Chronicles of America 

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.

[French schiste, from Latin (lapis) schistos, fissile (stone), a kind of iron ore, from Greek skhistos, split, divisible, from skhizein, to split.]

schistose schis'tose' (shis'tos') or schis'tous (-t?s) adj.

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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