National Geographic Magazine/Volume 1/Number 1/Classification of Geographic Forms by Genesis

by W. J. McGee

THE CLASSIFICATION OF GEOGRAPHIC FORMS BY GENESIS.

By W. J. McGee.

Scientific progress may be measured by advance in the classification of phenomena. The primitive classification is based on external appearances, and is a classification by analogies; a higher classification is based on internal as well as external characters, and is a classification by homologies; but the ultimate classification expresses the relations of the phenomena classified to all other known phenomena, and is commonly a classification by genesis.

The early geologic classification was based chiefly upon simple facts of observation; but with continued research it is found that the processes by which the phenomena were produced may be inferred, and, accordingly, that the phenomena may be grouped as well by the agencies they represent as by their own characteristics. Thus the empiric or formal laws of relation give place to philosophic or physical laws indicating the casual relations of the phenomena, and the final arrangement becomes genetic, or a classification by processes rather than products.

The phenomena of geography and geology are identical, save that the latter science includes the larger series: since the days of Lyell the geologist has seen in the existing conditions and agencies of the earth a reflection and expression of the conditions under which and the agencies by which its development has been effected; the far stretching vista of geologic history is illuminated only by knowledge of the earth of to-day; and the stages in geologic development are best interpreted in terms of geography. So a genetic classification of geologic phenomena (which is rendered possible and intelligible through geographic research) will apply equally to geography, whether observational or of the more philosophic nature which Davis proposes to call Systematic Geography, and which Powell has called Geomorphology. Such a classification is here outlined.

 

The various processes or movements with which the geologist has to deal fall naturally into two principal and antagonistic categories and five subordinate categories; and each category, great and small, comprises two classes of antagonistic processes or movements.

The initial geologic movements (so far as may be inferred from the present condition of the earth) were distortions or displacements of the solid or solidifying terrestrial crust, occurring in such manner as to produce irregularities of surface. These are the movements involved in mountain growth and in the upheavel of continents. They have been in operation from the earliest known eons to the present time, and their tendency is ever to deform the geoid and produce irregularity of the terrestrial surface. The movements have been called collectively "displacement" and "diastrophism," but in the present connection they may be classed as diastatic, or, in the substantive form, as deformation. Recent researches, mainly in this country, have indicated that certain diastatic movements are the result of transference of sediment—that areas of loading sink, and areas of unloading rise but it is evident that the transference of sediment is itself due to antecedent diastatic movements by which the loaded areas were depressed and the unloaded areas elevated; and the entire category may accordingly be divided into antecedent and consequent diastatic movements. A partially coincident division may be made into epeirogenic, or continent-making movements (so called by Gilbert), and orogenic, or mountain-making movements. Though there is commonly and perhaps always a horizontal component in diastatic movement, the more easily measured component is vertical, and when referred to a fixed datum (e. g. sea level) it is represented by elevation and depression.

The second great category of geologic processes comprehends the erosion and deposition inaugurated by the initial deformation of the terrestrial surface. By these processes continents and mountains are degraded, and adjacent oceans and lakes lined with their debris. They have been in active operation since the dawn of geologic time, and the processes individually and combined ever tend to restore the geoid by obliterating the relief produced by deformation. The general process, which comprises degradation and deposition, may be called gradation.

The first subordinate category of movements is allied to the first principal category, and comprises, (1) the outflows of lavas, the formation of dykes, the extravasation of mineral substances in solution, etc., (2) the consequent particle and mass movements within the crust of the earth, and (3) the infiltration of minerals in solution, sublimation, etc.,—in short, the modification of the earth's exterior directly and indirectly through particle movements induced by the condition of the interior. These processes have been in operation throughout geologic time, though they perhaps represent a diminishing series; they have added materially to the superficial crust of the earth; and it is fair to suppose that they have modified the geoid not only by additions to the surface but by corresponding displacements in their vicinity. The category may be tentatively (but rather improperly) called vulcanism, and the antagonistic classes of movements constituting it are extravasation and its antithesis. The vibratory movements of seismism probably result from both deformation and vulcanism under certain conditions.

The second subordinate category of processes is closely linked with all of the others. It comprises the various chemic and chemico-mechanical alterations in constitution and structure of the materials of the earth's crust. The processes have affected the rocks ever since the solidification of the planet, though probably in a progressively diminishing degree; and they have materially (but indirectly rather than directly) modified the internal constitution and external configuration of the earth. The processes may be collectively called alteration; and the antagonistic classes into which the category is divisible are lithifaction and decomposition in their various phases, or rock-formation and rock-destruction.

The third subordinate category of processes, viz: glaciation, is related to the second principal category; but since (1) it is probable if not actually demonstrable that under certain circumstances glacial grinding tends to accentuate preëxisting irregu- larities of surface, and since (2) it is well known that glacial deposition sometimes gives great irregularity of surface, it is evident that glaciation is not a simple process of gradation, but must be clearly distinguished therefrom. A considerable portion of the earth's surface has been modified by glaciation during later geologic times. The general process comprises glacial construction and glacial destruction.

There is a fourth subordinate category of processes, which is also allied to gradation, viz: wind-action, which may be made to include the action of waves and wind-born currents; but since the winds scoop out basins and heap up dunes, while the waves excavate submerged purgatories and build bars, it is evident that this category, too, must be set apart. The processes are only locally important as modifiers of the land surface of the globe. They comprise constructive action and destructive action.

There is a final category which is in part allied to alteration but is in part unique, viz: the chemic, mechanical, and dynamic action of organic life. Ever since the terrestrial crust become so stable as to retain a definite record of the stages of world-growth, life has existed and by its traces has furnished the accepted geologic chronology: at first the organisms were simple and lowly, and affected the rocks chemically through their processes of growth and decay, as do the lower plants and animals of the present; later, certain organisms contributed largely of their own bodily substance to the growing strata; and still later, the highest organisms, with man at their head, have by dynamic action interfered directly with gradation, alteration, and wind-action, and thus, perhaps, indirectly with the more deep-seated processes of world growth. The vital forces are too varied in operation to be conveniently grouped and named.

These categories comprise the various processes contemplated by the geologist, and collectively afford an adequate basis for a genetic classification of geologic science. Their relations are shown in the accompanying table:

Classification of Geologic Processes.

Principal Categories	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	1.—Deformation.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Antecedent<Epeirogenic	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Elevation.
				Consequent>Orogenic.		Depression.
		2.—Gradation.			$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Deposition.
						Degradation.
Subordinate Categories.			$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	1.—Vulcanism.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Extravasation.
						(Antithesis of Extrav.)
				2.—Alteration.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Lithifaction.
						Decomposition.
				3.—Glaciation.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Glacial construction.
						Glacial destruction.
				4.—Wind action.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Wind construction.
						Wind destruction.
				5.—Vital action.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Various constructive and
						destructive processes.

On applying this classification to geographic forms, the various phenomena immediately fall into the same arrangement. The continents, great islands, mountain systems, and non-volcanic ranges and peaks generally, the oceans, seas, and some bays, gulfs and lakes, evidently represent the diastatic category of movements. These greater geographic features have long been named and classified empirically, and can be referred to their proper places in a genetic taxonomy without change in terminology. The volcanoes, craters, calderas, lava fields, tuff fields, tufa crags, mesas, volcanic necks, dykes, etc., however modified by degradation, alteration, glaciation, or wind action, exhibit characteristic forms which have often received names indicative of their origin. The glacial drift with its various types of surface, the moraines, drumlins, kames, roches de moutonnées, rock basins, kettles, lacustral plains, aqueo-glacial terraces, loess hills and plains, etc., have been studied in their morphologic as well as their structural aspects, and the elements of the configuration commonly assumed have been described, portrayed, and appropriately named; and they take a natural place in the classification of products by the processes giving rise to them. The dunes, dust drifts, sand ridges, etc., and the wind-scooped basins with which they are associated, are local and limited, but are fairly well known and fall at once into the genetic classification of forms and structures. But all of these geographic forms are modified, even obliterated, by the ever prevailing process of gradation, which has given origin to nearly all of the minor and many of the major geographic forms of the earth. The forms resulting from this second great category of geologic processes have generally engaged the attention of systematic students, but their prevalence, variety and complexity of relation are such that even yet they stand in greatest need of classification.

Lesley thirty years ago regarded the mountain as the fundamental topographic element; Richthofen recognizes the upland and the plain ("aufragendes Land und Flachböden") as the primary classes of configuration comprehending all minor elements of topography; Dana groups topographic forms as (1) lowlands, (2) plateaus and elevated table lands, and (3) mountains; and these related allocations are satisfactory for the purposes for which they are employed. But the implied classification in all these cases is morphologic rather than genetic, and is based upon superficial and ever varying if not fortuitous characters; and if it were extended to the endless variety of forms exhibited in the topography of different regions it would only lead to the discrimination of a meaningless multitude of unrelated topographic elements.

In an exceedingly simple classification of geographic phenomena, the primary grouping is into forms of construction and forms of destruction; but it is evident on inspection of the table introduced above that such a classification is objectionable unless the greater geographic elements due to diastatic movements (in which the constructive action is veritable but different in kind from those in the other categories) be excluded, and this is impracticable without limiting the classification to subordinate phenomena. Moreover it is illogical and useless to unite the constructive phenomena of the remaining categories, since (1) the processes exemplify widely diverse laws, which must find expression in any detailed classification whether genetic or not, and since (2) the differences between the forms united are much greater than the differences between the forms separated in such a classification—e. g. the differences between a dune, a drumlin and a mesa (all constructive forms) are far greater than the differences between a fresh lava sheet and a deeply cut mesa, between a drumlin and the smallest drift remnant, or between a dune and a Triassic mound of circumdenudation; and this is true whether the distinction be made on analogic, homologic, or genetic grounds. Indeed it seems evident that while discrimination of constructive and destructive forms is necessary and useful in each genetic category, the use of this distinction as a primary basis of classification is inexpedient.

The classification of topographic forms proposed a few years ago by Davis, who regards "special peculiarities of original structure" as a primary, and "degree of development by erosion" a secondary basis, and Richthofen's arrangement of categories of surface forms as (1) tectonic mountains, (2) mountains of abrasion, (3) eruptive mountains, (4) mountains of deposition, (5) plains, and (6) mountains of erosion,^[1] in addition to depressions of the land (Die Hohlformen des Festlandes), are more acceptable, since they are based in part on conditions of genesis. But it is clearly recognized by modern students of dynamic geology that waterways are the most persistent features of the terrestrial surface; and the most widely applicable systems of classification of the surface configuration of the earth thus far proposed have been based substantially on the agencies of gradation. Thus Powell, Löwl and Richthofen classify valleys by the conditions of their genesis; Gilbert classifies drainage; and Phillipson, unduly magnifies the stability and genetic importance of the water parting, classifies the hydrography through the divides; and, although these geologists have not dwelt upon and perhaps have failed to perceive the relation, the same classification is as applicable to every feature of the local relief as to the streams by which the relief was developed.

In a general classification of the topographic forms developed through gradation, it would be necessary to include the forms resulting from deposition as well as degradation, and also to discuss the relation of base-level plains to antecedent and consequent relief; but in a brief résumé it will suffice to consider only the modifications produced by degradation upon a surface of deposition after its emergence from beneath water level as a regular or irregular terrane; and the influence of base-level upon the topographic forms developed upon such a surface may be neglected in a qualitative discussion, though it is quite essential in quantitative investigation.

The hydrography developed upon terranes affected by displacement both before and after emergence has already been satisfactorily classified. Powell, years ago, denominated valleys established previous to displacement of the terrane by faulting or folding, antecedent valleys; valleys having directions depending on displacement, consequent valleys; and valleys originally established upon superior and subsequently transferred to inferior terranes, superimposed valleys; and these valleys were separated into orders determined by relation to strike and again into varieties determined by relation to subordinate attitude of the terranes traversed. Gilbert adopted the same general classification, and so extended as to include certain special genetic conditions. Tietze, in the course of his investigation of the Sefidrud (or Kizil Uzen) and other rivers in the Alburs mountains of Persia, independently ascertained the characteristics of the class of waterways comprehended by Powell under the term antecedent; Medlicott and Blanford observed that many of the Himalayan rivers are of like genesis; and Rütimeyer, Peschel and others have recognized the same genetic class of waterways; but none of these foreign geologists have discussed their taxonomic relations. Löwl, who upon a priori grounds denies the possibility of antecedent drainage, has recently developed an elaborate taxonomy of valleys which he groups as (a) tectonic valleys, and (b) valleys of erosion (Erosionsthäler). The first of these categories is separated into two classes, viz: valleys of flexure and valleys of fracture, and these in turn into several sub-classes determined by character of the displacement and its relations to structure; and the second, whose genesis is attributed to retrogressive (rückwärts fortschreitende" or "rückschreitende") erosion, is vaguely separated into several ill-defined classes and sub-classes determined by structure, climate, and various other conditions. The second of Löwl's categories is also recognized by Phillipson. Still more recently, Richthofen, neglecting antecedent drainage, designated the superimposed class of Powell epigenetic, and formulated a classification of the remaining types of continental depressions (Die Hohlformen des Festlandes) as (a) orographic depressions (Landsenken); (b) tectonic valleys, and (c) sculptured valleys; and the last two categories are separated into classes and sub-classes, corresponding fairly with those of Löwl, determined by their relations to structure and by various genetic conditions.

These several classifications have much in common; their differences are largely due to the diversity of the regions in which the investigations of their respective authors have been prosecuted; but combined they probably comprehend all the topographic types which it is necessary to discriminate.

The American classification and nomenclature, particularly, is unobjectionable as applied to montanic hydrography; but it does not apply to the perhaps equally extensive drainage systems and the resulting topographic configuration developed on emergent terranes either (a) without localized displacement or (b) with localized displacement of less value in determining hydrography than the concomitant erosion, terracing and reef building; neither does it apply to the minor hydrography in those regions in which the main hydrography is either antecedent or consequent; nor does it apply even to the original condition of the superimposed or antecedent drainage of montainous regions.

Upon terranes emerging without displacement and upon equal surfaces not yet invaded by valleys, the streams depend for their origin on the convergence of the waters falling upon the uneroded surface and affected by its minor inequalities, and for their direction upon the inclination of that surface. They are developed proximally (or seaward) by simple extension of their courses by continued elevation, and distally by the recession of the old and the birth of new ravines; and since in the simple case it follows from the law of probabilities that the receding ravine will retain approximately the old direction and that the new ravines will depart therefrom at high angles, the drainage systems thus independently developed become intricately but systematically ramified and more or less dendritic in form. Löwl, Phillipson, Richthofen, and other continental, as well as different British and Indian geologists, and Lesley in this country, indeed recognize this type of drainage, but they do not correlate it with the montanic types; and Löwl's designation, derived from the manner in which he conceives it to be generated ("rückschreitende Erosion"), does not apply to either the completed drainage or the coincident topography.

Although its subordinate phases are not yet discriminated on a genetic basis, this type or order of drainage is sufficiently distinct and important to be regarded as coördinate with the type represented by the entire group of categories recognized by Powell and clearly defined by Gilbert. Such hydrography (which either in its natural condition or superimposed characterizes many plains, some plateaus, and the sides of large valleys of whatever genesis) may be termed autogenous; while the drainage systems imposed by conditions resulting from displacement (which characterize most mountainous regions) may be termed tectonic. Gilbert's classification of drainage may then be so extended as to include topography as well as hydrography, and so amplified as to include the additional type.

Drainage systems and the resulting systems of topography (all of which belong to the degradational class of forms) are accordingly.—

Type 1, Autogenous.
Type 2, Tectonic―
Order A, Consequent, upon
Class a, Displacement before emergence, and
Class b, Sudden displacement after emergence;
Order B, Antecedent; and
Order C, Superimposed, through
Class a, Sedimentation (when the superimposed drainage may be autogenous),
Class b, Alluviation or subaerial deposition, and
Class c, Planation (in which two cases the superimposed drainage may simulate the autogenous type).

In brief, the entire domain of geologic science is traversed and defined by a genetic classification of the phenomena with which the geologist has to deal; and the same classification is equally applicable to geographic forms, as the accompanying table illustrates:

Representative Geographic Forms as classified by Genesis.

Genetic Processes.			Geographic Forms.
Category.		Class.
Deformation	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Elevation	Continents, great islands, most mountain ranges, etc., not classified in detail.
		Depression	Oceans, great seas and bays, some inland valleys and lake-basins, etc., not classified in detail.
Gradation	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Deposition	Newly emerged ocean-bottoms (e. g., portions of the Coastal plain), playas and mountain-bound deserts, many flood-plains, marshes, etc., not classified in detail.
		Degradation	Drainage-systems and resulting topographic elements which are— 1—Autogenous (not classified in detail); and 2—Tectonic— Consequent, upon Displacement before emergence, and Sudden displacement after emergence; Antecedent; and Superimposed, through Sedimentation, Alluviation, and Planation.
Vulcanism	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Extravasation	Volcanic peaks, craters, lava-fields, tufa-crags, sinter-cones, volcanic necks, mesas, dykes, some mineral veins, etc., not classified in detail.
		(Antithesis of do.)	Sinks, caverns, some fissures, etc., not classified in detail.
Alteration	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Lithifaction	Minor features of certain topographic forms, e. g., reefs, crags, pinnacles, salients, out-cropping veins, some cataracts, etc., not classified in detail.
		Delithifaction	Minor features of certain topographic forms, e. g., pools and basins, reëntrants, some fissures and caverns, etc., not classified in detail.
Glaciation	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Glacial construction	Drift-plains, moraines of whatever character, drumlins, kames, aasar, drift-dammed lakes, loss-plains and ridges, etc., not classified in detail.
		Glacial deconstruction	Rock-basins. U-cañons, roches de moutonnées, etc., not classified in detail.
Wind action	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	Wind construction	Dunes, sand-ridges, bars, spits, etc., not here classified in detail.
		Wind deconstruction	Ponds associated with dunes, "blow-outs," "purgatories," etc., not classified in detail.
Vital Action	|	(Not discriminated)

1. (1) Tektonische Gebirge, (2) Rumpfgebirge oder Abrasionsgebirge, (3) Ausbruchsgebirge, (4) Aufschüttungsgebirge, (5) Flachböden, und (6) Erosionsgebirge.