Public domain
[p. 931]
The end of the glacial period. The termination of the glacial period is usually placed at the time when the ice-sheets disappeared from the lowlands in the middle latitudes of Europe and North America. Notwithstanding this conventional usage, it is to be noted that the ice-sheets had not then completely disappeared, and have not even now, for about 10% of the recently glaciated area of North America (chiefly in Greenland) is still buried in ice. These relics of the last glacial epoch show that the continent has not yet emerged completely from the glacial period.
Future glaciation. It is not absolutely clear that there may not be another increase of ice before the long series of glacial epochs closes, but the probabilities seem to be much against it. The declining series of oscillations already noted seems to have reached its last term, yet the factors that produce glaciation are too complex to warrant more than a comfortable presumption of future immunity until another great deformation shall have taken place.
The end of the deformation period. It is not wholly clear that the deformative period which began in the late Tertiary, and extended through the Pleistocene, is yet completed. We are accustomed to regard it as essentially passed, notwithstanding some movements still in progress; and, in the main, this position seems to be justified.
The movements of post-glacial times, however, are not to be ignored. The northeastern part of North America has been elevated relatively since the disappearance of the ice (p. 912). This relative rise is perhaps a reaction from the depression due to the weighting and cooling caused by the ice-sheets.
A recent movement in the region of the Great Plains seems to be suggested by certain physiographic features. Extensive tracts [p. 932] in central Kansas and Nebraska hear an aspect of pronounced topographic youth, suggesting that they have been lying, until recently, near the neutral horizon between erosion and deposition, and have lately been raised on the western side. In the Dakotas, there are broad gradation plains of abandoned river-courses which cross the present valley of the Missouri River. Their present gradients, and their elevation above the present river-bottoms of the region imply a westward elevation. These and collateral phenomena, taken with the remarkable movement of the Keewatin ice-sheet from what is now the lower to what is now the higher side of the plains, seem best satisfied by the view that until about the close of the Glacial Period the western side of the Great Plains was lower than now, relatively, or the eastern side higher. On the western side of the continent there is much evidence of recent movement, some of which appears to have taken place since the close of the Glacial Period, as usually defined. Similar phenomena are found in other continents.
It is not wholly clear, therefore, whether the present is to be regarded as a part of that period of deformation which had its climax in the Pliocene, of whether it is rather the initial stage of a period of quiescence now being entered upon.
The suggestions of existing physiography. The view that the earth is now passing into a period of quiescence is strengthened by the present physiographic features of the earth’s surface. These are an inheritance from the Tertiary deformations superposed upon pre-existing configurations, though they have been modified by gradational agencies since. They should tell us whether the face of the earth is that of a planet in the midst of deformation, or that of one recently deformed, and now in a more quiescent stale. Every stream should show whether it has just been rejuvenated, or has done some notable work since it was rejuvenated. Every coast should show whether the continental border stands forth in the manner characteristic of an earth-segment just crowded up by a deformative thrust, or whether it has made some not able prog in settling back, or in being cut back, to an inter-deformative state.
Most of the streams of the continents show that they have had time to do some appreciable work since they were rejuvenated. [p. 933] Falls which owe their origin to the deformations of the recent deformative period abound on all the continents, but most of them have canyons below, showing that the falls have been receding for a long time. The falls and canyons are often so related to slack water below as to show that the rejuvenating process stopped some time ago. The Falls of the Columbia, Congo, Zambesi, Brahmaputra, Yang-tse, and of a multitude of other rivers descending from the elevated portions of the continents, are illustrations in point. If the various criteria of topographic age are applied to the face of the continents, it will be seen that, while they betray, very generally, evidences of rejuvenation by deformation in relatively recent times, there is very little to indicate rejuvenation now in progress. The most declared evidences of topographic youth are in regions recently abandoned by the ice-sheets of the last glacial stage.[1]
If attention is turned to the borders of the continents, significant evidence is found in the fact that the real edges of the continents are almost everywhere submerged. They generally lie 100 fathoms or so below sea-level; that is, continental shelves almost universally border the continents. An area of 10,000,000 square miles, more than 15% of the true continental surface, is thus submerged. The submergence of the shelves took place so recently that they are marked by trenches, valleys, and embayments referable to rivers that formerly crossed them. These features imply that the continental shelves were out of water recently, and that rivers then reached the true borders of the continental platforms. They also imply a recent (if not present) general movement toward continental submersion.
The channels on the continental borders. The conclusion has generally been that the coastal tracts affected by valleys once stood high enough to allow streams to excavate the valleys. As commonly interpreted, this means that the tracts have in recent times stood some thousands of feet higher than now, and the submerged valleys have been much appealed to in support of the elevation hypothesis of glaciation. The submerged valleys of the continental [p. 934] shelves have occasioned much discussion.[2] To the view that the continental borders stood so high in recent times there are obvious objections. One of them is the difficulty of disposing of the water of the ocean when all the continents were lifted some thousands of feet. Another is the fact that some of these valleys descend into closed basins such as the deeper parts of the Mediterranean and Caribbean seas and the Gulf of Mexico, which must be supposed to have retained so much of their water as lies below the lowest notch in their rims.
The views of deformation here entertained afford a different mode of interpretation, in which lateral movement plays a larger part, and vertical movement a lesser part. This view cannot be elaborated here, but some of its elements may be suggested.
It is conceived that the continental protuberances, which stand up some three miles, on the average, above the sea-bottom, may have a movement somewhat akin to that of great ice-sheets, though much slower, and that they tend to creep slowly out toward the lower ocean basins. If such a movement took place, it would tend to carry the valleys of the coastal lands out under the sea. It is conceivable that the submerged valleys arose in this way. This conception of the behavior of continental borders is too new to be accepted without reserve, but if it is true, it helps to explain many difficult problems connected with the coasts.[3]
However the submerged valleys originated, it seems remarkable that they have not been filled with sediment, for the rivers must have been carrying detritus, and littoral currents must have swept drift into the channels. The efficient agent in keeping the valleys unfilled was possibly the tides. Their ebb and flow, particularly where the river-mouth broadened to an estuary, doubtless scoured the channel, and not improbably enlarged and deepened it where the configuration of the surroundings favored. Tides would seem to be especially effective in this work at the edge of the continental [p. 935] shelves. It seems not improbable that valleys in the outer edge of the continental shelf, and on the abysmal slope, are even deepened and widened by tidal scour.
In the seas, and on the land in the tropics, the life of the Pleistocene period appears to have passed by imperceptible gradations into that of the present. In the higher latitudes, the transition was marked by two exceptional features, the re-peopling of the lands laid waste by the ice-incursions, and the invasion of the human race. Whatever may have been true in the low latitudes where the human race perhaps came into ascendancy gradually, the appearance of man in the higher latitudes was an invasion, and from the point of view of other organisms, it was an irresistible inundation.
The re-peopling of the glaciated areas. The re-peopling of the northeastern half of North America by plants and animals after the retreat of the last ice-sheet was a great event of its kind. Certain plants that abounded in Europe before the glacial period were forced across the Mediterranean, or southeastward into Asia, and did not recross the barriers of water and desert when the climate of Europe, became mild again. No such barrier intervened in North America, so that the problem of re-peopling can be studied to better advantage in our continent than in Europe. There was, however, an ill-defined climatic barrier between the arid plain region of the southwest and the humid forest region of the southeast. There is abundant evidence that open plains and arid climates had developed in the middle latitudes of the west by the later part of the Tertiary, and that these have persisted, perhaps with brief interruptions, till now. The pre-glacial arid tracts seem to have had a distribution in the western part of our continent not unlike that of to-day, while the eastern half of the continent was then, as now, more moist, and covered with forests rather than herbaceous vegetation.
With the oncoming of the ice-sheets, the floras and faunas were driven southward, as described in the history of that period. [p. 936] In the west, the northern life was driven by ice behind, hemmed in by mountain and other barriers at the sides, and resisted by arid tracts in front. The arid tracts were themselves shifted in some measure, but the restraint of migration to the east and west became increasingly formidable as glaciers gathered on the mountain heights and occupied the passes. As the trend of the mountains was mainly north and south, they denned a series of meridional tracts which directed the life migrations. Even east of the mountains, climatic differences seem to have appreciably restrained east-west migration.
In the eastern half of the continent, the forests and forest-life were driven southward in a more unrestrained way, but for the greater part they kept within the eastern humid tract.
Following the last ice-retreat, the life of each of these sections moved northward, each biotic zone, arctic, subarctic, cold-temperate, and temperate, expanding as it went. It was as though the life-zones were elastic bodies which had been compressed to narrowlimits about the edge of the advancing ice, and then recovered their normal breadth by expansion northward as the ice withdrew. The arctic or tundra flora and fauna that had probably been crowded into a narrow zone fringing the ice-sheet, moved northward through about 20° of latitude, and expanded to a breadth of 600 or 700 miles in the northern part of the continent, and occupied the arctic islands where not covered by perennial ice and snow. The zone of this arctic flora and fauna now lies mostly north of 60°. The subarctic zone of stunted conifers moved northward about 12°, and expanded into a zone some 400 to 600 miles wide. The coldtemperate belt of deciduous and evergreen trees moved a less distance, but expanded almost equally, while the warm-temperate flora spread itself over the territory abandoned by the last.
With each of these vegetal zones went the appropriate fauna. The musk-ox, whose remains have been found skirting the glaciated area in Pennsylvania, West Virginia, Ohio, Kentucky, Oklahoma. Missouri, and Iowa,[4] has since retired to the extreme arctic regions. The reindeer, which had a similar distribution about the edge of [p. 937] the ice, occupies the barrens of the northern border of the continent; while the fur-clothed animals distributed themselves through the three northerly zones, most notably the subarctic zone of the conifers.[5]
The westward spread of these floras and faunas of the southeastern regions seems to have been meager. In the west, the southwestern arid and prairie floras and faunas seem to have had the better of the contest with the forest forms, and to have spread eastward in the mid-latitudes at the expense of the southeastern group; at least arboreous vegetation is found appreciably farther west in interglacial deposits than on the present surface. This does not seem to be equally true in the higher latitudes, where the trees of the eastern group are distributed far to the northwest.
The arid and semi-arid floras and faunas of the southwest seem to have pushed the more boreal and arboreous forms to the northward, or forced them to ascend the mountains; but the movement was less sweeping and more complicated than that of the east, because of topographic interference and the effect of the lingering mountain glaciation.
In this re-dispersion of the North American faunas and floras there is a world of suggestive detail of which only a small part has been worked out into clear definition. From the viewpoint of investigation, it is a rich and almost virgin soil, forming the turn-row, as it were, between the more cultivated fields of the geologic and biologic sciences. Among other things, it seems not improbable that studies on the rate of migration of plants to the northward, after the last glaciation, may afford a basis for estimating the length of post-glacial time.
The Dynasty of Man
Human dispersal. As yet there is little geological evidence relative to the place of man’s origin, or to the earliest stages of his development. Various considerations connected with his physical nature and his distribution seem to point to the warm zone of the [p. 938] eastern hemisphere, perhaps southern Asia or northern Africa, as the place of his appearance. There are some grounds for the inference that the earliest developments of those qualities that gave him dominance were associated with the open tracts of the sub-tropical zone, rather than with the forests of the equatorial belt. Subsequent history, as well as the nature of the case, teach us that extreme desert conditions and excessive heights are prohibitive, that semi-arid conditions of varying and precarious intensities lead to nomadic habits, sparse distribution, and limited social and civic evolution; while well-watered plains and fertile valleys, under congenial skies, invite fixed habitation and the development of stable civil and social institutions. Excessive humidity, dense forests, extreme ruggedness of surface, tend to limitation and repression among primitive peoples. Early in the history of the race, it is presumed that a warm climate was more favorable than a severe one. From these considerations and from historical evidence arises the presumption that the primitive centers of virile evolution and radiation of the race lay somewhere in the open or diversified parts of the warm tract of the largest of the continents. From this, or from some analogous tract in that quarter of the globe, there seem to have been divergent movements to all habitable lands of the earth.
A basal factor in the early evolution of civilization was the productiveness and availability of the soil. The passage from the condition of hunters and fishers, scattered in adjustment to the distribution of game and shifting with its changes, or from that of simple herders in sterile tracts roaming with the changes of pasture, was dependent essentially on agriculture, and was therefore influenced largely by the fertility of the soil and suitable climatic conditions. And so, conversely, among the agencies that have forced the migration of centers of civilization, loss of soil-fertility is one of the more important. In the lower latitudes, the upland soils are usually the residue left by the decomposition of the underlying rocks which has not been removed by surface-wash. With cultivation, wash and wind-drift are accelerated, and unless protective measures are employed, as has not been the case usually. the soils are swept away; and barrenness succeeds productiveness. [p. 939] There are areas in the Orient, once well settled, where nothing grows except such plants as find a foothold in the crevices of the rock. Soils with sandy subsoils have been washed away, leaving barren wastes, and the sands derived from the denuded subsoil have been driven by the winds over adjacent fertile tracts, and by burial have included them in the common waste. The explanation of much of the former richness and present poverty of Oriental peoples no doubt lies in this simple process. This impoverishment of soil threatens many peoples to-day, and is in process of actual realization.
Glaciated lands are comparatively new fields for civilization, and the soil-factor there has a character quite its own. 1. Near the centers of glacial radiation, the old soils were borne away, and new soils have not always developed in equal amount in their stead. A reduced fertility is the result. The half -decayed rock below was largely worn away, and a long period must ensue before a new soil will have become effective. These areas lie chiefly in high latitudes where other factors do not favor human development. 2. In regions of glacial deposition, which fortunately include the greater and the more southerly parts of the glaciated area, a deep sheet of comminuted rock-material, ready for easy conversion into soil by weathering and organic action, covers great plains, and has a gentle relief that aids in restraining its removal. In the peripheral belt of the glaciated area in North America, for a width of 400 or 500 miles, the subsoil of glacial flour and old soil, glacially mixed, has an average thickness of about 100 feet. A similar statement may be made of a large area in north-central Europe. The average thickness of the residuary soils of unglaciated regions similarly situated is about 5 feet. The twenty-fold provision for permanent fertility thus arising from glaciation seems likely to be a factor of importance in the localization of the basal industry of mankind, and of the phases of civilization that are dependent on it.
With the evolution of the industrial arts, resources which were neglected at first have come to play important parts in the distribution and in the activities of the race, among which are the long and growing lists of mineral resources. Chief among these are the metallic ores, the fossil fuels, the mineral fertilizers, and the structural [p. 940] and ornamental materials of stone and clay. These now influence man’s distribution and activities far more than formerly, and they are quite certain to be more influential still in the future.
The distribution and activities of men have also recently come to be affected by the distribution of rejuvenated streams that arose from the deformations of the late Tertiary periods, and by the stream-diversions of the glacial period, both of which have furnished sources of water-power heretofore neglected in the main. With little doubt, such native sources of power are to play an increasingly large part in human affairs as time goes on and the stored fuels are exhausted.
With the increasing complexity of human activities, the localization of the race will more and more depend on combinations of resources and of conditions; but it is difficult to see beyond the day when persistent fertility of the soil, under favorable climatic conditions, co-ordinated with great supplies of ores, fuels, and structural materials, will not constitute a decisive and controlling advantage.
Provincialism giving place to cosmopolitanism. The early history of human dispersal was marked by pronounced provincialism. The early peoples were much isolated by distance and by natural barriers, and they often interposed artificial barriers against free inter-communication, and hence against the development of a common cosmopolitan type. So long as hunting and fishing were the dominant pursuits, a wider and wider dispersion into small tribes was a necessary tendency, which was abetted by conflict of interests. That such artificial sources of provincialism were more effective than natural ones seems to be implied by the fact that while physiological differences sufficiently marked to readily characterize varieties were numbered by hundreds, dialects sufficiently different to prevent free intercourse were numbered by thousands. Provincial sentiment to-day manifests itself more conspicuously in language than in most other ways. The tendency to provincialism. however, has never gone so far as to divide the race into distinct species.
When efficient water-transportation was developed and the control of the sea attained, a period of cosmopolitan tendency was inaugurated. This has been greatly accelerated in the last few [p. 941] decades, supplemented by swift land-transportation and electric communication, and is rapidly involving the whole race in a cosmopolitan movement. Almost the whole world is already in daily communication, and almost all the races are more or less habitually intermingling by travel and trade. That this is to become more and more habitual until the whole race shall be in constant intercommunication, is not to be questioned. There will then have been inaugurated the most marked period of cosmopolitanism, in all senses of the term, which the world has ever witnessed. What all this will ultimately mean for the race we do not venture to predict.
Man as a geological agency. The earlier geologists were inclined to regard man’s agency in geological progress as rather trivial, perhaps because physiographic geology, in which his influence is chiefly felt, was then less cultivated than other phases with which he has little to do. The fact probably is that no previous agent, in an equal period of time, has so greatly influenced the life of the land, or the rate of land-degradation, as man has since the present agricultural epoch was well established. That this influence will be increased during coming centuries seems clear. The flora is rapidly passing from that which had been evolved by natural agencies through the ages, to that which man selects for cultivation or preservation, together with that which has taken advantage of the special conditions he furnishes. With the further progress of this movement, the native floras seem destined to an early extinction. The same may be said of the native faunas. The favored animals, under man’s care, flourish beyond precedent, while the others, so far as they are within his reach, are suffering rapid declines that look toward extinction. The life of the sea is less profoundly affected than that of the land, but even that does not escape modification. The most pronounced exceptions to man’s dominance, and those that bid fair to contest his supremacy longest, are found in organisms too minute to be easily controlled by him, and in organisms that, quite against his wish, flourish on the conditions he furnishes. But even the accelerated evolution of these organisms is a part of the profound biological revolution which attends man’s dominance.
[p. 942]
Man’s control has not thus far been characterized by much recognition of the complicated interrelations of organisms and of the consequences of disturbing the balance in the organic kingdom, and he is reaping, and is certain to reap more abundantly, the unfortunate fruits of ignorant and careless action. For the greater part, man has been guided by immediate considerations, and even these not always controlled by much intelligence. Thus great wantonness has attended his destruction of both plant and animal life. But a more intelligent as well as a more sympathetic attitude is developing, and will doubtless soon become dominant. A new era in control and in evolutionary selection is dawning. New varieties and races are being produced that not only depart widely from the parent stock, but diverge in lines chosen to meet given conditions, or to produce desired products. How far this may yet go it is impossible now to predict.
Prognostic Geology. The long perspective of the past should afford at least some suggestions of the future, but it must be confessed that the most important conjectures as to the future are dependent on interpretations of the past that are not yet certain. A word has been said relative to a possible return of a glacial epoch, but no sure prediction can be made. Question has been raised as to whether the deformations of recent times are over, but the answer remains uncertain. The duration of the earth as a habitable globe has been a common theme of prognosis. A final refrigeration as the result of the secular cooling of a once molten globe has been the usual forecast, and the final doom of the race has been a favorite theme for pseudo-scientific romances. But this all hangs on the doctrine of a former molten earth, if not on the doctrine of its origin from a gaseous nebula. Under the alternative conception of a slow-grown earth conserving its energies, conjoined with a more generous conception of the energies resident in the sun and the stellar system, no narrow limit need be assigned to the habitability of the earth. A Psychozoic era, as long as the Cenozoic or the Paleozoic, or an eon as long as the cosmic and the biotic ones, may quite as well be predicted as anything less. The forecast is at best speculative, but an optimistic outlook seems more likely to prove true than a pessimistic one. An immeasurably [p. 943] higher evolution than that now reached, with attainments beyond present comprehension, is a reasonable hope.
The forecast of an eon of intellectual and spiritual development comparable in magnitude to the prolonged physical and biotic evolutions, lends to the total view of earth-history great moral satisfaction, and the thought that individual contributions to the higher welfare of the race may realize their fullest fruits by continued influence through scarcely limited ages, gives value to life and inspiration to personal endeavor.
Jour. Geol., Vol. XII, p. 707. ↩︎
Spencer, Bull. Geol. Soc. Am., Vols. VI (pp. 103-140) and XIV (pp. 207-226); Am. Jour. Sci., Vol. XIX, 1905, pp. 1-15; and Am. Geol. XXIV, 1904, pp. 110-111; Hall, Trans. Victoria Inst., 1897 (pp. 305 324); and 1900 and 1902, and Geog. Jour., 1899; Nansen, Rep. Arc. Expl., p. 232; Davidson, Proc. Cal. Acad. Sci., Vol. I, 1897, pp. 73-103. ↩︎
For fuller statement of the hypothesis see the larger treatise of the authors, Vol. Ill, pp. 519-530. ↩︎
Hay’s Catalogue of Fossil Vertebrates in North America, Bull. 179, U. S. Geol. Surv., 1902. ↩︎
Some of these and other features are suggestively discussed by C. C. Adams, The Post -Glacial Dispersal of the North American Biota, Biol. Bull., Vol. IX, 1905, pp. 53-71. ↩︎