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Existing Geological Action of the North American Lakes
Posted By Dennis Partridge On In Native American | No Comments
That species of action which is supposed to have brought the surface of the earth into its habitable condition is comprised in the era of physical revolutions which are long past. By what causes, and according to what laws, these changes were produced, and their effects on the superposition and relation of strata, constitute no small part of the considerations of geology. Seas, rivers, mountains, and plains, are conjectured to have been left by those ancient revolutions, all of which preceded the historical epoch. It has been observed that the post-diluvial action of rivers flowing into the sea, and carrying down the usual accumulations of matter resulting from disintegration and gravitation, has added much to the area of their alluvions. Volcanic forces are continually exerting an action upon continents and islands; the beds of certain rivers are perceived to be elevated; large portions of the shores of the ocean curtailed of their limits; and, in this manner, the configuration of the earth is subject to large and appreciable alterations. All this is the result of a species of action, which is very strikingly exemplified by the North American Lakes.
It is known that the quantity of water on the earth s surface is much greater in a new and forest region, where solar evaporation is hindered, than in old and long cultivated countries. No one will pretend that the quantity of water brought down by rivers is not diminished by these curtailments of the dominions of the forest. There was a time, within the habitable period, when the rivers of this continent ran higher than at present.
1. This existing action is of two kinds, both of which are remarkably exhibited in the area of the Lakes; namely, the action of general fluviatile drift or outflow, and the action of disintegration and atmospheric phenomena. The Mississippi possesses the drift power in a high degree. By its present overflowing it is destined to be always raising its bed and banks. It lays the Rocky and the Alleghany Mountains under constant tribute for this purpose; and, if the present rate of deposition be maintained, the day is not far distant when the vast nascent deposits at its mouth, which are now covered with grass and water, will be known as some of the very best rice lands in America. Far less amount of labor in forming dykes and embankments than it has required to rescue Holland from the German Ocean, would now convert that tract of rich river-drift into a fertile and populous region.
2. Of the second species of action, that arising from disintegration and atmospheric phenomena, there is no instance on the same scale as is observed in the Great Lakes. I have selected the broad expanse of Lake Superior to exemplify this power. Hundreds of miles of uninterrupted wind and wave-power are here displayed. This sheet is one vast reservoir of elemental action: not only its large area and great computed depth have served, most fully, to develope this power, but this effect has been promoted by the very unequal degree of hardness of the rocky structure of its sides and bed; and it is within the scope of modern observation that, owing to this action, its boundaries have, under the actual fluctuations of its level, suffered great mutations. Being the only one of the series of lakes (with a partial exception in favor of Lake Huron) which has primitive borders and Alpine scenery, these effects are the more striking, and have imparted to portions of the coast a scenic grandeur, and boldness of outline, which are unparalleled.
This lake may be considered as occupying an interstice between the most northerly portions of the great diluvian and sedimentary formations of the Mississippi Valley, and the crystalline and vitreous rocks of British America. This ancient line of junction may be followed down its outlet, through the Straits of St. Mary s, into Lake Huron, and is continued along parts of its north and northeasterly shores north of the fossiliferous strata of the Manatouline chain. Lake Superior is, however, the most impressive field of remark, whether we refer to the ancient periods of its volcanic or oceanic energies, or the remarkable powers of elementary action still possessed by it.
The western and northern sections of this lake exhibit the strongest proofs of ancient action and upheaval. A colossal dyke of trap appears to have crossed the lake about two-thirds of its length from east to west. Admitting, (what appears to be very probable,) that the bed of the lake west of this dyke was originally produced by the sinking down of the strata, forming an anti-clinal axis, and the consequent elevation of its shores, we may attribute to the disturbing force of winds the central breach of this barrier, which has been subsequently widened by the ordinary force of the waters driven by the strong west and north-west winds, at a period when its water-line stood at one of its highest levels; so that, at this time, Isle Royal, Beaver Island, Ship Island, and the elevated precipitous range of Keweena Point, all of which consist of members of the trap rock, are the only existing monuments of this ancient dyke. The heavy beds of trap boulders east of this point, reaching in blocks of large magnitude to St. Mary’s Falls, and the northern shores of Lake Huron, strongly denote the probability of such action. Another proof of the extensive destruction of the central trap chain is drawn from its mineralogy. This rock, (the trap,) as modern discovery denotes, is, everywhere, the true repository of the veins of copper ore, and of native copper, for which the shores of this lake have been so long noted. By their prostration, their mineral contents have been scattered far and wide, along with other debris, and hence masses of the metal, and its ores, are found along portions of the coast, where the strata not only give no indication of being metalliferous, but, geologically, forbid the expectation. Hence also the abundance, along parts of the Superior coasts, of fragments and abraded masses of agates, zeolites, amethysts, and other imbedded trap minerals, which were originally contained in the amygdoloid.
Action upon the harder rocks and their contents, is still very perceptible, particularly along the western face of the great point of Keweena, which is now known also to be one of the best repositories of native copper and copper ores.
At numerous points of this coast, the waves have acted on crevices or breaks in the stratification, until deep passages have been worn into the coast, and interior bays formed, into some of which, vessels of considerable burden can sail; and they form a very welcome shelter, in stormy weather, to the many row-boats, which visit these remote points in the prosecution of the fur, fishing, and copper trade.
But the most extensive scene of the existing energies of this lake, is witnessed upon its grauwackes and sandstones, which have been broken up, comminuted into fine sand, and piled up in elevated ridges, or spread out over wide plains along its southern margin. A coast of winding bays and headlands, which measures, by a reduced computation, four hundred and fifty miles, upon this single section, may be conjectured to have encountered heavy inroads from waves and currents forced across the lake by north winds, or acting diagonally from the north-east, or north-west. By far the most extensive field of this action occurs between the easterly termination of the crystalline series of rocks, at, and near Granite Point, and their reappearance in the elevated mountain ranges of Gros Cape, at the head of St. Mary’s straits. The vast sand dunes, on this section, to which the French couriers du bois applied the name of Les Grandes Sables, constitute a most unique and picturesque object. Their perfect aridity, and great height above the lake, which has been computed at three hundred feet, and the general parallelism of the tops of the series of hills, strongly fix attention. These elevations are found, however, to rest on beds of clay, loam, and gravel, of a compact structure, and to be only buried beneath a coating or upper stratum, of loose yellow sand, which has been, manifestly, washed up by the waves, and driven land-ward by the winds. Tempests of sand are thus formed, which spread inland, bury or kill the tallest trees, and carry destruction and desolation in their track. Such is also the lake action of Huron and Michigan, the two next descending of the series of the lakes. Dunes are at first formed, which spread inland, carrying sterility over many thousands of acres of land, formerly fertile, and well wooded; and the tendency of this peculiar atmospheric formation is constantly to extend its limits, and arrest the progress of vegetation.
Another effect of this sand-drift is, by obstructions of the water-courses, to form ponds and lagoons, at the temporary or fixed points of their termini, on the arable land, and thus to destroy, and render unfit for the use of man, other large belts of country; besides which, these arrested waters are the prolific sources of noxious vapors, generating extensive disease in the vicinity. Evidence of the comparatively recent era of this atmospheric formation is seen in the prostrated and buried trees, fresh water shells, and other organic substances, in a perfectly unaltered state, which are, in some localities, noticed in digging at great depths, and sometimes exposed by recent eruptions of the waves. Such are the evidences on the east shores of Lake Michigan, between St. Joseph’s and Grand Traverse Bay.
Another formation, due to lake action, and not to diluvial action, which cannot be mistaken, but of earlier age, is found in the large sandy plains along the lake shore, as between the Takwymenon, on Lake Superior, and Grand Sables. These plains bear a growth of pines, poplars, and birch, which but slightly conceal their comparatively recent origin. On examining and penetrating these tracts, ridges of sand occur, lying in win-rows, as if recently formed by the winds and waves. The depressions between these often embody water in the shape of small lakes, ponds, and marshes, which constitute the favorite retreat of the small fur-bearing animals.
The power of attrition possessed by Lake Superior and the other Great Lakes is so complete, upon the sandstone series, as to allow full scope to the principle of gravitation in the re-arrangement of the comminuted and upheaved materials. Large portions of the magnetic oxide of iron exist in the northern sandstones. As these surcharged strata are ground down, in the great laboratory of the Lakes, this oxide is liberated from its silicious connection, and reproduced upon the shore in separate and pure beds of iron-sand, which are, not infrequently, a foot in thickness, and line the beach for miles. Such is the appearance of the coasts at Nezhöda and Mesácoda rivers.
A remarkable appearance has been produced at the Presque Isle River, which attests the power of attrition possessed by the waters of that stream. The river, within half a mile of its mouth, drops into a vast pot-hole of grauwacke rock, by a fall of about sixty or seventy feet. This cavity is eighty feet over, and in the summer season, when the water is low, produces an astounding spectacle of a striking cast. By going a little higher, the river is seen to have worn its bed for a depth of more than a hundred feet, perpendicularly, into the same rock.
The actual process, both of degradation and resistance, in the lighter colored and non-metallic sandstones, is nowhere better observed, perhaps, than along the walled and abraded coast locally known under the name of the Pictured Rocks. About twelve miles of this mural coast is most completely fretted and riddled into curious architectural forms and caves, by the force of the equinoctial gales. Colossal caverns, into which large boats can enter, are formed under the impending rock, and it requires but little aid from the imagination, in passing along these shores, to behold, in their head-lands, and rounded columns, and toppling pinnacles, the most imposing array of ancient ruins.
The annexed view (Plate 44) is taken, looking outwardly, from one of the principal caverns; it was sketched while seated in a twelve-oared barge, within the principal or labyrinthian cavern west of the point called Doric Rock.
It may be mentioned, before closing this paper, that there are several phenomena in the Lakes, in addition to those named, which deserve future philosophical notice.
3. Tidal Phenomena. One of the most general of these is the appearance of a tidal current in the Straits of Michillimackinac, and the several points along the chain of lake waters, where bays intersect the main mass; as well as in the effect produced in the general levels of the surface. The cause of this has been but imperfectly investigated, but it appears to be due to the currents of wind as affected by general problems of temperature.
4. Perforated Stones. The striking effect, resembling a reacting current, of the mass of Lakes Huron and Michigan, was early noticed. That this effect is not confined to the surface alone, but affects deeper masses of the water, appears to be proved by curious detached masses of limestone drawn up in the straits, by the fishermen s nets, from great depths.
These perforations of the boulders of limestone from the bottom of Lake Huron are very curious, and instructive of the mode of aqueous attrition. By examining them, it will be perceived that the most of the stone is completely perforated with cavities. Some of these extend through the mass; others part way; a few are flattened or irregular. On a more minute inspection, it will be perceived that each orifice consists of annular rings; as if the impressions were left by a boring instrument, or, (what may furnish the true solution,) by some small inorganic substance, as a minute pebble, which the water has kept in motion.
As these curious masses are drawn up from deep water, at 70 to 80 fathoms, in those jets of current which are formed by the influx and afflux of the waters of the straits, it seems clear that these singular perforations were formed by the oscillatory motion of very small pebbles.
The limestone itself is of the compact semi-crystalline character, which is common in Lake Huron, in inferior situations. Some of this compact limestone, examined in situ, is found to exhibit small open punctures, as if left by the point of a penknife. But these punctures may be supposed to be the impressions of pre-existing crystalline matter, now decayed. They seem to owe their forms to minute crystals of the sulphate of strontian.
5. Temperature Of The Lakes. It is found, by experiment, that the rays of light passing through transparent bodies of lake-water, which is, of course, fresh water, do not in any degree heat them. Is not this phenomenon one cause of the coldness of the lake-waters? The infusion of muriate of soda in seawater, by giving it the properties of absorbing heat, may tend to warm it; and hence, in the tropics, the sea is warmer.
6. Crystallization In The North. Hitherto, the primitive rocks discovered near the shores of Lake Superior have yielded few imbedded minerals, or crystalline bodies. But there is reason to suppose that further researches and discoveries will disclose them. It is believed that the primitive or crystalline district contains granitic beds, highly crystalline in their structure. A mass of drift granite at Green Bay contains a vein of highly crystalline matter, in which the plates of mica are large, shining, and distinct, and of a green color. It embraces very beautiful crystals of black tourmaline, common garnet, and a green massive mineral, which is apparently prase. A block of black mica, observed at Drummond Island, is manifestly brought from the primitive district, north or west of that point. It is crystallized in well-defined hexahedral prisms. A block of mica slate near Elm Creek, Lake Huron, yields staurotide. These, if we admit a current of water, or water bearing ice, as the disturbing force, may be supposed to have been transported from the region referred to; and indicate a range of crystalline strata in the north and west, quite varied and interesting.
7. Continental Abrasion. If we are to regard the lakes as a grand geological triturating apparatus, converting its loose and shore-rocks into a pulverulent state, it may be anticipated that their action on the configuration of the shores will be very considerable, in the course of long periods. What is lost in this process in one place, from their rock area, is found to augment the quantity of alluvial soil in another; which, in time, renders the whole area suitable for agriculture. Thus the plough gradually, but surely, follows the tempest and the hurricane; while the absolute indestructibility of matter is man s guarantee under every change.
8. Integrity Of Matter. The absolute quantity and cubical area of material matter of these immense areas is still the same. The elements of which they are composed are seen to be indestructible. No change of combination or position is Been to take from, or add to, the material aggregate. If physical matter, under the force of tempests, could be destroyed, as well as change its forms, there would result an annihilation of a part, or molecule, of the original accretion of elements. Wild as their rage sometimes is, casting vessels on high on these Lakes, the entire volume of them yet retains its integrity.
9. Lake Refraction. The phenomenon of light, as seen on these Lakes, offers a still more familiar instance of changes in the position of matter, without adding to, or diminishing, its bulk. And in this, as in other departments of physical forms, while the instances vary, there are no evidences to show that in the resplendent refractions that visit these Lakes in their curious mirages, and boreal displays, and brilliant sunset scenes, there ever was a combination which did not vindicate the wisdom, exactitude, and beauty of nature s laws.
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