Geomorphology

“ The records which it is the task of the geologist to decipher, are for the most part written in sand and mud—the deposits of the ocean, for in by far the larger number of formations into which the stratified part of the earth's crust has been divided, and which form his only guide to the history of the past, he can detect no trace of land. Hill and valley have alike disappeared, and the character of their scenery and inhabitants he can often but dimly conjecture from the nature of the sediment and of the drifted terrestrial relics that may chance to be found among strata wholly marine. ”

“ A CENTURY OF GEOLOGY.—STEPS OF PROGRESS IN THE INTERPRETATION OF LAND FORMS
By HERBERT E. GREGORY The essence of physiography is the belief that land forms represent merely a stage in the orderly development of the earth’s surface features; that the various dynamic agents perform their characteristic work throughout all geologic time. The formulation of principle and processes of earth sculpture was, therefore, impossible on the hypothesis of a ready-made earth whose features were substantially unchangeable, except when modified by catastrophic processes. ”

“ Dynamic geology deals with the agents and forces which have been concerned in making the earth what it is. To dynamic geology belongs the consideration of the activities of rivers, the atmosphere and the ocean, the forces concerned in volcanic action, crustal movements, etc. Petrography is that phase of geology which deals with the rocks as such. Structural geology is that branch of the subject which deals with the positions of rock-strata and with the relations of rock formations to one another. Structural geology might be defined as the architecture of the crust of the earth. ”

“ Passing from the nature and mode of origin of the sedimentary strata to the manner of their arrangement in the Earth’s crust, we must endeavour to sketch in the merest outline the methods of stratigraphical geology. The surface of the Earth in some places stands out in the form of bare masses of rock, roughly hewn or finely carved by Nature’s tools of frost, rain or running water ”

“ All the fossiliferous formations may be studied by the geologist in two distinct points of view: first, in reference to their position in the series, their mineral character and fossils; and, secondly, in regard to their physical geography, or the manner in which they now enter, as mineral masses, into the external structure of the earth; forming the bed of lakes and seas, or the surface and foundation of hills and valleys, plains and table-lands. Some account has already been given on the first head of the Tertiary, the Cretaceous, and Wealden strata ”

“ GENERAL TOPOGRAPHY.
The main topographical features of Britain may be arranged as mountains, tablelands, valleys, and plains. All our Mountains are the result of erosion on areas of land successively upheaved above the sea. In the development of their forms, the general outlines have [Pg 134] been mainly determined by erosion, independent of geological structure; while the details have been chiefly guided by structure, but partially also by the rate and kind of erosion. Ruggedness, for example, has resulted primarily from structure, but has been aggravated by greater activity of erosion. ”

“ Under dynamical geology are included important subdivisions, such as seismic geology, or the study of earthquakes, and vulcanology, or the study of volcanoes. Another large subject, glacial geology, belongs within the broad frontier common to both dynamical geology and physiography. A relatively new subdivision of geological science is orientational geology, which is concerned with the trend of earth features, and is closely related both to physiography and to dynamical and structural geology. ”

“ Hidden under the farm vegetation or masked by the woods are scattered outcroppings of rock which have been the basis of the geologist’s judgment in preparing the map. Experience shows that in order to bridge the wide gap between the geology in the landscape and the patches of color upon the map something more than mere examination of the colored sheet is necessary. ”

“ Having at length mastered the more abundant rocks and organic remains of his neighbourhood, let him try to trace out - x - the connexion of the different strata across the country, so as to understand its structure. For this purpose it will be necessary to examine every ravine and natural exposure of the rocks, along with quarries, ditches, railway-cuttings, and, in short, the whole surface of the district. A general notion of the geology of the place will, not perhaps be of very difficult attainment ”

“ Also the coasts of continents and islands suffer continual wear and tear at the hands of sea waves; and thus the supply of sediment is increased.
When the geologist comes to study the great rock-masses—hundreds, and even thousands, of feet in thickness—of which mountain-ranges are composed, he finds all those kinds of rock which we have just been considering,—sandstones, shales (or hardened clays) , pebble-beds, and limestones,—and endeavours to picture to himself their gradual growth in the ways we have described. ”

“ This law is traced in the very geological qualities of the chain, not in the later form. The [96] outer form is often very deceptive, the very convulsions which indicate the surer signs having served to obliterate what we should suppose the most prominent marks. The present of mountains must often be studied in the light of their past. Orography must be interpreted by geology. But the geological surveys of the earth are as yet very imperfect; the outer form has often to be accepted as the only guide. ”

“ In this little valley we see slow changes now in progress. We find also in the composition, the structure, and the attitude of the rocks, and the land forms to which they have been sculptured, the record of a long succession of past changes involving the origin of sand grains and their gathering and deposit upon the bottom of some ancient sea, the cementation of their layers into solid rock, the uplift of the rocks to form a land surface, and, last of all, the carving of a valley in the upland. ”

“ A knowledge of the geology of the land is of the first importance; that is, not only a knowledge of the range and extent of each formation and its subdivisions, which may be called geographical geology, but also how far and to what extent the various lands do depend upon the substratum for their soil, and the local variations in the chemical or mineralogical character of the substrata themselves, and which may be called the differential geology of soils. ”

“ Without the aid of Geology the geographer cannot, in fact, discriminate between mountains of elevation and mountains of denudation; hence geographical terms so constantly in use as mountain-range and mountain-chain are very often applied by writers, ignorant of geological structure, to elevated regions which have no claim to be described either as chains or ranges. Some knowledge of Geology, therefore, is essential to us if we would have correct views of many of the grandest features of the globe. ”

“ And to the practised eye of the geologist, a fine landscape is not simply a pleasantly or grandly diversified surface, but it has depth; for he reads in the superficial lineaments the structure of the rocks out of which they are carved. ”

“ When beds of sand, clay, and marl, containing shells and vegetable matter, are found arranged in a similar manner in the interior of the earth, we ascribe to them a similar origin; and the more we examine their characters in minute detail, the more exact do we find the resemblance. Thus, for example, at various heights and depths in the earth, and often far from seas, lakes, and rivers, we meet with layers of rounded pebbles composed of flint, limestone, granite, or other rocks, resembling the shingles of a sea-beach or the gravel in a torrent’s bed. ”

“ Thus mountain chains and mountain systems were created. Often their summits, worn down by frosts and rains, disclose the core of rock which, ages before, then hot and fluid, had underlain the crust and bent it upward into mountain form. Now, cold and hard, these masses are disclosed as the granite of to-day's landscape, or as other igneous rocks of earth's interior which now cover broad surface areas, mingled with the stratified or water-made rocks which the surface only produces. ”

“ As valleys approach base-level they develop flats. As the valleys and their flats widen, and as their tributaries increase in numbers and size, a stage of erosion is presently reached where but little of the original upland surface remains. The country is largely reduced to slopes. In this condition the drainage and the topography which it has determined are said to be mature. Mature topography is shown in contours in the figures of Plate V, and in the northern part of Plate VI, where slopes, rather than upland or valley flats, predominate. Fig. ”

“ To obtain a general idea of the nature and method of our science before beginning its study in detail, we may visit some valley, such as that illustrated in the frontispiece, on whose sides are rocky ledges. Here the rocks lie in horizontal layers. Although only their edges are exposed, we may infer that these layers run into the upland on either side and underlie the entire district; they are part of the foundation of solid rock which everywhere is found beneath the loose materials of the surface. ”

“ The moment, therefore, that one of us beheld the ranges of the Jura, with their combs and offsets, their vast escarpments, and far-glittering white gaps, he was at home among friends, where geologists, born among them, felt that they were strangers. For the valleys of the Jura were filled with later formations full of fossils, which the Appalachian valleys never are. There much is hidden, here all is told. Fossils themselves in the Jura distracted study from the topography.”
Geomorphy, like its antecedent, the science of topography, is an American innovation, and for the same reasons. ”

“ Other effects on topography.—Inequalities in the hardness of rock develop certain peculiarities of topography other than those of valleys. The less resistant portions of a land area more or less distant from streams are worn down more readily than those which are more resistant. If great areas of high land be capped with hard rock they are likely to remain as plateaus after surrounding areas of less resistance are brought low. ”

“ But modern observations have tended continually to dispel this delusion, and the geologist is now convinced that at no given era of the past have the boundaries of land and sea, or the height of the one and depth of the other, or the geographical range of the species inhabiting them, whether of animals or plants, become fixed and unchangeable. ”

“ The layered appearance of sedimentary rocks is one of their most characteristic features and these rocks are said to be bedded or composed of many individual beds of sedimentary rock. Sandstone, composed of sand size particles; shale, originally mud; and limestone, once lime-rich mud, are examples of sedimentary rocks.
7
Metamorphic rocks result when igneous or sedimentary rocks are subjected to abnormal heat and pressure. Folding or faulting [2] of rocks within the earth’s crust or deep burial beneath overlying rocks or sediments commonly produce metamorphism. ”

“ This kind of observation will enable us to find proofs everywhere that the surface of the land has not always been what it is to-day. In some districts, for example, when the dark layer of soil in which vegetables grow is turned up, there may be found beneath it sand and gravel full of smooth, well-rounded stones. Such materials are to be seen in course of formation where water keeps them moving to and fro, as on the beds of rivers, the margins of lakes, or the shallow shores of the sea. ”

“ And hence if the geologist would form an accurate conception of the origin and structure of many of the stratified rocks, he must study them not in hand specimens neatly trimmed and arranged along the shelves and drawers of a cabinet, nor even in the ponderous blocks daily exhumed by the quarryman, but along some surf-beaten cliff-line or down some precipitous ravine where the rock for centuries has been exposed to the wear and tear of the elements. ”

“ The elements of structure are set forth in a more elaborate form in structural geology when slaty structure, lode structure and stratified structure are the themes, and where flexures, faults, fractures and displacements are set forth in describing the structure of mountains, plateaus, hills and plains as slates, lodes and strata, giving figure to the topographic features and the endless variety and beauty of the topographic landscape. ”

“ Speculative geology is romance, and does not merit the name of science; yet is science obliged to borrow her theory of soils. The alternation of heat and cold, moisture and dryness, decomposed the mountains of primitive, secondary and tertiary formation; rains, and the laws of gravity, brought these from places of more, to places of less elevation—where, by mechanical mixture and chymical combination, the present substrata were formed. ”

“ To assume that in one large area inhabited exclusively by a single assemblage of species any important revolution in physical geography can be brought about, while other areas remain stationary in regard to the position of land and sea, the height of mountains, and so forth, is a most improbable hypothesis, wholly opposed to what we know of the laws now governing the aqueous and igneous causes. ”