Popular Science Monthly/Volume 53/August 1898/Topographic Features Due to Landslides
|TOPOGRAPHIC FEATURES DUE TO LANDSLIDES.|
IT is well known that masses of rock sometimes break away from steep slopes and descend as avalanches or landslides into adjacent depressions.
The topography of a region where landslides occur is changed in two ways—i. e., by the removal and by the accumulation of material. The contours of a mountain, plateau, or other land form that stands in relief, are altered by the removal of material; as, for example, when a portion of the border of a plateau falls away, a re-entering angle or curve is produced; or, when a rock avalanche occurs on a mountain side, a high-grade gorge or depression may result. The material composing a landslide comes to rest in ridges and piles which have certain characteristic shapes. The most noticeable feature in such instances is the backward slope of the surface of the displaced material after it comes to rest. The surface of a landslide, whether composed mainly of a single block or of a heterogeneous mass of loose material, slopes toward the cliff from which it came. This backward slope tends to the formation of basins in which water accumulates, and lakes and swamps result. The backward slope referred to appears to be due to friction between the moving mass and the rocks beneath, which retards the progress of the material at the bottom and in front, so as to allow the material which comes later and at a higher level to slide over it. In a heterogeneous mass of fallen rocks there appear to be several planes of shear along which differential motion has taken place.
The changes produced by landslides are usually considered, even by geologists, I believe, as of a local character and of minor importance in the topography of most regions. Recent studies by the writer of the geography of the lava-covered region of Oregon, Washington, and Idaho, however, have shown that the phenomena referred to furnish an explanation of the origin of a class of topographic forms which occur not only on mountain sides and plateau margins, and among the hills at the bases of such elevations, but in certain instances in the minor features of broad and approximately level plains.
The landslides or avalanches which sometimes rush down the sides of mountains are frequently, and probably in most instances, composed of loose rock and soil. The most frequent condition leading to such catastrophes is the saturation of the material with water. Landslides usually follow heavy rains. The famous Willey land slide in the White Mountains in 1826, and the still greater one which descended into Biraki Ganga River, near Gohna, India, in 1893, and gave origin to Gohna Lake, are instances of this nature. A large number of landslides, however, are due to geological rather than climatic causes, and it is to this class that I wish to invite attention.
It frequently happens that a layer of hard rock rests on softer or more easily soluble beds. When steep escarpments are formed of two strata having this arrangement, conditions are produced which favor the breaking off of masses of the hard upper layer and their descent to the foot of the escarpment in landslides. There are other conditions, such as the thickness of the hard layers and the manner in which they are jointed, and the consistence of the soft beds—whether slippery clays, loose volcanic lapilli, etc.—which modify the process. The main or controlling conditions referred to are furnished at hundreds of localities in the region occupied by what is known as the Columbia lava, and drained by Columbia River. In that region sheets of basalt, ranging from two to five hundred feet and more in thickness, alternate with or overlie sheets of clay, shale, volcanic tuff, etc., which in many instances are hundreds of feet thick. The basalt was poured out in a molten condition and spread over the land in horizontal sheets. On cooling, these sheets acquired a columnar structure due to joints, usually at right angles to the top and bottom surfaces. This columnar structure facilitated the breaking away of great masses of basalt, when, for various reasons, portions of the sheets form the summits of bold escarpment.
In part, the Columbia lava, and the softer beds interleaved with it, have remained undisturbed, and are now practically horizontal over thousands of square miles. In adjacent areas of a great extent the beds have been broken by extensive fractures and the blocks thus formed variously tilted. Over still other extensive regions, particularly in the Cascade Mountains, the originally horizontal sheets, aggregating several thousand feet in thickness, have been raised into dome-shaped uplifts, at least one of which is nearly circular, while others are more nearly elliptical, the major being several times as long as the minor axis. Some of these domes, if unaffected by erosion, would have an altitude above their immediate bases of from five to eight thousand feet.
Where the Columbia lava is still essentially horizontal, as in southeastern Washington, it has been dissected by streams which now flow in magnificent cañons with clifflike walls from a thousand to four thousand feet high. In the region referred to as having been extensively fractured, cliffs have been produced by the tilting of the blocks thus produced. The dome-shaped uplifts have been broken by weathering and by the work of streams in such a manner as to remove their more elevated portions, and leave their truncated bases as a series of cliffs facing inward toward the center of the uplift. Thus, in many ways, and owing to still other conditions noted in part below, the region of the Columbia lava has become a land of great escarpments.
The fact that the escarpments referred to are formed of the edges of layers of hard basalt, which are traversed by joints at right angles to the planes of bedding, and also the occurrence of layers of soft rocks beneath the hard, cliff-forming layers, furnish conditions unusually favorable for landslides. In fact, landslide topography, as it may be termed, is nearly as characteristic of the Columbia lava region as are its magnificent cliffs. The topographic features due directly to landslides themselves are probably seldom recognized, while the long lines of frowning escarpments obtrude themselves on the attention of even the least observant. The fact is, however, that the cliffs in many instances have resulted from the breaking away of large rock masses, and will in time be destroyed by the same process.
The alternation of lava sheets and of lacustral sediments, etc., is not a marked feature of the entire country occupied by the Columbia lava, and for thisgreat variations occur in the extent to which the escarpments of that region have been affected by landslides. In southeastern Washington, for instance, sedimentary or other soft layers between the lava sheets are relatively unimportant, and throughout scores and even hundreds of miles of canon walls appear to . In this portion of the field the evidences of former landslides have not been noted.
In striking contrast with the region just referred to is the great dome from which the Wenatchee Mountains have been sculptured. (The Wenatchee Mountains are situated in the central portion of Washington, and on the eastern flank of the still vaster and much elongated Cascade dome. Mount Stuart and a number of associated peaks composed of dense granite form the center of the Wenatchee dome, and now stand in bold relief, owing to the removal of softer beds from about them.) In this instance the central portion of a dome fully fifty miles in diameter, has been removed and the truncated edges of the hard layers composing it left in prominent escarpments which sweep about the central core of granite in vast irregular curves. At least four sheets of Columbia lava, varying in thickness from three to four or five hundred feet, formerly extended over a large portion and possibly covered the entire region where the Wenatchee dome was upraised, but have been eroded away from its central portion. The uncovering of the region referred to, embracing fully one thousand square miles, has been accomplished by the slow recession of inward-facing cliffs, due principally to their having fallen from time to time in landslides, and the gradual decay and removal of the fallen blocks by streams and percolating waters. This process is till in progress, and a series of topographic changes from fresh landslides to rolling, prairielike lands with deep, rich soils and features characteristic of old land surfaces, can be easily traced. The reader must not infer, however, that the entire area from which the successive sheets of Columbia lava have been removed has a low relief. The streams have cut deeply into the rocks beneath the lowest lava sheet, and produced a markedly different series of land forms, in which sharp ridges and deep, narrow valleys are conspicuous elements. A belt of country marked by landslide topography which was gradually smoothed out, owing to the decay and erosion of the fallen blocks of basalt, receded with the slow retreat of the encircling cliffs and was replaced by exceedingly rugged topography.The nature of the changes produced by landslides and the subsequent decay of the fallen masses and their melting down, as it were, into an undulating plain with undrained basins, is graphically displayed at many localities adjacent to the still receding escarpments. Favorable localities for this study are furnished by Table and Lookout Mountain, to the northward of Ellensburg, Washington, or on the southeastern margin of the truncated Wenatchee dome. Standing on Lookout Mountain, for instance, one beholds toward the southeast a gently sloping table-land which rises toward his station. The surface of this inclined table is formed of a sheet of Columbia lava, but not the older sheet, which dips southeast at an angle of four or five degrees. On its northwestern margin this table-land breaks off so as to form a precipice from a thousand to twelve hundred feet high. In many places this escarpment is vertical, but its lower slopes are masked by talus. Below this palisadelike escarpment are many others of a similar character, but of less height and seldom over half a mile in length. The lower escarpments are formed of the edges of blocks of lava which have broken away from the main escarpment from time to time and plowed their way down into the valley. The fallen blocks are inclined at angles of ten to fifteen degrees toward the cliffs from which they fell. At the base of the main escarpment there is a series of irregular depressions or basins, which connect one with another more or less perfectly, and are bounded on their northwest margins by the backward-sloping blocks. In three of these basins at the present time there are small lakes without visible outlets. Each lake is four or five acres in area, and, except where the basins have become partially filled with talus from the overshadowing cliffs to the eastward, are deepest on that side. The western edges of the fallen blocks rise some two or three
hundred feet above the surfaces of the lakes, situated on their depressed borders. A view of one of these interesting lakes is given in Fig. 1.
To the northwest of the basins holding lakes, just referred to, there is a series of ridges and hills inclosing undrained basins, which extends about two miles from the base of the main escarpment and gradually decreases in height at the same time that the minor features in their relief become more and more subdued. This belt of ridges and basins finally merges by insensible gradations into a tract of undulating, prairielike land, two to three miles broad. The edges of the more recent of the fallen blocks stand out as sharp-crested ridges, with gentle slopes toward the great cliffs from which they fell, but present precipitous escarpments of bare rock toward the valley. As one descends the series of ridges and hills, the cliffs become less and less sharply defined and soon give place to rounded swells. Old lake basins change to swampy areas, and at a still greater distance become grassy dells.
As already stated, there is a gradual transition from the still hilly region to the undulating plain, at the northwest base of Lookout Mountain, where the relief has been smoothed out and only gentle, flowing outlines attract the eye. On the margin of the plain adjacent to the lower hills there are obscure ridges, on which there are many rounded and much-weathered bowlders of basalt, but a mile farther westward the soil is exceedingly fine and homogeneous, and scarcely a stone can be found. Such pebbles as do occur are of basalt, rounded by decay. A characteristic feature of the plain, now cleared of the scattered groves of pine that formerly covered it, and sown with wheat, is the presence of shallow, undrained basins, with low, gently swelling hills between them. This tract of country, eight or ten square miles in area, lies between Teanaway River and Swank Creek, but is entirely without stream channels. The scanty rain is absorbed by the deep, porous soil.
The undulating, prairielike lands just described have resulted from the slow disintegration and decay of blocks of basalt which fell as landslides during the slow recession of the thick lava sheet and of the soft volcanic tuff beneath, which once covered the region. The undulating surface of the wheat lands, with undrained basins, illustrates the old age of landslide topography. A view of this undulating plain is reproduced in Fig. 2. The hills seen in the distance owe their origin to another sheet of Columbia lava, the lowest of the series, which slopes toward the observer, and breaks off in steep slopes to the northward.An ideal section through the margin of Lookout Mountain is
shown below. The section crosses one of the lake basins at the base of the mountain, and is continued northwestward through the belt of hills and basins to the plain into which they merge. In this diagram an attempt has been made to indicate the breaking down and rounding of the fallen blocks, and their gradual change to an undulating plain.
In some instances a landslide plows its way out into a valley for a mile or more from the base of the cliffs from which it came, and
forces up a series of ridges and mounds about its margin. These ridges have a striking resemblance to terminal moraines left by the recession of glaciers, but the scars on the adjacent escarpment or mountain sides and the associated hills and basins plainly show their origin.
The sequence of topographic changes described above, so well illustrated at Lookout Mountain, is typically and characteristically displayed at hundreds of other localities in the same general region, but is not confined to the basin of the Columbia. With minor modifications due to local conditions, it may be recognized in many lands where bold escarpments occur. Where a humid climate prevails, however, and streams occupy the valleys, the old age of landslide topography is seldom reached.
The Columbia lava, it will be remembered, was spread out during a series of inundations of molten rock and has an area of approximately two hundred and fifty thousand square miles. Previous to the opening of the tens of thousands of fissures through which the molten rock reached the surface, the country had a rugged topography due to erosion. The lava covered the plains and entered the valleys in the mountains so as to give them level floors. Hills, ridges, and mountains were in some instances partially or wholly surrounded by the fiery flood and became capes and islands. Isolated eminences of the old land rise through the sheets of lava which cooled and hardened about them, in much the same manner that nunalakas break the monotony of the borders of the Greenland ice fields. When these islands in the sea of lava are of resistant rocks, like quartzite, which withstand the attacks of the destructive agencies of the air better than the encircling basalt, they still remain in bold relief, as is illustrated by Steptoe and Kamiack buttes, described elsewhere by the present writer.
When, however, the projecting portions of the nearly submerged peaks and ridges were of granite, volcanic tuff, limestone, etc., which weather more rapidly than the surrounding lava, they have wasted away so as to give origin to basins, valleys, and canons, with boundary walls of basalt. The sheets of basalt in these escarpments in many instances rest on less resistant rocks, and a recession of the cliffs due to the breaking away and falling of large masses of their capping layers takes place. The fallen blocks disintegrate and waste away in the manner described above, and the canons and valleys increase in size. The ground plans of the depressions originating and enlarging in this manner, vary according to the shapes of the islandlike rock masses which have been removed; some of the depressions are nearly circular, others are greatly elongated, and now have the characteristics of flat-bottomed cañons with vertical walls.
The remarkable circular valley surrounded by an almost continuous palisade in eastern Oregon, known as Grande Ronde Valley, from which a river of the same name flows northward to join the Snake, is an illustration of the class of topographic forms produced in the manner described above. I can not testify from personal observation as to the nature of the soft rocks beneath the lava in the walls of Grande Ronde Valley, but other similar valleys, less regular in outline, near at hand, have resulted from the removal of islandlike masses of soft volcanic tuff.
Another unique feature in the topography of the region drained by the Columbia is the Grande Coulee in what is known as the Great Plain of the Columbia, or more familiarly as the "Big Bend country," in central Washington. . The Grande Coulée is a flat-bottomed cañon some thirty miles long and varying in width from two to four miles. In its vertical walls, usually about three hundred and fifty feet high, the edges of several sheets of Columbia lava are exposed. This great trench through the but little disturbed plain of lava was in existence previous to the Glacial epoch, and furnished an avenue of escape for Columbia River which was dammed by a glacier. At the southern end of the Grande Coulée, as can be seen from Couleé City, the lava sheets on its eastern side dip gently eastward, while the beds comprising its western wall are apparently horizontal. This fact led me to infer that the Grande Coulée, like several other similar but smaller canons in the lava, is due to stream erosion along a line of fracture. At the northern end of the cañon, however, granitic rocks form a portion of its walls, and stand as isolated towerlike masses within it. Some of these towers are capped with horizontal lava sheets. When the lava was poured out it surrounded a granite ridge having the position of the Grande Coulée, but probably not extending as far south as the depression since formed. The weathering and removal of the granite gave origin to a trenchlike depression with vertical walls, composed of basalt above and granite below. The more rapid crumbling of the granite led to the breaking away of the jointed basalt resting on it, and the widening of the depression in the manner already noticed.
From the brief and inadequate description I have given of certain of the more striking features of Washington and Oregon it will be seen that landslides have modified the topography of the region occupied by the Columbia lava in several ways. There are yet other changes in the geography of that most interesting and instructive land due to the same causes. Chief among them are the obstructions to the streams formed by landslides, and the production of lakes and rapids. At several localities in the upper Columbia masses of rock which have fallen from the cliffs bordering the stream obstruct its course. There are now no lakes along the course of the river due to this cause, but terraces above rocky rapids, show where such water bodies previously existed.
Perhaps the most interesting fact brought out by the study of landslide topography is that certain broad, nearly level areas, now covered with deep, rich soil, and in the autumn golden with the sheen of ripened grain, owe the minor features in their relief to ancient landslides. The hills, with broadly rounded summits, and the shallow undrained basins between, in such regions are an inheritance from a time when long, precipitous escarpments, by their slow recession, left the land covered with a rugged, confused mass of fallen blocks. A review of the facts concerning the minor features in the relief of the broad wheat lands of southeastern Washington, in the light of the conclusions here presented, leads to the suggestion that some of the ridges and basins of that region may be due to the recession of cliffs produced by stream erosion, [any portions of the deeply decayed surface of the basaltic plateau of southeastern Washington resemble closely the old landslide topography in the valley to the northwest of Lookout Mountain, shown in the accompanying illustration.
- Published by permission of the Director of the United States Geological Survey.
- A in Southeastern Washington. Water Supply and Irrigation Papers of the United States Geological Survey, No. 4, 1897, pp. 37-40.
- A Geological in Central Washington. By Israel C. Russell. United States Geological Survey Bulletin, No. 108, 1893, pp. 90-92.
- A in Southeastern Washington. By Israel C. Russell. Water Supply and Irrigation Papers of the United States Geological Survey, No. 4, 1887, pp. 58-69.