DIFFERENCES IN AGE OF THE VARIOUS GLACIAL DEPOSITS
55. Difference in Degree of Weathering Shown by Deposits of Different Ages
Great differences in age probably exist between the deposits left by the earlier invasions of the ice sheet and those left by later advances. In Iowa, for instance, where all of the invasions mentioned have left deposits, the morainal material deposited by the Kansan advance of the ice sheet has weathered downward to depths varying form 12 to 30 and even 40 feet, and the Iowan and Wisconsin deposits scarcely show any conspicuous signs of weathering at all. [p. 123]
[Photo: View of gullied lands at top of hill country northeast of Hole’s creek bridge, looking
westward from a point on the western side of the main gully. The gully lies southwest of the
abandoned barn on the crest of the Walden ridge.] [p. 124]
By weathering is meant the change in color and the disintegration or decay of the minerals, resulting from exposure to air and water under the influences of considerable changes of heat and cold. The change in color usually is toward yellow or reddish brown. The feldspars in the granites change to kaolin and have a dull whitish instead of a more or less colored and glassy appearance. The hornblendes and biotite micas assume a more or less rusty appearance. Other changes take place, readily enough recognized by the mineralogist, but only crudely explainable to those who have given no previous attention to minerals.
In the older deposits, those of the Kansan invasion, the rocks have changed in color and have disintegrated, so that frequently it is necessary to break them open in order to secure sufficiently unaltered specimens to be able to identify them, while the rocks in the more recent deposits usually preserve their component minerals in a readily recognizable form, even at the surface.
Evidently the Kansan drift must be very much older than the Wisconsin. Recent studies by Frank Taylor would admit the elapse of even as much as 50,000 years since the retreat of the Wisconsin ice sheet began. Considering that this great elapse of time has been insufficient to produce any considerable change in the rocks deposited by the Wisconsin invasion, the deposits of the Illinoian advance must be much older.
But what shall we say of the age of the deposits made by the advance of the Kansan glacier? Weathering down to a depth of 8 feet requires more than twice the time necessary to produce weathering to a depth of 4 feet. Weathering proceeds much more slowly as the distance from the surface increases. Several hundred thousand years can not be too long a time to assign to the earliest deposits left by the continental glaciers.
56. Difference in the Degree of Surface Erosion Shown by Deposits of Different Ages
Differences in age between the deposits of the different advances of the continental glaciers are indicated also by the degree of change which has affected the general topography of the land surface left behind after each of these advances by the retreating ice sheet. [p. 125]
The Kansan morainal deposits, for instance, have been deeply carved by streams since the retreat of the ice sheet from these deposits. In some cases, river valleys more than 100 feet deep and over a mile wide have bee cut into the Kansan deposits. Deep, open ravines, from which branch lateral ravines with gentle gradients, intersect the original morainal deposits in all directions, leaving intermediate areas characterized by rounded hills and ridges. It is the kind of topography known to students of physical geology as mature, with mature streams, valleys, and hills.
Contrast this with the changes to be noticed in the case of the Illinoian deposits. Here steep rain-cut gulches extend only for comparatively short distances, never exceeding several miles, back from the margin of the Illinoian morainal deposits and from the few large river courses. Three-fourths of the area covered by the Illinoian drift is not affected at all by stream action. It is an area of youthful topography.
Still more striking is the difference noticed on approaching the deposits left by the Iowan and Wisconsin advances. Here the glacial deposits usually still preserve their original contours. Very little cutting by stream action is in evidence. The topography is that of extreme youth, from the standpoint of a student of physical geography. No vital change has taken place since its original deposition.
57. The Gravel Ridge Territory South of Dayton Is Characterized by Very Youthful Topography
The territory between Calvary cemetery, Carrmonte, the Moraine farm, and Delco Dell belongs to the area of late Wisconsin deposits. Here also there is evidence only of the most youthful topography. The contours of the hills, ridges, hollows and valleys have suffered scarcely any change in form since they were left by the retreating ice sheet forty thousand or more years ago. There are no rain-cut gulches excepting where started recently by denuding the land of its forest covering. Very few streams have manage to establish themselves as yet, and these have not had sufficient time to carve channels of greater depth than several feet below the natural level of the ground. In fact, the youthfulness of the topography is the most [p. 126] striking feature of this area. But when youthfulness is measured in periods of forty thousand years, what is meant by old age, in geological studies, becomes more impressive.
58. Differences in Age Suggested Also by the Interglacial Deposits
The elapse of time represented by the different interglacial intervals is suggested by the different degrees of weathering shown by the immediately preceding and succeeding glacial deposits, and by the relative amount of change which has affected their surface topography.
The character of the deposits made during the interglacial periods themselves also may suggest different lapses of time. For instance, during the Aftonian interglacial interval, peat beds accumulated in some localities in Iowa to a thickness of 3 feet. The material consists chiefly of tamarack and moss. Many generations of plant growth must have been necessary to produce peat deposits of such thickness. Moreover, as may be seen in the following paragraph, there is evidence of abundant life during this Aftonian interglacial interval. But and abundance of animal life requires an abundance of vegetation, and it must have required many years for this vegetation to establish itself after the retreat of the Kansan ice sheet.
Forests were present also during the Yarmouth and Sangamon interglacial intervals. Their presence is detected chiefly in the peat deposits, but parts of tree trunks may be included in the immediately overlying till.
I have noted already the presence of fragments of wood in the peat deposit one mile west of Germantown, and in the till at the quarries northeast of Beavertown.
The greater thickness of the peat deposits during the Aftonian interglacial interval suggests not only the presence of considerable moisture during this period, but also a relatively greater lapse of time.
59. Differences in Age Suggested Also by the Character of the Animal Life Enclosed in the Interglacial Deposits
Possibly the most interesting evidence of the great age of the Aftonian deposits is given by the nature of the animal life then existent. In recent [p. 127] years, a considerable number of animal remains have been discovered in the Aftonian gravels, in Iowa. These include two or three species of elephant, one or two species of mastodon, one species of camel, two species of horses, and various other less familiar animals, some of large size and belonging to such genera as Megalonyx and Myolodon, names which have no meaning except to students of ancient fossil life.
All of these four-footed animals living during the Aftonian interglacial interval have one characteristic in common. All of the elephants, camels, horses, deer, and other animals then living, even when given familiar names, are different from the elephants, camels, horses, deer and other animals now living. They belong to entirely different species. This itself is an evidence of the great age of the Aftonian deposits.
Some of the Aftonian animals have been replaced by other species more or less closely related to the forms then living. Others have left no descendents, or close relatives, and have become entirely extinct.
On the other hand, during the Yarmouth interglacial interval some of the species of animals which still are in existence made their appearance. These include the wood rabbit and the common skunk. Associated with these more modern animals there still remained, during the Yarmouth interval, many of the more ancient types, which since have become extinct.
In fact, many of the animals now extinct must have persisted until quite recent times, geologically speaking, since their remains occur in swamps resting on the top of the late Wisconsin morainal deposits. Their age is at least more recent than these deposits. To this group of animals, now extinct, but persisting at least until the close of the late Wisconsin advance, belong the elephant and mastodon, and other less familiar animals, such as the megalonyx, tapir, peccary, musk-ox, and giant beaver.
60. The Methods of Identifying Fossil Animals
When we say that we have found the remains of some animal that lived many thousand years ago we mean merely that we have found the bones or part of the bones belonging to the skeleton. The other parts almost invariably decay. [p. 128]
In the frozen soils of northern Siberia, the bodies of the large elephant known as the hairy mammoth have been found with the flesh so well preserved that it was eaten by dogs. The most instructive specimen was found on the banks of the Beresowka river, in eastern Siberia. Its remains were taken to St. Petersburg. It appears that this animal, while quietly grazing, fell into a hole in the treacherous soil, and was injured so badly that it died within a short time. The body was soon frozen, and was covered by ice and soil so at so remain in practical cold storage for thousands of years. So well was it preserved that even its internal organs could be dissected and studied. The whole body was covered with yellowish brown hair, that on the breast and sides of the neck being long. The finer hair or wool, from 2 to 4 inches in length, formed a dense coat over the entire body, and among this wool grew the courser and much longer hair, some of it attaining a length of 20 inches. The stomach was filled with the food the animal had just eaten and between the jaws was a mass of material which it had not had time to swallow before it died. Even the food in the stomach was preserved sufficiently well by the cold to permit the identification of many of the plants which the mammoth had eaten.
Various persons have claimed also to have found hair associated with the bones of another elephant know as the American mastodon. The hair decays much later than the flesh.
It has been asserted also that parts of the skin of prehistoric animals have been found associated with the skeletons where these had been mired in salt licks, or swampy places impregnated with salt, in which case the salt acted as a preservative.
As a rule, however, the remains of prehistoric animals occur only in the form of skeletons. If these skeletons always were complete, the identification of the animals frequently would be comparatively easy. As a rule, however, only parts of skeletons, and sometimes even only isolated bones or teeth are preserved. In these cases the identification of the remains is much more difficult, and requires the skill of those specialists who call themselves osteologists. These men, almost always connected either with some college or with some museum, make such a study of each bone of each animal that they frequently can identify an animal by a single [p. 129] bone. In such cases the bone possesses some characteristic not possessed by any other animal. Even when the exact identity of the animal can not be established, the group to which it belongs often can be determined. For instance, the animal may be identified as belonging to the group of cats, or horses, or deer, without determining the exact animal, since the group of cats includes the lion, tiger, leopard, jaguar, and many other animals, in addition to the ordinary cat. The group of horses includes also the ass, and zebra. The group of deer includes also the moose and elk and other animals. In the case of each bone there is some point at which the nerves of blood-vessels enter, some ridge to which the muscles are attached, some smooth surface at the joint, always in the same place and of the same form in the same bone of the same type of animal. By means of these characteristics, when known, the animals are identified.
Frequently, however, it is not necessary to identify an animal from a single bone. In fact, all the more satisfactory identifications are based upon the presence of several bones, sometimes of almost all the bones of an animal. The fact to be emphasized is that in the hands of an expert much can be learned regarding the identity of an animal even from a single bone. The teeth frequently serve as excellent means of identification, for instance, in the case of elephants, horses, deer, and various other animals. Perhaps the most satisfactory single part of the skeleton is the skull. The skull, of course, is composed of many bones, but these are so firmly grown together and possess so many peculiarities in different animals that the finding of a single skull almost invariably is sufficient to identify the animal with precision.
61. Where Prehistoric Animals Are Found
When animals die on the fields or in the woods, all except the skeleton soon decays, but even the skeleton disappears within a comparatively short time. You may have noticed that you do not often come across the skeletons of animals in the woods although animals are continually dying, and only a few ever are buried. Only in very dry deserts do skeletons remain when exposed on the surface for many years. [p. 130]
When buried in the ground, especially in clayey soil, through which water can not readily circulate, skeletons are preserved much longer. When mired in bogs, so as to become covered with the mud, especially if the bogs are very peaty, skeletons may be preserved many years
In general skeletons are preserved for thousands of years when embedded in clayey soil or mud, far from the changeable influences of the weather. There are comparatively few such places. They consist chiefly of swamps, bogs, lakes, and the estuaries or deltas of rivers. Here the animal after sinking to the bottom has a chance of being covered by the mud, or sand, and it is chiefly in places which once were muddy or sandy that the remains of prehistoric animals are found. The enormous prehistoric reptiles, of which you may have read, which are found in the western states, occur chiefly in the deltas or estuaries along the shores of prehistoric lakes, which long ago have disappeared.
62. The Length of Time Involved in the Advance and Retreat of the Ice Front During the Wisconsin Invasion
In a recent paper read before the Geological Society of America, Mr. Frank Taylor presented the following estimates of the time involved in the closing episode of the history of the continental ice sheet. If about 40,000 years be estimated at the time necessary for the advance of the continental ice sheet, during the Wisconsin invasion, to its most southern limits, followed by 40,000 years during which the ice front retreated until it exposed the entire area now covered by the Great Lakes, and if to this be added 10,000 years for the time from the beginning of the Wisconsin invasion to the present day. Of the period of retreat, about 8,000 years are assigned as the time necessary for the Miami lobe to have melted back from the vicinity of Cincinnati as far as Defiance, in northern Ohio. This was followed by a period of 17,000 years during which the ice melted as far back as Port Huron, in Michigan, and the earlier glacial lakes in the Erie basin were formed. Next came a period of 15,000 years, during which the an- [p. 131] cient glacial lake Algonquin covered the present sites of Lakes Superior, Michigan, and Huron, finding its outlet for most of this time chiefly by way of Georgian bay and the Trent channel into the basin of the present Lake Ontario. Of the time which has elapsed since the melting back of the ice sheet sufficiently to permit the discharge of the lake waters into the St. Lawrence river, about 6,500 years belong to the history of the Nipissing lakes, including about the same territory as Lake Algonquin, but emptying by way of the northern end of Georgian bay and the Ottawa river channel instead of by way of the much more southern Trent channel, and Mohawk river. For more than 3,000 years the Great Lakes are supposed to have had their exits very much as at the present day.
This estimate of a total of 90,000 years for the lapse of time since the beginning of the early Wisconsin invasion may be matched by another presented by Professors Chamberlin and Salisbury, of Chicago University, who estimated the time which has elapsed since the climax of the late Wisconsin invasion as somewhere between 20,000 and 60,000 years, while the climax of the early Wisconsin was attained between 40,000 and 150,000 years. By the climax of these invasions is meant their greatest distance south. In the case of the early Wisconsin, this distance farthest south extended to Hartwell, a few miles north of Cincinnati. The greatest distance south in the case of the late Wisconsin that the gravel ridges south of Dayton were formed. These estimates do not take into account the time used by the invasions to reach their most southern boundaries during their periods of advance, but only the time occupied during their retreat.
63. The Duration of the Glacial Ice Age
Chamberlin and Salisbury estimate the times elapsed since the various glacial invasions of earlier age than the Wisconsin as follows:
Since the climax of the Iowan…. 60,000 to 300,000 years
Since the climax of the Ilinoian…140,000 to 540,000 years
Since the climax of the Kansan…300,000 to 1,020,000 years
Since the climax of the Pre-Kansan…A still greater number [p. 132]
Very little value is placed on these estimates by the professors cited beyond the fact that they give some sense of the proportion involved in the different time periods.
64. Distance of Retreat of the Ice Front During Interglacial Periods
In the case of the Alpine glacier examined by Tyndall, the rate of motion of the central part of the ice reached a maximum of 37 inches per day. Estimating the rate of motion of the continental ice sheet in crossing the comparatively level state of Ohio at one foot per day, it would require about 2,440 years to travel from the northern boundary of Ohio to the most southern point reached by the early Wisconsin invasion, near Hartwell, north of Cincinnati, and 2,200 years to reach Germantown, at the southern margin of the late Wisconsin invasion. These numbers, however, make no allowance for any melting back at the glacial ice front, which must have enormously retarded the rate at which the ice front advanced, since the latter depends not only upon the rate of motion of the ice, but also upon the rate at which the ice front is melted back. Evidently the advance of the ice front was much less on the average than 365 feet per year. If one-tenth of this rate of advance were considered ample, then 24,400 years would be required for the ice to advance across the state as far as Hartwell, and 22,000 years to reach Germantown. Neither estimate makes allowance for the time occupied by the retreat of the ice front during interglacial times. It seems scarcely likely that the continental glacier disappeared entirely from our continent between the Iowan and Wisconsin, or between the early and late Wisconsin invasions. In fact, it is improbable that during these intervals the ice front retreated even over an interval as great as that represented by the state of Ohio. Comparatively moderate retreats would be sufficient to account at least for what is known of these later interglacial periods. [p. 133]
[Photo: View from Eastview ridge, on the Moraine farm, across the Locust farm valley. The Locust farm buildings are on the extreme right and the National Cash Register Club buildings are seen on the left. This part of the valley is drained by the stream passing through the Ex-Governor Cox farm.] [p. 134]
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