Popular Science Monthly/Volume 3/July 1873/Nature and Origin of the Drift-Deposits of the Northwest II
|NATURE AND ORIGIN OF THE DRIFT-DEPOSITS OF THE NORTHWEST.|
STATE GEOLOGIST OF MINNESOTA.
II. Origin of the Drift.
THE first records of exact observations pertaining to the drift seem to have been made in the first quarter of the present century, and are wholly confined to the appearances and positions of the bowlders, or "travelled rocks," that constitute a striking object to the scientific observer throughout the northern portions of Europe and America. In 1819 the memoirs of the Wernerian Society of Edinburgh announced the transportation by ice of a large piece of conglomerate 4x6x8 feet a distance of 260 yards in one night. It was deposited in the sands on the shores of a little bay on the Mersey Firth.
A similar account is published in the American Journal of Science and Arts, 1822, as occurring at Salisbury, Connecticut.
After the year 1820, exact observations were stimulated in this country by the publication of the American Journal of Science and Arts, which from time to time called attention to the various phenomena of the drift. The earliest investigations of note were made by De Saussure, Pallas, and De Luc, on the Continent of Europe, and by Sir James Hall in Scotland. These observers coincided in the opinion that the existence of the "travelled rocks" must be explained by the occurrence of devastating currents of water, or débâcles, from the north, which transported them from their original places. This theory was advocated, sometimes with slight modifications, by the revered Dr. Edward Hitchcock, of Massachusetts; by Dr. Benjamin Silliman, of Connecticut; by Dr. Hildreth, of Ohio; Lapham, of Wisconsin; J. N. Nicollet, of Minnesota; and by Von Buch, Studer, Buckland, and De la Beche, of Europe. Von Buch, seeing that one débâcle, proposed by De Saussure, would not account for all the phenomena, supposed there were several. De la Beche believed this vast inundation from the north was the immediate result of a sudden upheaval of the polar regions, turning the waters of the Arctic Ocean southward with great violence. This cause was also accepted by Prof. Buckland and Dr. Silliman. This theory is the same as that known as diluvion. Hence the groovmgs on the rocks were first known as diluvial marks.
Contemporary with the débâcle theory was that of Chabrier, who believed the bowlders came from the atmosphere. This theory seems not to have met with very much countenance, and soon ceased to be regarded.
In 1828 Peter Dobson, of Connecticut, proposed the germ of what became an important and long-lived theory, viz., that floating ice, in the form of vast sheets, carried great quantities of gravel and stones, and distributed them wherever they were stranded. This suggestion, aided by the quick indorsement of Sir R. I. Murchison, grew into that known as the iceberg theory, which survives to the present day. This last necessitates the submergence of the continent beneath the quiet waters of the ocean, and here diverges from the débâcle theory which requires turbulent waters. The iceberg theory received many prominent and able advocates. Among them may be named Sir Charles Lyell, Sir Roderick I. Murchison, Peter Dobson, John L. Hays, C. T. Jackson, Sedgwick, of England; W. C. Redfield, of New England; Prof. Mather, of Ohio; Dawson, of Canada; and a great many others.
Before, however, the iceberg theory had grown into prominence, Mr. De Kay, of New York, proposed another, which at least has the advantage of allowing the continent to stand still instead of sinking it several thousand feet below the ocean-level.
Mr. De Kay claimed that the bowlders originated at or near the places in which they now lie, that they are the remains of ancient peaks of primitive rock that have since been demolished by earthquakes and by atmospheric forces, the sites covered by detritus, and concealed from the observer. This was announced in 1828, but it made no headway, probably from the fact that these bowlders lie on the surface not only where primitive rocks abound, but also over broad areas where the primitive rocks are buried thousands of feet below later sedimentary formations, such formations being intact over the whole area.
In 1837 Prof. Louis Agassiz propounded that theory known as the glacier theory in a paper read before the Helvetic Society of Natural History in his native country, Switzerland. It is thus concisely stated by Mr. Charles McLaren: "It was deduced from a careful study of the phenomena attending glaciers. . . . The Swiss philosopher advanced step by step. He satisfied himself that in the Alpine valleys, where glaciers still exist, they once rose to a higher level, and extended farther down into the low country than they now do. Next he discovered indications of their former existence on Mont Jura and over the whole Swiss valley; and, connecting these with similar indications found in the Vosges, the Scandinavian mountains, and elsewhere, and with the well-known fact of sheets of ice covering the northern shores of Siberia, and entombing the remains of extinct species of animals, he came to the conclusion that, at a period, geologically speaking, very recent, all the Old World north of the 35th or 36th parallel had been enveloped in a crust of ice. Whence the cold came which produced this effect, and why it afterward disappeared, are questions he did not feel himself bound to answer." This theory had been suggested before by Venetz, but had been applied by him only to the region of the Alps. Prof. Agassiz afterward more fully worked out his theory, giving facts, and careful measurements, and calculations, in his famous work entitled "Études sur les Glaciers."
Prof. Agassiz supposes that the eastern Alps were upheaved when the coating of ice was on the surface, this being the last cataclysm that has visited Europe. By this upheaval of the Alps the ice was disturbed, like the rocky formations. This was accompanied or followed by a higher temperature, and the thawing of the ice, which produced torrents and consequent valleys of erosion. The floods which followed the upheaval of the Alps were sufficient to float icebergs containing blocks of rock that might be deposited in different places, the water being at least 300 feet deep, and the agent that carried and deposited the fine drift in the valleys below. The catastrophe which enveloped the northern regions in ice was sudden, according to Prof. Agassiz, but the retreat of the glacier was slow.
This theory, so novel, so startling, met with various acceptance. By some it was loudly scouted as the product of the imagination solely; and was classed by Prof. B. Studer, a savant of the Continent of Europe, with the poetical Indian legends, wherein the periods of heat and life are made to alternate with periods of freezing and death. But its force lay in the inherent evidence of candor, and honesty in the statement of facts about which there could be no dispute. By the most enlightened geologists, both of the Old World and the New, it was received as a flood of light cast on what had before been dark and unexplained; and it was accepted with some caution and exceptions by such men as Prof. Buckland, Sir Charles Lyell, and Prof. Edward Hitchcock. At the present day but few geologists can be found in this country who do not admit the reality of the glacial epoch.
But, while it is true that but few geologists can be found in this country who do not admit the truth of the glacier theory of Prof. Agassiz, it is also true that a great many, perhaps the majority, also adhere to the iceberg theory of Peter Dobson. The two theories at first came in violent conflict. They diverged at the outset. One required the continent below the ocean, and the transportation of bowlders and other drift by floating ice; the other required it elevated high above the ocean, and the transportation of the drift by ice in the form of continental glaciers. How, then, can the same person hold to both theories?
Soon after the promulgation of the glacier theory by Prof. Louis Agassiz, Mr. Charles McLaren attempted to make it harmonize with the iceberg theory. He was seconded in the effort by Prof. Edward Hitchcock, who invented the term aqueo-glacial, to express the force, or forces, that operated to disperse the materials of the drift. In explaining the meaning of that term, he says, however, he cannot admit the glacier theory of Prof. Agassiz, and apply it unqualifiedly to this country; but, while acknowledging himself greatly indebted to Agassiz, he thinks that icebergs were the principal agents in transporting the drift. In the years 1841 and 1842 Sir Charles Lyell visited this country. His observations on the drift, published in various scientific journals, and repeated in his book of "Travels in North America," in 1845, furnish the basis for the most plausible union of these two theories. He divides the drift epoch into four parts:
1. The period of emergence of the land, during which some of the bold, rocky escarpments of the continent were formed.
2. A gradual subsidence and moderate submergence of the interior portions of the continent, during which icebergs floated over the surface of the ocean, grinding and marking the rocks.
3. The deposition of the clay, gravel, and sand, of the drift, with occasional fragments of rock, the last through floating ice.
4. Period of reëlevation and formation of lake-terraces.
Although not professedly aiming to reconcile the iceberg theory with the glacier theory of M. Agassiz, Mr. Lyell's eminent authority would not permit the total extinguishment of the iceberg theory, and his generalizations have, perhaps, had more influence in directing the efforts of others in such reconciliation than the writings of any other man. It required but very slight changes in Mr. Lyell's method of dividing the history of the drift to evolve, in its present aspect, the latest theory of geologists touching the origin of the drift-deposits. Mr. Murchison, of England, coincides with Mr. Lyell in the submergence, or iceberg theory. Mr. J. D. Dana advocates the glacier theory in its fullest extent; but, although adopting also the term Champlain, he is far from admitting the recent enlargement of that epoch, so as to bring the continent beneath the water of the ocean as required by the supporters of the combination theory.
Professors E. W. Hilgard, of Mississippi, and J. S. Newberry, of Ohio, are among the most prominent advocates in this country of this new theory, resulting from the combination of the glacier and the iceberg theories. Dr. A. Winchell, of New York, also advocates the same.
It is as follows:
First. The glacier epoch proper.
During this epoch the continent was considerably elevated above its present level, especially in the north. This either produced, or was accompanied by, a greater degree of cold, the effect of which was to bring over the continent the vast sheets of ice in the form of continental glaciers, required by the hypothesis of Prof. Agassiz. During this epoch the rocks were scored, and many deep valleys were excavated. Large bowlders were transported to regions farther south.
Second. The submergence of the continent, attended by an amelioration of the climate and the disappearance of the glaciers, or their retreat to the far north. The assortment and stratification of the drift, produced by the glaciers, and the deposition of the great mass known as Erie clay, and other clayey portions of the drift-sheet. This condition of the continent was attended by the appearance of numerous icebergs which floated over the submerged land, and aided to transport the coarse drift, according to the hypothesis of Peter Dobson.
Third. The emergence of the continent with a halting progress, producing terraces and ridges marking the ancient levels of the ocean.
These three steps have been named by Prof. Dana, in their order, the Glacial Epoch, the Champlain Epoch, and the Terrace Epoch.
The studies of Professors Agassiz and Tyndall on the glaciers of the Alps, and of Dr. Kane on those of Greenland, have so fully demonstrated the adequacy of glaciers to produce all the effects attributed to them by the theory of Agassiz, that it is now very generally admitted that, wherever those phenomena are seen, glaciers must have existed.
Geology, having demonstrated thus the necessity for a period of cold, to account for the phenomena of the surface of the earth, labored under the difficult task of accounting for such change of climate on philosophical principles. It is here that the community of the sciences is beautifully illustrated. Astronomy comes to the aid of her younger sister, and Geology receives from her the solution she could not herself compass.
Astronomy makes known three great irregularities in the motions of the earth, requiring thousands of years each for their recurrence. They are—
1. The "precession of the equinoxes," combined with the revolution of the line of the apsides, produces a progression in longitude, of the aphelion place of the earth, bringing about a coincidence of the winter-solstice with the aphelion once in 21,356 years. At the present time our aphelion position occurs in the summer season.
2. Variation of the inclination of the earth's axis to the plane of its orbit. This passes through a double oscillation in about ten thousand years. Its effect is to carry the solstitial point through a small variation in latitude, and to that extent prolonging or withdrawing the influence of the sun's rays in the polar regions.
3. Change in the eccentricity of the earth's orbit. This irregularity passes from maximum to maximum once in about one hundred thousand years. Its effect is to lengthen those seasons nearest the earth's aphelion, and to shorten those nearest its perihelion. At the present time, our aphelion, occurring in the summer season, lengthens the warm months to that amount that, combined with the shortening effect of perihelion in winter, makes a difference of about eight days between the summer and winter months of the year.
The conditions favorable for polar glaciation are as follows:
1. Winter-solstice in aphelion.
2. Obliquity of the ecliptic at minimum.
3. Eccentricity of orbit in maximum.
The coincidence of these three causes would produce the greatest glaciation. The least multiple of their periods of recurrence is about forty-two million years. The second cause may combine a great many times with either of the others within that interval. Its effect being quite inferior to that of either of the other two, it may be disregarded, and the time required for the combination of the other two would then be 4,200,000 years, which would necessarily be preceded and followed by a number of approximations. These would occasion corresponding periods of increase and diminution in the degree of cold that would not reach the maximum cold incident to their coincidence. Probably the most powerful of the causes enumerated is the occurrence of the winter-solstice in aphelion, which alone may have produced the last glacial epoch. In that case changes in the prevalence of the ice would be due to the operation of cause No. 2, or the obliquity of the ecliptic. The earth is now 622 years past its period of south-polar glaciation in the operation of cause No. 1, and is entering upon its period of north-polar glaciation. Its last glacial epoch in the operation of this cause occurred in the Northern Hemisphere in its acme of intensity at a period 11,300 years ago.
The effect of these irregularities in the motions of the earth on the climate has been ably discussed by Mr. James Croll, of the Geological Survey of Scotland ("Transactions of the Geological Society of Glasgow," vol. ii., part iii., p. 177), and detailed calculations on the periodicity of these variations have been made by Mr. Stockwell, of Cleveland, Ohio.
Let us endeavor to picture the recurrence of one of these coincidences, and to rehearse some of the phenomena of an actual period of continental ice.
The precipitation of the winter season is all preserved on the ground in the form of snow and ice. It constitutes what has been denominated névé. The advent of the summer season is not powerful enough to melt the accumulations of the long winter. The névé is simply converted into granulated ice. Another winter adds to the thickness left by the preceding. Another summer changes it to ice. Some water may be the result, but it is congealed in the streams, or perhaps escapes to the ocean. This succession is continued, with a slow increment of cold, through thousands of years. The ice-mantle becomes continental in its extent. Its thickness reaches hundreds of feet. Toward the pole this may be increased to thousands or tens of thousands. It has a great weight. It presses upon itself. Its lower portions yield to the inequalities of the rocky surface. The mass seeks the valleys. It slides down the mountain-sides, carrying the débris which it detaches in its descent. It covers the broad plains. The accumulations toward the north, ever increasing, press out toward the south, the foot of the ice-sheet. A general movement is developed by reason of the gravity of the mass, the fracturing and regelation of its parts, and the molecular forces that allow it to yield under pressure. Each recurring summer develops more or less water. This water perhaps enters the openings and crevasses, and washes out some of the obstructions, facilitating the general progress. The main water-sheds separating valleys serve also as ice-sheds. The valleys are more rapidly dug out by the rasping and ploughing movement of the glacier than are the highlands. In the valleys the ice flows most rapidly. In the valleys, also, the ice is prolonged much farther into warmer latitudes than on the highlands. Southward, prolongations of the ice-sheet follow the north-south outcropping edges of argillaceous formations. Lake Michigan lies in one of these troughs. Lake Huron lies in another. Lakes Erie and Ontario are only shallow basins dug out of soft rocks by ice that passed southwestwardly. The shale-bed that gave rise to Lake Ontario also determined the location of Georgian Bay and of Green Bay. The basin of Lake Erie is much shallower toward the west end than toward the east, and it finally runs out altogether by reason of the westward attenuation, and finally the entire disappearance of the salina formation in which it is largely excavated. The ice was then thrust up on to harder rocks that form the basis of Northwestern Ohio and Northeastern Indiana. Lake Michigan was terminated southwardly by the eastward trend of the rocky outcrops at an angle that the ice could not follow. The Red-River flats of Minnesota correspond to the Winnipeg basin in the same way that the Black-Swamp district of Ohio does to the Lake-Erie basin, or the prairie district surrounding the southern end of Lake Michigan does to the basin of that lake.
It must be remembered, however, that throughout the continuance of the ice-period the motion of the ice brought it finally into a climate where it could not exist as ice. It gave rise to countless streams of water. The broad, level country of the Northwest was not sufficiently irregular to gather the ice, and consequently not the water, into valleys having a north-south direction. The water from the ice acted all along the ice-foot with a comparatively uniform energy. If it was ultimately gathered into large streams, it must have been at considerable distances from the glacial field. It must be remembered also that the accumulated precipitation of the entire year over broad, continental areas was preserved from thaw till it arrived at the latitude of the limitation of the glacier, and there its full volume was discharged. It was as if the entire precipitation of the continent—say from the latitude of Chicago to the north-pole—were concentrated on a belt of territory, say of fifty miles in width, running east and west across the continent, and having the direction of the marginal line of the ice-foot. Thus a constant sheet of turbid, running water would act on all objects over which the ice-foot retreated.
We must not forget, in recalling to our imagination the scenes and events of the ice-period, to inquire what were the position and the condition of the drift to which it gave origin.
In regions far to the north, the eye probably would not be able to discern any object except that of the universal ice. The surface of the ground would be thousands of feet below the traveller, if we may be permitted to presume so hardy a human being. Like Dr. Kane exploring the great Humboldt Glacier of Greenland, he would meet with countless obstacles and dangers. But those obstacles would consist of hummocky ice, or crevassed ice, or perpendicular ice-walls. He would see no soil, no rocks, no vegetation, no animal life. The winds would whistle, storms would rage, snow would be drifted about, and the ineffectual sun would rarely venture to smile on the dreary waste. Farther to the south, the explorer would find isolated spots of bare ground. He would see about them the accumulated débris of bowlders, gravel, and dust, from constant winds, spread more or less over the ice-field, staining its painful whiteness, and showing the more grateful aspect of earth and stones. Another hundred miles farther south, and he finds the evidences of the dissolution of the ice-sheet multiplying. Occasional streams of water run on the surface of the ice, or plunge into some of its openings. Deep gorges reveal multitudes of fragments of rock frozen into the ice, and occasional bands of dirt and gravel embraced in the solid ice. The surface is everywhere dirty, or perhaps muddy, from the wasting away of the surface of the glacier. He meets frequent openings, in which generally water may be seen or heard. Into these gorges the débris slides down the sloping sides, increasing the insecurity of his footsteps. Still farther south, the general surface is covered with a pulpy earth, mingled with stones and bowlders. The ice is evidently much attenuated. The areas of firm, uncovered terra firma are wonderfully increased in size and frequency. The ice itself is crowded into the valleys, or, if it be in a broad, level tract, like the State of Minnesota, the surface is covered with the débris of the conflict of ice with earth, the ice itself being visible only in those places where crevasses reveal it, or where deep gorges are worn by running streams. Travelling still farther south, the explorer would come upon large areas in which he would not be able to know whether the glacier underlay the superficial drift or not. If he were to stop on one of those wide areas, and make his latitude and longitude certain, by a series of astronomical observations, he might find to his surprise, after a few years' residence, that his observatory and apparatus had been bodily carried, by an imperceptible motion, some rods to the south. If he were to penetrate the earth on which his foothold seems so steadfast, he might find, equally to his surprise, that he was still riding on the surface of a vast ice-sheet, the earth and soil of which may have furnished him annual crops of potatoes and barley. In other places in the same latitude he would find the ice laid pare over considerable areas, washed clean by the drainage incident to the dissolution of the glacier. The turbid streams would be vastly larger than those which occupy the same beds to-day. They would run with tenfold more violence. The drift-materials would be freed from the clayey portions, and be spread along their channels in curious and varying assortment. In some places the thickness of the whole sheet of drift would be brought under this washing and stratifying process. In others, the ice gently dies out, and lets it down on the rocky surface without any change from the condition in which it lay on the glacier.
If, at last, the explorer travels far enough south to actually leave the area of the glacier, what is the condition of the surface? It is plainly one of glacier-drift. In some places he will find the various parts, such as gravel-stones, sand, clay, and bowlders, confusedly mingled, showing no assortment or stratification. The clay which has resulted from the grinding action of the glacier on the surface on which it lay, from dust blown over the ice by violent winds, as in the Alps, and from the sediment washed on the ice from the higher knobs that first became uncovered, fills all the interstices so closely as to make of the mass an impervious and uniform hard-pan. This has generally been denominated "unmodified drift." It is that which escaped the assorting action of the water issuing from the glacier. In other places, this unmodified drift would be superficially assorted, showing the effect of running water after its deposition or in the act of deposition. Probably very much of that portion of the drift that lay in the course of the broad Mississippi, yet south of the limit of the glacier, would be superficially worked over, losing much of its clay. We actually see vast tracts on the Upper Mississippi, and even in the latitude of St. Paul, in which the surface consists, to a considerable depth, principally of stratified, sand and gravel. He would also find parallel ridges of drift-materials, consisting largely of the coarser portions, and showing stratification where water passed over or through it in being deposited. Some such ridges would still retain the most or all of the original clayey portions. This would be the case where the drainage was not powerful. Such ridges mark the places at which the retreat of the ice was temporarily stopped by a period of greater cold, the slow advancing of the ice under the propulsive forces already named serving to heap up a greater amount of detritus all along the ice-margin. These ridges are known as moraines, and they occur in all parts of the drift-latitudes. They are developed on a very grand scale in Northwestern Ohio.
There is still one important point in this discussion that must not be omitted. It is plain to see that, in some parts of the Northwest, the advance of the continental glacier would be up gentle slopes, instead of descending an incline. These slopes, of course, present obstructions to the movement of the ice in those directions. It is true, also, that the continental glacier would tend to level the country and obliterate such northward slopes. But, in the later part of the ice period, the valleys would be the last relinquished, and would be deeper dug by isolated branches or spurs from the main ice-sheet, which would conform in their direction to the contour of the valleys they might occupy. All glaciers, however, whether continental or local, would avoid an ascent if there were any other passage. Now, when a glacier, propelled by a force exerted far to the north, meets with a gentle slope toward the north, the water which issues from its foot Will be confined in a lake about the foot of the ice, and will rise to the height of the lowest outlet. Into this lake may flow streams of considerable size, bringing their sediment from the south, east, or west, according to the topography. Here we should have, then, a constant accession of drift from two sources, the chief of which would be, of course, the glacier itself. As this drift is brought under the influence of standing water, its fine parts are floated away by currents and waves, to be spread over the bottom of the lake in horizontal laminations, the principal portion, and notably the bowlders, sinking at once to the bottom unassorted. Thus, by the continued slow withdrawal of the field of ice, the result is a layer of unassorted drift overlaid by a thickness of handsomely-laminated fine clay and sand. This combination of circumstances must have occurred south of Lakes Erie and Huron, producing the Black Swamp and the Cottonwood Swamp in Ohio and Michigan, about the south end of Lake Michigan, and over an extensive fiat south of the Winnipeg basin.
All the phenomena of the drift in the Northwest are, hence, attributable to the approach, long duration, and slow disappearance of the glacier-ice of Prof. L. Agassiz. It certainly seems unwarrantable to propose upward and downward movements of the crust, involving the submergence of the continent, when one simpler cause can be shown sufficient to produce the known effects. The submergence of the New-England coast to the depth of about seventy feet is all that Prof. J. D. Dana finds warranted by a vigorous inspection of the drift-deposits about New Haven, Connecticut. The four-hundred-foot "beach," near Montreal, may have the same origin as the so-called "beaches" that rise several hundred feet higher in the State of Ohio. The Champlain and Terrace Epochs find no application to the drift in the Northwest, as those terms are defined and used in the East. There is abundant evidence throughout the West of a former higher stage of the rivers. This higher stage may, however, be explained by referring it to the large increase of water incident to the melting of the glacier only after reaching the latitude of a warmer climate. The terraces have not, moreover, in the Northwest, generally that system or uniformity of height and arrangement necessary to warrant their reference to successive reductions in the volume of water, but are usually due to a variable resistance offered by the banks or rocks in which they occur, arising from their stratification.
No well-authenticated fossil remains from the hard-pan drift have yet been met with. Statements have been published of the finding of fossil remains in the unmodified drift in various parts of the Northwest, but they are generally based on the reports of non-scientific observers, and must be taken with great caution, unless verified by a geologist who has definite ideas of what "modified" or "unmodified" drift is. It would not be improbable that, near the southern margin of the icefield, the remains of vegetation, and even of animals, should be involved in the drift undergoing transportation, but their structures are too fragile to withstand the grinding incident to the general progress of the ice, and would not bear transportation from northern latitudes. Much uncertainty is also thrown on the true age of vegetable remains reported from the drift in the Northwest, by the wide-spread but hardly distinguishable clays of the Tertiary and Cretaceous formations, which contain modern species of wood and leaves, associated with marine fossils.
Believing, therefore, in the glacier origin, directly, of the Post-Tertiary deposits of the Northwest, it is impossible to concur in the supposition recently expressed by an eminent authority relative to the probable blending of those deposits with the marine deposits of the Tertiary: "A careful study of these modern deposits" (meaning the Quaternary) "will undoubtedly show consecutive links by which it was united to the Tertiary period, in the same manner as the Cretaceous and Tertiary are connected." It is difficult to conceive how the sedimentary deposits of an epoch of submergence, like the Tertiary, which abound in marine fossils, can show, however carefully studied, consecutive links of connection with an epoch of débris transported and deposited through the agency of vast continental glaciers.
St. Anthony, Minnesota, March, 1873.
- Prof. F. V. Hayden, in "Geology of Wyoming."