of rocky ribs rising
between clear lakes that fill the hollows. The lakes are drained by
rapid rivers which wind this way and that in hopeless confusion as
they strive to move seaward over the strangely uneven surface left
by the ice. Such a land is good for the hunter and trapper. It is
also good for the summer pleasure-seeker who would fain grow strong
by paddling a canoe. For the man who would make a permanent home it
is a rough, inscrutable region where one has need of more than most
men's share of courage and persistence. Not only did the climate of
the past cause the ice to scrape away the soil, but the climate of
the present is so cold that even where new soil has accumulated the
farmer can scarcely make a living.
Around the borders of the Laurentian highland the ice accomplished a
work quite different from the devastation of the interior. One of
its chief activities was the scouring of a series of vast hollows
which now hold the world's largest series of lakes. Even the lakes
of Central Africa cannot compare with our own Great Lakes and the
other smaller lakes which belong to the same series. These
additional lakes begin in the far north with Great Bear Lake and
continue through Great Slave Lake, Lake Athabasca, and Lake Winnipeg
to the Lake of the Woods, which drains into Lake Superior. All these
lakes lie on the edge of the great Laurentian shield, where the ice,
crowding down from the highland to the north and east, was
compressed into certain already existent hollows which it widened,
deepened, and left as vast bowls ready to be filled with lakes.
South and southwest of the Laurentian highland the great ice sheet
proved beneficial to man. There, instead of leaving the rock naked,
as in the Laurentian region, it merely smoothed off many of the
irregularities of the surface and covered large areas with the most
fertile soil.
In doing this, to be sure, the ice-cap scoured some hollows and left
a vastly larger number of basins surrounded in whole or in part by
glacial debris. These have given rise to the innumerable lakes,
large and small, whose beauty so enhances the charms of Canada, New
England, New York, Minnesota, and other States. They serve as
reservoirs for the water supply of towns and power plants and as
sources of ice and fish. Though they take land from agriculture,
they probably add to the life of the community as much in other ways
as they detract in this. Moreover glaciation diverted countless
streams from their old courses and made them flow over falls and
rapids from which water-power can easily be developed. That is one
reason why glaciated New England contains over forty per cent of all
the developed water-power in the United States.
Far more important, however, than the glacial lakes and rivers is
the fertile glacial soil. It comes fresh from the original rocks and
has not yet been exhausted by hundreds of thousands of years of
weathering. It also has the advantage of being well mixed, for
generally it is the product of scrapings from many kinds of rocks,
each of which contributes its own particular excellence to the
general composition. Take Wisconsin as an example. (R. H. Whitbeck,
"Economic Aspects of Glaciation in Wisconsin", in "Annals of the
Association of American Geographers," vol. III in (1913), pp.
62-67.)
Most parts of that State have been glaciated, but in the
southwest there lies what is known as the "driftless area" because
it is not covered with the "drift" or glacial debris which is
thickly strewn over the rest of the State. A comparison of otherwise
similar counties lying within and without the driftless area shows
an astonishing contrast. In 1910 the average value of all the farm
land in twenty counties covered with drift amounted to $56.90 per
acre. In six counties partly covered with drift and partly driftless
the value was $59.80 per acre, while in thirteen counties in the
driftless area it was only $33.30 per acre. In spite of the fact
that glaciation causes swamps and lakes, the proportion of land
cultivated in the glaciated areas is larger than in the driftless.
In the glaciated area 61 per cent of the land is improved and in the
driftless area only 43.5 per cent. Moreover, even though the
underlying rock and the original topography be of the same kind in
both cases, the average yield of crops per acre is greater where the
ice has done its work. Where the country rock consists of limestone,
which naturally forms a rich soil, the difference in favor of the
glaciated area amounts to only 1 or 2 per cent. Where the country
rock is sandy, the soil is so much improved by a mixture of
fertilizing limestone or even of clay and other materials that the
average yield of crops per acre in the glaciated areas is a third
larger than in the driftless. Taking everything into consideration
it appears that the ancient glaciation of Wisconsin increases the
present agricultural output by from 20 to 40 per cent. Upwards of
10,000,000 acres of glaciated land have already been developed in
the most populous parts of the State. If the average value of all
products on this area is reckoned at $15 per acre and if the
increased value of agricultural products due to glaciation amounts
to 30 per cent, then the net value of glaciation per year to the
farmers of Wisconsin is $45,000,000. This means about $300 for each
farmer in the glaciated area.
Wisconsin is by no means unique. In Ohio, for instance, there is
also a driftless area. (William H. Hess, "The Influence of
Glaciation in Ohio," in "Bulletin of the Geographical Society of
Philadelphia," vol. XV (1917), pp. 19-42.) It lies in the southeast
along the Ohio River. The difference in the value of the farm land
there and in the glaciated region is extraordinary. In the driftless
area the average value per acre in 1910 was less than $24, while in
the glaciated area it was nearly $64. Year by year the proportion of
the population of the State in the unglaciated area is steadily
decreasing. The difference between the two parts of the State is not
due to the underlying rock structure or to the rainfall except to a
slight degree. Some of the difference is due to the fact that
important cities such as Cleveland and Toledo lie on the fertile
level strip of land along the lake shore, but this strip itself, as
well as the lake, owes much of its character to glaciation. It
appears, therefore, that in Ohio, perhaps even more than in
Wisconsin, man prospers most in the parts where the ice has done its
work.
We have taken Wisconsin and Ohio as examples, but the effect of
glaciation in those States does not differ materially from its
effect all over southern Canada and the northern United States from
New England to Kansas and Minnesota. Each year the people of these
regions are richer by several billion dollars because the ice
scraped its way down from Laurentia and spread out over the borders
of the great plains on the west and of the Appalachian region on the
east.
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