Popular Science Monthly/Volume 3/June 1873/Nature and Origin of the Drift-Deposits of the Northwest I
|THE DRIFT DEPOSITS OF THE NORTHWEST.|
STATE GEOLOGIST OF MINNESOTA.
I. Nature of the Drift.
IN the March number of this journal, Mr. Elias Lewis calls attention to the occurrence of bowlder-like masses of clay in stratified gravel, at Brooklyn, N. Y. In the progress of the geological survey of Ohio, similar masses of gravelly clay were met with in the northwestern portion of the State, lying in the stratified gravel and sand that constitute the long ridges which have often been pronounced "lake-beaches." These occurrences, and a great many others that militate against the popular theory that those ridges are attributable to the action of the waters of Lake Erie, and the stratification of the drift generally over the "interior continental basin" to the action of a wide-spread lake, or of the ocean, made it necessary to reinvestigate the drift-deposits thoroughly, for the purpose of deducing from the drift itself such a theory of its origin as would stand the application of all the facts. Such reëxamination has resulted, in the opinion of the writer, in the confirmation of the glacier theory of Prof. L. Agassiz, and the consequent abandonment of the iceberg theory of Peter Dobson. It has also shown the baselessness of the assumption of some who would extend the Champlain epoch of Prof. J. D. Dana, so as to bring on, after the period of the glacier, a submergence of the continent beneath the ocean. It is proposed to review, in a non-technical way, the phenomena of the drift of the Northwest, and to offer a few thoughts on the glacier theory, and its application to the explanation of those phenomena.
In general, the term drift applies to whatever lies on the surface of the rocky framework unconsolidated, whatever be its origin or lithological character. Glacial drift is that which has been transported by the agency of ice, or by ice and water, from regions farther north, and spread over the surface of the country. It may embrace bowlders, gravel, and clay. These substances may be arranged in stratification, and nicely assorted, or they may be confusedly mixed. When stratified and assorted, they have sometimes been denominated modified drift; when not assorted, unmodified drift. But these terms require considerable caution in their use, since they have been differently applied by different writers, depending somewhat on the supposed cause of the assortment witnessed in modified drift, and since the assorted and non-assorted portions of the drift are not uniform, either in their positions in the great mass of the deposit, or in the characters they generally possess.
The character and nature of the drift in the Northwest are very largely misapprehended. This is true, not only among those who might not strictly be regarded as geologists—such as surveyors, engineers, lecturers, and public literati—but even among those who have given considerable attention to the study of fossils and rocks. These misapprehensions, so generally spread among the people, are largely due to the industry of the authors of certain theories concerning its origin, in spreading their views before the public. A plausible theory, moreover, has a great influence in its own favor.
A pretty careful study of the drift in this State, and in others embraced in what may be called the continental basin, east of the Mississippi, has shown it to consist, in general, of the following parts, in descending order:
No. 1. Surface Soil.—This, of course, presents all the varieties due to local influences. Over large portions of the Northwest it is a fertile black loam, highly arenaceous, and supplied with a considerable proportion of carbon in a state of minute subdivision. This arenaceous loam passes into a more gravelly loam on the brows of knolls and in rolling land. It is also sometimes replaced by a gravelly clay. This is the case in large portions of the State of Michigan, and in Central and Southern Ohio. This is the fact in Northern Indiana and in Central Minnesota. The gravel prevails in wooded and rolling districts. In treeless districts the sandy element is more common, making a black loam. In valleys and along streams the soil is alluvial. It is invariably fine, nearly free from stones and bowlders, and very fertile. It is what is popularly known as "made land," and comprises those parts of the drift of the highlands that are susceptible of transportation by running water. That which is known as the "bluff-formation," lining the Mississippi, both in Minnesota and in the States farther south, consists of alluvium, washed into the great valley by innumerable streams from the adjoining country, at a time when the volume of the river was immensely greater than now. The same materials are now spread over the farms of Southern Minnesota, over much of Iowa and Illinois, over Northern Missouri and all the Far West, to the Rocky Mountains. It lies there also in the form of fine sand, and constitutes the loamalready described. Its thickness at points remote from the river is dependent on the facilities for natural drainage and wash. It may be six inches, or it may be six feet. Along the banks of the Mississippi it presents, not infrequently, perpendicular sections of six hundred feet. Its firmness in maintaining its position in such exposed bluffs is due to the infiltration of the cements of lime and iron while in the process of deposition, or subsequently. It is more largely developed along the Missouri than along the Mississippi. There are other places where the surface-soil may be peaty, from the preservation of dead vegetation. Extensive level tracts, that are submerged a large part of the year, may present a peaty soil. Very often also in such peaty places there will be found patches of highly-calcareous soil, resulting from the accumulation of fresh-water shells, or from the precipitation of the carbonate of lime from waters that enter the marsh from limestone districts.
But, whatever the character of the surface-soil, it must be borne in mind that it is accidental, and is always superinduced by causes that have operated since the advent of the drift. Its influence is strictly superficial, rarely exceeding three feet below the natural surface.
No. 2.—We come now to consider that which lies below the surface-soil. If we omit from this enumeration the "bluff-formation," and the alluvium of other streams which sometimes has a considerable thickness, we shall have two different substances, equally pertaining to the drift, and occupying the same relative position in different localities, that claim notice:
1. A clay subsoil.
2. A gravel or sand subsoil.
Now, although these are mentioned as appearing first beneath the surface-soil, it must not be understood that they appear there invariably, nor even usually. It is probably true that throughout the greater portion of the Northwest they are entirely wanting, and that feature of the drift prevails which will next be considered. They are mentioned here because they constitute an essential part of the drift, and must not be overlooked in giving its character and composition.
By the first, here denominated a clay subsoil, is not meant a gravelly clay, or one in which stones are present. It is, rather, a close, plastic, fine clay, with little observable sand. It is impervious to water, and is benefited by artificial drainage. It prevails in much of Southwestern Michigan and Northwestern Ohio. It occupies a large tract in Northeastern Illinois and Northwestern Indiana. It also probably underlies the Red River flats in Minnesota, and perhaps a belt of land rudely conforming to the shore of Lake Superior at its western extremity. When shafts are sunk through this clay subsoil, so as to reveal its composition and arrangement, it is seen to be handsomely laminated horizontally. The individual layers are separated by thinner layers of fine sand. Those of clay are usually about two inches in thickness, but may be no more than one-eighth of an inch; the layers of sand are rarely more than half an inch in thickness, and are apt to be less than an eighth. The aggregate thickness of these alternating layers of clay and sand is sometimes a hundred feet or more. Let it be noticed that these areas of clay subsoil are those in which there is a gentle descent, and drainage to the north or northeast into some one of the great interior lakes of fresh water. The relation this fact bears to the origin of this clay subsoil will be considered farther on.
The gravel or sand subsoil is that which is found in some tracts of rolling land where the drift is heavy, and at points more remote from the valleys of northward drainage, or in the upper portions of those valleys. As a general rule, when present, it will be found on a higher level than that in which the subsoil is clay. It pertains to the interior country like the central part of the southern peninsula of Michigan, the central and southern portions of Ohio, Indiana, and Illinois, and some parts of Central and Northern Minnesota. The area and location of this kind of subsoil are more irregular and more uncertain than the areas of clay subsoil. Such gravel and sand deposits often lie in belts traceable for a great many miles, especially where the general surface is smooth, and the underlying rock of uniform hardness, the country adjoining being, on either side of the belt, one of a clay subsoil, or one formed by No. 3. Such belts are sometimes three or four rods wide, or they may be much wider, and are rolling and slightly raised above the adjoining clay land. Sometimes, instead of lying in belts, such rolling, gravelly land is spread out over areas of no definite shape or limit. The sand or gravel constituting the subsoil in these rolling tracts is, like the clay of the clay subsoil, stratified and assorted. But the layers here are rarely horizontal. They show the most various alternation and change of dip. No two sections could be taken that would give the same succession of parts. The sand sometimes lies in heavy deposits fifteen or twenty feet thick, with lines of deposition running in curving and vanishing layers in all directions. Sudden transitions occur from sand to gravel, or from gravel to bowlders. Sometimes, also, bowlders are found embedded in the gravel; again, nests of bowlders are seen isolated from the rest, and packed closely by themselves. There is also very often a mingling of gravel and sand, with no clay, without stratification, as if the two had been dumped together, after having been first thoroughly washed and assorted. Occasionally, also, in this stratified gravel and sand, may be seen irregular masses of gravelly clay or hard-pan, comparable to those mentioned by Mr. Lewis at Brooklyn. Such gravelly clay sometimes embraces stones of considerable size. Near the bottom of this stratified gravel and sand there are also, often, upward protrusions of the underlying member of the drift (No. 3), somewhat wedge-shaped or oblique, so as to embrace on the lower side a portion of the stratified gravel and sand. Again, the line of junction between the gravel and sand, and the hard-pan of No. 3, may be marked by an unusual accumulation of coarse drift materials, such as stones and bowlders. These may be mostly surrounded by the gravel and sand of No. 2, or they may be mostly embedded in No. 3. The thickness of No. 2 is exceedingly variable. It is usually less than forty feet in level tracts, but it may be more than a hundred, depending on the duration of the cause that brought it there, and its operation at that point. It sometimes probably entirely replaces No. 3 and No. 4, and lies on the rock. The bowlders found within it are generally not scratched, but sometimes they are scratched, evidently by glacier-action. A great number of glaciated bowlders in this member of the drift have been seen at an excavation near the Falls of St. Anthony.
The following diagrams, Figs. 1 and 2, will express more fully the arrangement of the strata in this member of the drift, and give an idea of the manner of union with the succeeding member below. Fig. 1 is sketched from Nature, and shows a section of the laminated clay exposed in a railroad-cut near Toledo, Ohio:
|a.||Horizontal laminations of fine clay and sand.|
|b.||Oblique laminations of fine clay and sand.|
|c.||Detached masses of hard-pan clay, variously mingled and united with the laminated clay.|
|d.||The upper portion of the great hard-pan sheet.|
Fig. 2 is also sketched from Nature, and represents the alternation of parts as seen in No. 2, and manner of junction with No. 3 at the Falls of St. Anthony. No. 2 here consists of the stratified gravel and sand which constitutes the surface of the drift (immediately below the soil) in large portions of the State of Michigan, Central and Southern Ohio, Northern Indiana, and Central Minnesota. It also forms the principal component of the well-known ridges in Northwestern Ohio, popularly but erroneously styled "lake-beaches." The materials are usually much water-worn, but, at the Falls of St. Anthony, many of the bowlders embraced in No. 2 are conspicuously glacier-marked, a circumstance which plainly indicates the agency which transported and deposited the whole mass in which they occur.
No 3—The great deposit that follows No. 2, whether it be of clay or of gravel and sand, is that designated in common usage "hard-pan." It constitutes the chief member of the drift throughout the Northwest. It is rarely found entirely wanting, whereas the foregoing are very often wanting. It seems to be the parent member of which the former two are offshoots, or modifications. It sometimes has a thickness of more than two hundred feet, and rises to the surface forming the basis of the soil. It consists of a heterogeneous mixture of clay and gravel-stones, with bowlders of northern origin. It is nearly impervious to water, and occasionally, but rarely, shows a rude arrangement in alternating bands, as if, in a plastic state, it had been folded upon itself. Such arrangement discloses no assortment of the
|a.||"Bluff-formation," alluvial, unstratified, mostly sand 6-8 feet.|
|b.||Stratified, fine sand 6-20 do.|
|c.||Gravel and stones, in isolated pockets, unstratified.|
|d.||Bowlders and gravel, the former distinctly glaciated.|
|e.||Hand pan, or "unmodified" drift seen 10 do.|
|f.||Massive, fine sand.|
|g.||Rude arrangement within the hard-pan.|
|h.||Sandy and stony, with rude stratification.|
|i.||Hid from view by sliding sand.|
General section of the drift from toledo, ohio to shelby county, ohio
|a.||No. 2. (clay subsoil).|
|b.||No. 2. (gravel and sand subsoil).|
|c.||Glacial stratification within the hard pan.|
|d.||Hard pan, glacier drift, holding striated bowlders—(No. 3.)|
|e.||Gravel, sand, and bowlders, holding water.—(No. 4.)|
one of them. For example, in Ohio (Northwestern Ohio) it is very close and clayey, the upper part being free from bowlders and stones. In Minnesota (Central Minnesota), the stones and bowlders are more generally disseminated throughout the whole, and it shows much more sand. Hence, in the latter State it will allow the slow passage of water through it—a fact exceedingly fortunate for the agricultural capacity of the soils that are based on it.
Perhaps there should be added to this description of the drifts another member, which may be denominated No. 4.
No. 4.—This, however, is so inconstant and so often runs into the last, that it is hardly worthy of special designation. When present, it lies below No. 3, and immediately over the rock. It consists of gravel and bowlders, more or less mingled with clay. It is the great water-reservoir that supplies deep wells, and through it there is a constant slow drainage into deep valleys and excavations. It gives rise to springs at the base of the river-bluffs, and to artesian wells, when the confining stratum of hard-pan above is penetrated in regions of favorable slope. In Northwestern Ohio is a most wonderful series of artesian wells that depend entirely upon this fortunate combination of circumstances. This member of the drift sometimes consists largely of assorted materials, as sand and gravel. This is indicated by the issuing of considerable quantities of such sand from artesian wells that penetrate it, when the force of the current of water is sufficient to bring it to the surface. Instances have occurred of the collapse of the overlying clay and hard-pan, on the removal in this way of large quantities of sand. Along the upper side of this member, the materials are apt to be firmly cemented by iron and carbonate of lime, into a very firm, even rocky crust. Such cemented gravel and sand is seldom over two feet thick, yet well-borers, on reaching it, often mistake it for the bed-rock. They pass through it, and usually find a copious flow of excellent water. The announcement is then promulgated that that well was drilled through the rock. Hence it is a popular error that, in order to obtain water of the purest quality, it is necessary to sink a well "through the rock." This member, though, is not always found in penetrating the drift to the rock. It may be here stated, also, that, when No. 4 is present, the bed-rock does not show so plainly the marks of glaciation; but, when No. 3 extends to the rock, the surface of the bed-rock is almost invariably marked with the well-known glacier-etchings.
Having taken this survey of the component parts of the drift, it will be well to bring them into a general view, as illustrated by the preceding diagram (Fig. 3), in which the figures represent the same members as in the previous illustrations. It shows a general section of the drift, from Toledo to Shelby County, Ohio, and is based on extended observations, the results of which are to appear in the forthcoming reports of the "Geological Survey of Ohio." The six ridges here represented in profile can be traced from 30 to 130 miles, running into Indiana. Toward the north, in Michigan, they unite by convergence, but one or two being visible in that State. Another article will treat the interesting question of the origin of the drift.