Talus

“ In a moist climate a soluble rock, such as massive limestone, may form talus little if any faster than the talus weathers away. A loose-textured sandstone breaks down into incoherent sand grains, which in dry climates, where unprotected by vegetation, may be blown away as fast as they fall, leaving the cliff bare to the base. Cliffs of such slow-decaying rocks as quartzite and granite when closely jointed accumulate talus in large amounts. ”

“ Il ne se fait presque qu'un seul talus depuis leur sommet jusques dans les basses vallées, ou sur la plaine. Aussi l'état de ces montagnes est-il déjà presqu'entièrement fixé: on y voit très peu de rochers nuds qui s'éboulent, excepté, auprès des rivières. C'est dans ces lieux-la que l'ouvrage tarde le plus à se finir. Le bas des talus est miné par l'eau; leur surface s'éboule donc, pour ainsi dire, sans cesse, et laisse à découvert les rochers des sommets, qui par la continuent aussi à s'ébouler. Mais les vallées s'élargissent enfin ”

“ For example: if one went so, up that gully and to that chimney there, then one might come out high among those stunted pines that ran round in a sort of shelf and rose still higher and higher as it passed above the gorge. And then? That talus might be managed. Thence perhaps a climb might be found to take him up to the precipice that came below the snow; and if that chimney failed, then another farther to the east might serve his purpose better. And then? Then one would be out upon the amber-lit snow there, and half-way up to the crest of those beautiful desolations. ”

“ Between the beach and the line of columns there is a bosky talus, more thickly covered with brushwood than is at all common in the Hebrides, and scarce more passable than the rough beach at its feet. And at the bottom of this talus, with its one gable buried in the steep ascent,—for there is scarce a foot-breadth of platform between the slope and the beach,—and with the other gable projected to the tide-line on rugged columnar masses, stands the cottage. ”

“ La plupart de ces terrasses successives sont de grandes plaines, dominées par des rochers qui s'éboulent, et forment des talus. Si dans la succession des siècles, les éboulemens de ces bandes de rochers en amphithéâtre finissoient sans emporter les plaines qu'ils soutiennent, et que les torrens eussent creusé leur lit pendant ce tems là à quelque distance des talus tout seroit fini par cette première operation. Mais il y a peu de hautes montagnes où les arrangemens soient si simples: souvent ces bandes empiètent les unes sur les autres en s'éboulant, et alors le repos est bien différé. ”

“ Apparently the winds had first bent over all the stems of the clump; for whenever I saw one of them from the north, it showed a smooth, clean upward sweep. On the south side the snow first fell in a sheer cliff; then there was a hollow which was partly filled by a talus-shaped drift thrown in by the counter currents from the southern pit in which we were stopping; the sides of this talus again showed the marks that reminded of those left by the spoon when butter is roughly stroked into the shape of a pyramid. ”

“ Talus.—Wherever the earth’s surface rises in steep cliffs, the rock fragments derived from frost action, or by other processes of disintegration, as they become detached either fall or slide rapidly downward until arrested upon a flatter slope. Upon the earlier accumulations of this kind, the later ones are deposited, until their surface slopes up to the cliff face as steeply as the material will lie—the angle of repose. Such débris accumulations at the base of a cliff (Fig. 158) are known as talus, and the slope is described as a talus slope, or in Scotland as a “scree.”
[153]
Fig. ”

“ Talus—The mass of rock fragments lying at the base of a mountain cliff, formed by the accumulation of pieces brought down from above by the action of gravity, frost, rain, etc.; equivalent to "scree" or "debris."
Till—A stiff clay containing boulders of all sizes up to several tons weight; often smoothed and striated by glacial action. ”

“ An important point to be noticed, and which is evident from two of the reproduced photographs (Figs. 3 and 4) , is that the talus has never reached up so high as to check the disintegration at the mouth of the gorge of the Niagara shale and limestone which form the upper one hundred feet of the face, and which exhibit the maximum amount of enlargement which has taken place. ”

“ But if we examine the final result of this double waste, we shall see that when this ultimate ratio of equality is established throughout the upper and lower talus and the natural slope of the hill, the road must disappear altogether, instead of maintaining, as it often does, a breadth of 70 feet upon an uniform slope of the face of the hill. There is another circumstance in their construction equally repugnant to this hypothesis. In no one instance is the surface level, or even nearly so, as the profiles will show. ”

“ Up behind Bludenz you may see a row of limestone cliffs called Elser Schröfen, whose foot is defended by a “talus” of rubble which has slowly dropped down from the heights above; and a pretty thing it is, by{146} the way, when you look closely at natural features like this talus, to observe with what flawless accuracy they have been constructed; how these fragments of detritus pass in due order through all gradations of size down the slanting surface, from minute particles like sand at the top to the mighty blocks that form their base. ”

“ If the construction of soil goes on upon a steep cliff, the quantity retained on the slope may be small, but at the base we find a talus, composed of the fragments not held by the vegetation, which gradually increases as the cliff wears down, until the original precipice may be quite obliterated beneath a soil slope. ”

“ A kind of soil has accumulated on this talus, which is generally covered with vegetation. Trees of considerable size—oaks, pines, firs, cedars, maples, bay and dwarf oak, and lesser shrubs, are frequent. Although this debris is scarcely observed in a general view, its height above the bottom of the valley is in many places from three hundred to five hundred feet next to the cliff, from which it slopes some distance into the valley. In a few places the bases of the cliffs appear as if exposed nearly to the level of the valley. The valley proper is generally level through its entire length. ”

“ In some cases the surface-water disappears altogether before the outer border of the fan is reached, by sinking into the débris.
Fig. 170.—A talus cone. North Greenland Coast. The talus cone reaches the sea-level. Drawn from photograph.
Alluvial fans and cones have various forms, and often attain considerable dimensions. Their angles of slope depend on the amount of reduction of velocity which the depositing water suffers, and the amount and kind of load which it carries. The maximum slope of the cone is the angle at which the loose material involved will lie. ”

“ The wearing away of the fallen fragments of the cliffs is much hastened by the waterfalls which in time of heavy rains fall over the rim rock, their force being greatly augmented by the height from which the water is precipitated. The fragments continually falling from the roofs of the caves form a talus that extends from the floors of the caves down the side of the cliff. The cliff-dwellings are erected on the top of this talus. ”

“ The talus, pitted with the shallow pans called 'women's washings,' shows signs of hard work, probably dating from the days when every headman had his gang of 'pawns' and slaves. Rising at the head of the creek, itself a natural gold-sluice, its bed and banks can carry any number of flumes, which would deposit their precious burden in cisterns near the river. I need hardly say they must be made movable, so as to raise their level above the inundation. Here the one thing wanted would be a miner accustomed to 'hydraulicking' in California or British Columbia, Australia or South Africa. ”

“ As a rule such a talus, if art-containing, will have a large percentage of shop and quarry-shop refuse, for the reason that the exposed gravels, and the banks and beds of rivers cutting them, furnish, as a rule, a good deal of the raw material utilized by workers in stone, and the shops in which the work was done are usually located upon the slopes and outer margins of the terraces. ”

“ A little further on, the northern mountains send down a projecting spur, and an open tract is seen to the south. The mountains behind the alluvial platforms rise very abruptly, and present towards the plain, steep, almost perpendicular slopes, which, from the peculiar nature of the rock, a very fragile slate, are covered by a steeply-sloping mass of débris almost to the top. This talus, indeed, on some of the cliffs behind Lari, seems to rise to the very summit of the ridges. ”

“ It may be answered that the natural decay of the road would consist in the sliding down of the upper talus into the road, so as gradually to diminish its slope and to fill the interior angle, while a similar waste of the lower one would round the exterior or salient angle. ”

“ Although several thousand years have been required for their formation most talus accumulations in the park are still actively growing, especially in spring and early summer when rocks are pried loose by the alternate freezing and thawing of moisture within fractures. The artillery-like crack made when a falling rock crashes to the base of a high cliff is a familiar sound to anyone who has spent much time in the mountains. ”

“ The valley and its branches having been eroded out of basalt, the mountains and valleys are characteristic formations—pyramidal peaks, steep escarpments, deeply-cut gorges, with roaring torrents rushing down in a series of waterfalls and broken cascades; there are vast quantities of scree on the [33] mountain-sides covering the terraces of the lava flows, and accumulations of similar material at the foot of the mountains, forming a talus. Moraine heaps are also numerous. ”

“ On the slopes of the mountains he is hindered by thickets and masses of débris, on highland and plain alike by prostrate trees and a tangle of bushes, in the hollows and valleys by standing and flowing water, by brooks and swamps. Wide-spread talus from the rocks, blocks and boulders rolled into heaps and layers form barriers on all the hills; lichens and mosses form a web over the rocks, and treacherously conceal the numerous gaps and clefts between them; a young undergrowth is rooted between and upon the old possessors of the soil ”

“ It is evident that the loose stony and earthy matter that accompanies the trees will speedily sink to the bottom and rest at the foot of the slope somewhat like an ordinary sub-aerial talus, but not so the trees. The impetus of their fall must launch them afloat and impel them towards the middle of the estuary, where they will be spread about and continue floating, until by saturation they become dense enough to sink. They will thus be pretty evenly distributed over the bottom. ”

“ The agency of glaciers in producing permanent geological changes consists partly in their power of transporting gravel, sand, and huge stones to great distances, and partly in the smoothing, polishing, and scoring of their rocky channels, and the boundary walls of the valleys through which they pass. At the foot of every steep cliff or precipice in high Alpine regions, a talus is seen of rocky fragments detached by the alternate action of frost and thaw. ”

“ The ice-falls descending over the northern sides are almost continuous one with another, but the southern steep faces are nearly bare; evidently the sun gets a good hold on them. There must be a good deal of melting and rock weathering, the talus heaps are considerable under the southern rock faces. Higher up the valley there is much more bare rock and stratification, which promises to be very interesting, but oh! ”

“ The escarpment q p, from its position and height, is easy to fortify. Its length is 700 mètres; its mean elevation above the plain, 18. It presents to the river a talus of less than 45 degrees. The Romans made in this talus, along the crest, a trench, forming wall and fosse. Its length was 700 mètres.
4. From the point of Epeisses to the escarpments of Etournel, extent 6 kilomètres.—From Epeisses to Chancy the Rhone flows in a straight line, and presents the appearance of a vast fosse, 100 mètres wide, the walls of which have an inclination of more than 45 degrees. ”

“ In many the precipice at the top has been slanted back by the weather, and the chips falling to the bottom have continued the incline in the form of a long talus, which still remains only because in all the centuries that have passed since the chips began to accumulate, the stream has never been working on that side. ”

“ At the pass of Escrinet in France, on the northern escarpment of the Coiron hills, near Aubenas, I have seen a breccia in the act of forming. Small pieces of disintegrating limestone are transported, during heavy rains, by a streamlet, to the foot of the declivity, where land shells are very abundant. The shells and pieces of stone soon become cemented together by stalagmite into a compact mass, and the talus thus formed is in one place fifty feet deep, and five hundred yards wide. ”

“ On the lower slopes, even under the tapestry of verdure which nature delights to hang where she can over her naked rocks, we may detect the same pervading influence of the joints upon the forms assumed by ravines and crags. Each kind of stone, too, gives rise to its own characteristic form of scenery. Massive crystalline rocks, such as granite, break up along their joints and often decay into sand or earth along their exposed surfaces, giving rise to rugged crags with long talus slopes at their base. ”