At about the time when the subaërial origin of valleys and escarpments was being established in England, the explorations and surveys of our western territories were undertaken, and a flood of physiographic light came from them. One of the earliest and most important of the many lessons of the West was that Playfair’s law obtained even in the case of the Grand canyon of the Colorado, which was visited by the Ives expedition in 1858. Newberry, the geologist of the expedition, concluded that both the deep and fissure-like canyon and the broader valleys enclosed by cliff-like walls “belong to a vast system of erosion, and are wholly due to the action of water.” Although he bore the possibility of fractures constantly in mind and examined the structure of the canyons with all possible care, he “everywhere found evidence of the exclusive action of water in their formation.” This conclusion has, since then, been amply confirmed by Powell and Dutton, although these later observers might attribute a significant share of the recession of cliffs in arid regions to wind action. In a later decade, Heim demonstrated that the valleys of the Alps were not explicable as the result of mountain deformation, and that they found explanation only in river erosion. By such studies as these, of which many examples could be given, the competence of rivers to carve even the deepest valleys has been fully established; yet so difficult is it to dislodge old-fashioned belief that Sir A. Geikie felt it necessary to devote two chapters in his admirable ‘Scenery of Scotland’ (1887) to prove that the bens of the Highlands were not so many individual upheavals, but that the glens were so many separate valleys of erosion; and as able an observer as Prestwich, a warm advocate of the erosion of ordinary valleys by their rivers, maintained (1886), with the results of our western surveys before him, that fissures were probably responsible for the origin of the deep and narrow canyons of the Colorado plateau.
The tumultuous forms of lofty mountains ‘tossed up’ as they seem to be when viewed from some commanding height, are, in by far the greater number of examples yet studied, undoubtedly the result of the slow erosion of the valleys between them; but it should not be forgotten that regions of very recent disturbances—as the earth counts time—may possess strong inequalities directly due to deformation. The tilted lava blocks of Oregon have already been mentioned. The bold forms of the St. Elias Alps, also described by Russell, are regarded by him as chiefly produced by the tilting of huge crustal blocks on which erosion has as yet done relatively little work. An altogether exceptional case is described by Dutton, who says that on the margin of one of the “high plateaus of Utah a huge block seems to have cracked off and rolled over, the beds opening with a V and forming a valley of grand dimensions.” ‘Rift valleys,’ or trough-like depressions produced by the down-faulting of long, narrow, crustal blocks with respect to the bordering masses, are occasionally found, as in eastern Africa, where the ‘Great Rift valley’ has been described by Gregory. Trough-like depressions of similar origin, but much more affected by the degradation of their borders and the aggradation of their floors, are known to European geographers in the valleys of the Saône and of the middle Rhine. But no rift valley, no depression between the tilted lava blocks, resembles the branching valleys that are produced by the erosive action of running water.
Thus far, while much attention had been given to the work of rivers, little or no attention had been given to the arrangement of their courses. It seems to have been tacitly assumed that the courses of all streams were consequent upon the slope of the initial land surface. The explicit recognition of this origin, indicated by the provision of a special name, ‘consequent streams,’ was an important step in advance due to our western geologists. The discovery soon followed that rivers have held their courses through mountain ridges that slowly rose across their path; the rivers, concentrating the drainage of a large headwater region upon a narrow line, cut down their channels as the land was raised. This idea first came into prominence through Powell’s report on the Colorado River of the West (1875), in which he gave the name, ‘antecedent,’ to rivers of this class. He believed that the Green river, in its passage through the Uinta mountains, was to be explained as an antecedent stream. Much doubt has, however, been thrown upon this interpretation. Other accounts of antecedent rivers have been published, and to-day the Green is not so safe a type of antecedence as the Rhine below Bingen, the Meuse in the Ardennes, or several of the Himalayan rivers in the gorges that they have cut through the youngest marginal ridges of the range.
Rapidly following the establishment of these two important classes of valleys came the recognition of the very antithesis of antecedent rivers in those streams which have grown by headward erosion along belts of weak structure, without relation to the initial trough lines. To these the term ‘subsequent’ has been applied. It is frequently in association with streams of this class that drainage areas are rearranged by the migration of divides, and that the upper waters of one river are captured by the headward growth of another. This is accomplished by a most beautiful process of inorganic natural selection, which leads to a survival of the fittest and thus brings about a most intimate adjustment of form to structure, whereby the more resistent rock masses come to constitute the divides, and the less resistent are chosen for the excavation of valleys. Many workers have contributed to the solution of problems of this class; notably Heim, in his studies of the northern Alps (1876), and Löwl, who showed that, in folded mountain structures of great age, the original courses of streams might be greatly altered through the development of new lines of drainage (1882). A valuable summary of this subject is given by Philippson in his ‘Studien über Wasserscheiden’ (1886). The extraordinary depredations committed by the waxing Severn on the waning Thames have recently been set forth by Buckman. The turning of side branches from the slender trunk of the Meuse has been recognized in France. Many remarkable instances of stream captures have been found in the Appalachians, where the opportunity for the adjustment of streams to structures has been exceptionally good. Hayes and Campbell have, on the other hand, emphasized the importance of drainage modifications independent of the growth of subsequent streams on weak structures, but governed by a slight tilting of the region, whereby some streams are accelerated and their opponents are retarded. It should be noted that the proof of the adjustment or rearrangement of drainage marks a victory for the uniformitarian school that is even more significant than that gained in the case of the antecedent rivers; for in one case a growing mountain range is subdued by the concentrated discharge of a large drainage area; but in the other case, the mountain slowly melts away under the attacks of the weather alone on the headwater slopes of the growing valleys.
The reason why all these studies of land carving are of importance to the geographer is that they greatly enlarge the number of type forms that he may use in descriptions, and that they recognize the natural correlations among various forms which must otherwise be set forth in successive itemized statements. The brief terminology learned in early school days, somewhat enlarged by a more mature variety of adjectives, is usually the stock of words with which the explorer tries to reproduce the features of the landscapes that he crosses, and as a result his descriptions are often unintelligible; the region has to be explored again before it can become known to those who do not see it. The longitudinal relief of certain well-dissected coastal plains, or the half-buried ranges of certain interior aggraded basins, may be taken as examples of forms which are easily brought home and familiarized by explanation, but which commonly remain remote and unknown under empirical description.
It may be urged that in many geological discussions from which geography has taken profit, consideration is given to form-producing processes rather than to the forms produced. This was natural enough while the subject was in the hands of geologists; but geographers should take heed that they do not preserve the geological habit. The past history of land forms and the action upon them of various processes by which existing forms have been developed, are pertinent to geography only in so far as they aid the observation and description of the forms of to-day.
Further illustration of the growing recognition of form as the chief object of the physiographic study of the lands is seen in the use of the term, ‘geomorphology’ by some American writers; but more important than the term is the principle which underlies it. This is the acceptance of theorizing as an essential part of investigation in geography, just as in other sciences. All explanation involves theorizing. When theory is taken piecemeal and applied only to elementary problems, such as the origin of deltas, it does not excite unfavorable comment among geographers. But when the explanation of more complicated features is attempted, and when a comprehensive scheme of classification and treatment, in which theorizing is fully and frankly recognized, is evolved for all land forms, then the conservatives recoil, as if so bold a proposition would set them adrift on the dangerous sea of unrestrained imagination. They forget that the harbor of explanation can only be reached by crossing the seas of theory. They are willing to cruise, like the early navigators, the empirical explorers, only close along shore; not venturing to trust themselves out of sight of the land of existing fact; but they have not learned to embark upon the open ocean of investigation, trusting to the compass of logical deduction and the rudder of critical judgment to lead them to the desired haven of understanding of facts of the past.
One of the bolder explorers of the high seas of theory is Powell, who defined in the term ‘baselevel’ an idea that had long been more or less consciously present in the minds of geologists, and which has been since then of the greatest service to physiographers. Powell and his followers, especially Gilbert, Dutton and McGee, have consistently carried the consequences of subaërial erosion to their legitimate end in a featureless lowland, and have recognized the controlling influence of the baselevel during all the sequence of changes from the initial to the ultimate form. It is not here essential whether such a featureless lowland exists or ever has existed, but it is absolutely essential to follow the lead of deduction until all the consequences of the theory of erosion are found; and then to accept as true those theoretical deductions which successfully confront the appropriate facts of observation. Only in this way can the error of regarding geography as a purely observational natural science be corrected. Following the acceptance of the doctrine of baselevels came the method of reconstituting the original form initiated by deformation, as a means of more fully understanding the existing form; for only by beginning at the initial form can the systematic sequence of the changes wrought by destructive processes be fully traced and the existing form appreciated. This had often been done before in individual cases, but it now became a habit, an essential step in geomorphological study. Naturally enough, the terms of organic growth, such as young, mature, old, revived, and so on, came to be applied to stages in the development of inorganic forms; and thus gradually the idea of the systematic physiographic development of land forms has taken shape. This idea is to-day the most serviceable and compact summation of all the work of the century on the physical geography of the lands. It recognizes the results of deformation in providing the broader initial forms on which details are to be carved. It gives special attention to the work of destructive processes on these forms, and especially to the orderly sequence of various stages of development, recognizing that certain features are associated with youth, and others with maturity and old age. It gives due consideration to the renewed movements of deformation that may occur at any stage in the cycle of change, whereby a new sequence of change is introduced. It gives appropriate place, not only to the forms produced by the ordinary erosive action of rain and rivers, but to the forms produced by ice and by wind action as well; and it co-ordinates the changes that are produced by the sea on the margin of the land with the changes that are produced by other agencies upon its surface. It considers not only the various forms assumed by the water of the land, such as torrents, rapids, falls and lakes, appropriately arranged in a river system as to time and place, but also the forms assumed by the waste of the land, which, like the water, is on its way to the sea. In a word, it lengthens our own life, so that we may, in imagination, picture the life of a geographical area as clearly as we now witness the life of a quick-growing plant, and thus as readily conceive and as little confuse the orderly development of the many parts of a land form, its divides, cliffs, slopes and water courses, as we now distinguish the cotyledons, stem, buds, leaves, flowers and fruit of a rapidly-maturing annual that produces all these forms in appropriate order and position in the brief course of a single summer.
The time is ripe for the introduction of these ideas. The spirit of evolution has been breathed by the students of the generation now mature all through their growing years and its application to all lines of study is demanded. It is true that the acceptance of inorganic as well as of organic evolution is often implied rather than outspoken; yet evolution is favorably regarded, as is proved by the eagerness with which even school boards and school teachers, conservatives among conservatives, hail the appearance of books in which the new spirit of geography is revealed. In the last years of the century, the school books most widely used in this country have made great advance in the explanatory treatment of land forms. Tarr’s Physical Geographies and Russell’s monographic volumes on the ‘Lakes,’ ‘Glaciers,’ ‘Volcanoes’ and ‘Rivers’ of North America, all presenting land forms in an explanatory rather than an empirical manner, have been warmly welcomed in this country. Penck’s ‘Morphologie der Erdoberfläche’ (1894), although largely concerned with the historical development of the subject, presents all forms as the result of process. De Lapparent’s ‘Leçons de géographie physique’ (1886) treats land forms generically; and a second edition of the book is called for soon after the first. ‘Earth Sculpture,’ by James Geikie (1899), and Marr’s ‘Scientific Study of Scenery’ (1900), carry modern ideas to British readers. There can be little doubt that the books of the coming century will extend the habit of explanation even further than it has yet reached.
This review of the advance of the century in the study of land forms, the habitations of all the higher forms of life, might have been concerned wholly with the concrete results of exploration, as was implied in an earlier paragraph. Travels in the Far East of the Old World, or in the Far West of the New, have yielded fact enough to fill volumes. But such a view of the century has been here replaced by another; not because the first is unimportant, for it is absolutely essential, but because the second includes the first and goes beyond it. Not the facts alone, but the principles that the facts exemplify, demand our attention. These principles, founded upon a multitude of observations, are the greater contribution of the closing to the opening century in the study of the Forms of the Land.