Popular Science Monthly/Volume 31/July 1887/The North American Lakes
|THE NORTH AMERICAN LAKES.|
IN America, as in the Eastern Continent, the North is the land of lakes. A line from the mouth of the St. Lawrence to the western end of Lake Erie, and thence to the mouth of the Mackenzie, lies through and near a succession of lakes unequaled in number and aggregate area by any other like extent on the earth. The great North American depression, extending northward from the Gulf of Mexico, bifurcates at about the fortieth parallel, one branch trending northeastwardly to the Atlantic Ocean, and the other northwestwardly to the Arctic, lying nearly at right angles to each other, and in approximate parallelism to the mountain-ranges and shore-lines of the continent.
The forty-second parallel holds, to the north of it, nearly all the North American lakes, while to the south are numerous lake-basins, some of them rivaling even Superior in extent. These have been drained of their waters by the deepening channels of their effluent streams; or, as in the arid regions of the Southwest, by evaporation.
If we define a lake as, what geologically it actually is, a local digression of the surface, and treat the pressure or absence of water as only one of its accidents, we shall find the South, no less than the North, to be a land of lakes.
Lake-basins may be due
1. To local sinkings of the surface.
2. To excavations, notably by glaciers.
3. To the extinction of volcanoes, their craters filling with water.
4. To the breaking down of cave-roofs by earthquakes or other causes.
To the first and second of these agents are probably due nearly all the existing North American lakes, in some the one and in some the other acting as principal. In all the larger lakes there has evidently been a local sinking of the surface, the glacier having been only auxiliary. The numerous small lakes in Middle and Western New York lying in the direction of the glacier-flow, and having frequent groovings on their adjacent walls, have been credited wholly to the glacier. But, as nearly the whole of this lake-region lies within the Niagara limestone formation, it is not improbable that the falling of cave-roofs may have greatly aided the work of the ice-plow. Many small lakes and ponds, as in Kentucky, Tennessee, and Southern Indiana, are due wholly to the falling down of cave-roofs. In South-Eastern Missouri and Eastern Arkansas are lakes and lakelets where these roofs were shaken down by the earthquake of 1811. Lakes Borgne and Pontchartrain have been captured from the Gulf by the delta of the Mississippi; while numerous small lakes, called bayous, have been formed by changes in the river-bed, the deposit of sediment at both their inlets and outlets having kept them filled with water. Crater-lakes are not infrequent. These basins, but containing no water, abound in New Mexico, Arizona, and Southern California, while many of the beautiful lakes of the Italian Peninsula are but the filled craters of extinct volcanoes.
"Why is the North the land of lakes? In order intelligently to answer this question, let us see what has been going on at the South. Between the Alleghanies and the Blue Ridge is a long, narrow valley extending nearly the whole length of these parallel ranges; and, but for the breaks in whose walls, the whole extent must have been a basin of water. At Harper's Ferry, the Potomac, and near the Natural Bridge, the James River, have broken through the Blue Ridge carrying the waters of the upper half of the valley to the Atlantic. Farther to the south the Kanawha and the Tennessee drain the lower half into the tributaries of the Gulf of Mexico. These last-named rivers have also broken through the Cumberland Mountains, draining another considerable valley between these and the Alleghanies. Could these several outlets be closed, as they probably once were, large lakes would again rapidly form, and the work of abrasion and drainage begin anew. Could the Knobs and the Muldro Hills unite again at the falls of the Ohio, a large shallow lake would form, covering parts of Ohio, Indiana, and some of the fairest portions of Kentucky. Should the bluffs of the Wabash come together at the mouth of the Salimony, another shallow lake would result, whose outlet would probably be the Maumee. Commencing at Richmond, Indiana, itself situated in a small lake-basin, and thence northeastwardly half-way across Ohio, is a succession of shallow depressions once filled with water, and through which still flow the streams whose unceasing work has cut through their margins and emptied them of their contents. In many of these ancient lake-basins the draining is not yet completed, the lowest parts being still marshes or ponds of water. The Mohawk and the Connecticut flow each through a bead-roll of small lake-basins, walled around by solid rock. Through their margins the rivers for untold ages have been deepening their channels until the lake-bottoms have become dry land, and the homes of men. The Great Lakes themselves have, from a like cause, been much reduced from their former dimensions. The evidences are abundant that Lakes Michigan, Erie, and Huron are but the relics of what was once a large body of water, covering all the intervening and much of the adjacent lands. The work of depletion is still going on. Not only is Niagara deepening its channel and sinking thereby the surface of Lake Erie, but by the gradual recession of the falls a much greater work is prophesied. It is only a question of time when Erie will be robbed of its waters, and the other Great Lakes reduced to insignificant parts of their present dimensions. Lake Pepin, now but an expansion of the Mississippi, was once a much larger body; and Peoria, a similar widening of the Illinois, once spread over the adjacent level lands equaling in area that of Lake Champlain.
Doubtless, in many of these lake-outlets, natural fractures and marginal depressions have not only given direction to the effluent streams, but greatly aided in the work of abrasion. The evidence, however, of the former greater extent of these lakes is abundant and apparent. The railroad from Lafayette, Indiana, northward cuts through several low lake-margins, marking the gradual retreat of the waters; and runs within sight of several sand-hills similar to those on the lake-shore near Michigan City. There are evidences that the Illinois was once the outlet of Lake Michigan and the Wabash that of Erie, carrying the waters of these lakes to the Gulf of Mexico.
It is now generally conceded that the whole northern part of the continent, reaching southward in some places to the thirty-eighth parallel, once wore an ice-cap of immense thickness, through which only the mountain-peaks projected. I have already alluded to the work of the ice-plow in the excavation of lake-basins. I am now about to give to the glacier the credit of their preservation when formed.
Although it has been found that the glacier flows like the water of a river, only more slowly, the ice, except when wedged in between two walls, as the Mer de Glace, could not have been confined in narrow channels, and can not, therefore, have grooved out long, tortuous river-beds. The abrasion and drainage were indeed going on, but by a slow process, as compared to the work of the released and active waters.
When the ice-field began to disappear it gradually receded northward, first uncovering that part of the drift-region in which the lakes have been wholly drained. The southern half of the continent has had even larger rivers than now fed by the ice and snow of the gradually disappearing glacier. The length of time during which these rivers were doing their work of excavation, while the North was still wearing its ice-crown, can be only approximately guessed. It was, however, a long time—a time compared with which the historic period dwindles into a few days. During these long ages, the outlets of the more southern lakes were being sunken by the slow disintegration of the rock; and, doubtless, while the ice-king still reigned in the North, these now "sweet vales of Avoca" had become dry land, and possibly the homes of men.
The physical geography of the country is a strong witness to the truth of this hypothesis. To the south of the drift, where the rivers have been flowing ever since the continent was above the sea, the lake-bowls have been drained to their bottoms; in the southern part of the drift they have been only partially drained, while to the north they are still filled with water. This exactly accords with the hypothesis.
It has been argued that the southern half of the continent has been longer from under the sea, and therefore that the rivers have had more time for deepening their channels. Exactly the converse is the truth. The Laurentian Hills are not only the oldest land on the continent, but, so far as now known, the oldest on the planet.
It may be admitted, without affecting this hypothesis, except it be to re-enforce it, that the great weight of the accumulated ice must have sunk the more northern region in some parts below the sea-level, and that in its gradual melting these rose again, preserving their equilibrium.
The gradual recession of the ice northward, and therefore the first uncovering of the southern half of the drift-region, must have taken place. That this recession was slow, and during a long period of years, must be true. That during this long time the rivers must have been deepening their channels and emptying the lake-basins, is so manifest as to need only to be stated. It we should term that portion of the drift-region, south of forty-one and a half degrees, sub-glacial, we shall find the southern part of it wholly drained, the middle and northern part of it only partially so, the lakes and lakelets increasing in number and magnitude on approaching its northern boundary.
But the "dry lakes" of the Pacific slope, what of them? Their margins are still intact. True; but it is because they are dry lakes that their margins have not been cut through and no rivers connect them with the sea. These lake-basins, too, were once filled with water; the rain-supply was not equal to the evaporation, and hence their gradual drying up.
Another cause for the destruction of lakes, too important to be left out of the account, is the continuous deposit of sediment on their bottoms. While their effluents are continuously sinking their surfaces, their affluents are no less industriously raising their bottoms by deposits from the land. The bottom of Lake Superior at its deepest place is about three hundred feet below the sea-level; it can not, therefore, be wholly drained by its outlet. But its supplying streams are constantly sifting sediment on its bottom. The St. Louis River has a large delta, making access to Superior City so difficult as to require annual dredging. It has been estimated that the sediment yearly carried to the Gulf by the Mississippi is sufficient to raise a square mile two hundred and forty-one feet, or equaling a cubic mile in a little less than twenty-two years. Where the Rhône enters Lake Geneva, its water is loaded to its capacity with sediment; where it leaves the lake, it is crystal clear. This solid matter is being continuously deposited, raising the bottom, while the deepening channel is sinking the surface. In Indiana and Ohio are numerous shallow lake-basins, now dry land, their bottoms, level as a floor, with often several feet of rich alluvium, still bearing testimony to the agencies that have despoiled them of their waters. The celebrated Walnut Level of the former State is but an ancient lake-basin, and it is to this deposit of sediment that it owes its far-famed fertility. It is not improbable that by the time the Falls of Niagara shall have broken through the rim of Lake Superior, the sinking surface of the water may reach the rising bottom only a little above the ocean-level.
But why should the lakes begin to increase in size and frequency at about the forty-first parallel? The answer is to be found in the relative amount of snowfall during the glacial epoch. More snow falls at the south end of Hudson Bay than at Boothia Felix; more at Cape Farewell than at Cape Hatherton; more at twenty degrees south of the Arctic Circle than at any parallel north of it. The line of greatest snowfall, like the isothermal line, is irregularly extended, depending greatly on the wind-currents. The water of the southern winds condenses and falls as they reach the colder latitudes. Allowing the line of greatest snowfall to pass through Hudson Bay, it must have been far to the south of it during the ice period. At or near the forty-second parallel the glacier probably attained its greatest thickness. Here it intrenched itself to stay; and for a very long time the winter snows must have compensated for the summer thaws. While, therefore, that part of the drift-region lying farther to the south was uncovered, and the water-courses actively at work digging out their beds and draining the land, the whole country to the north was a field of ice. Simultaneously the ice and the line of greatest snowfall receded northward. As the day's greatest heat is not when the sun is on the meridian, but an hour or two later; as the summer's greatest heat is not when the sun is at its greatest altitude, but a month or two later; so it is probable that the highest average temperature has not yet been reached, and that the line of greatest snowfall is still receding toward the poles. This fact, if it be one, must presage for Arctic explorers wider and more open fields of work a thousand years hence than to-day.
The cause of the saltness of some American lakes is too patent to require many words of explanation. It is probable that, when the continents were raised from the sea, the lake-basins had been already formed, and came up, therefore, brimful of water. In the northern and eastern part of the continent, where the supply from rain and snowfall exceeds the loss by evaporation, the salt, being continuously carried away through their outlets, has become so diluted as to be an imperceptible quantity. In arid regions, as the Pacific slope and the country about the Caspian, where the evaporation was in excess of the supply, the water-level of the lakes continuously sank until, on account of the diminished extent of surface, the equilibrium of loss and gain was attained. Hence the exceeding saltness of Great Salt Lake, the Dead Sea, etc. For a like reason the water of the Mediterranean contains more salt relatively than that of the ocean. Evaporation exceeding the supplies from the rivers and rainfall, it requires a constant current through the Strait of Gibraltar. The same is true of the Red Sea, causing a like current through the Strait of Bab-el-Mandeb. Other salt or brackish lakes probably owe their saltness to the supplies from the land. Water being the most general of all solvents, the rains gather up the chloride of sodium from the soils and the disintegrating rocks, and where the streams fall into lakes whose only outlet is evaporation, the land itself must be a constant source of saline supply, and their waters must become more and more salt, until their capacity as a solvent has been reached.
The Utah Basin must once have been filled to its brim with ocean-water. The outlet has been evaporation. The lake, receding to its present level, has left many evidences of its former extent.
To the drying up of salt lakes is probably due the presence of rock-salt, often found in great quantities in regions of little rainfall.
I come, lastly, to the trend of the North American lakes. A good map, and especially one on the Mercator projection, will show that lakes are not dotted promiscuously here and there, with no regard to system. They have with each other a trend of direction often as well defined as mountain-ranges, or the coast-lines of continents. As already shown, the great American depression bifurcates at about the fortieth parallel, and nearly at right angles, into northeastern and north-western branches, whose lines of direction lie respectively in approximate parallelism with the far-off Appalachian and Rocky Mountain ranges, and with the still farther-off Atlantic and Pacific shore-lines.
Geologists, and especially physical geographers, have noted the fact that the mountain-ranges, the shore-lines of continents, and the islands with each other, have lines of trend mostly northeastward or northwestward. The lakes of North America have similar trends of direction, and therefore form an integral part of the great system upon which the planet itself is built. This is as should be expected. That the line of greatest depression should have an approximate parallelism with the adjacent greatest upheaval is but a physical necessity.
Many thousands—perhaps millions—of years before man was on the earth, the Laurentian Hills were raised above the dark waters, their origin dating back to the very dawn of life on the planet. Both the primordial continent and the primordial life were the prophets—this of the higher orders of organism, and that of the continents and islands yet to be. This original continent has held its own. It took the initial in the building up of North America. Its two lines of direction had the form of a right angle, one projecting northeastward and the other northwestward.
This is the normal plan. It is the structural arrangement, not of this continent only, but of all continents; and the lake-depressions, conforming to the general system, are an additional witness to the common underlying laws and forces of which the earth itself is a grand phenomenon.