Russel Wallace : Alfred Russell Wallace (sic)
If this description applied to any real locality we should have, on a small scale, all the features which characterise an 'inaccessible valley,' the sides formed by rocky precipices, while at the upper and lower ends are narrow gorges rendered impracticable, either by waterfalls, or by the stream filling up the channel at its narrowest portion where the vertical side walls leave no foothold. Persons who know Exmoor thoroughly say, however, that there is no such valley in any part of the district, and that the talented author has, in this portion of his work, drawn on his imagination for his facts. Nor, so far as I am aware, has such a valley been described in any part of the British Isles, or even in that land of rock-girt valleys and narrow gorges, Switzerland. In fact, considering how very common are each of the four elements required to form an inaccessible valley, it is remarkable how few such valleys exist in any part of the world. These elements are, either a waterfall or a water-blocked gorge at each end, and both sides to be walled by a continuous line of precipices. Valleys with rocky walls on one side and a narrow gorge for outlet are frequent, but then the opposite side has slopes which [[p. 392]] render it easily accessible. Not unfrequently there is a ravine with waterfalls as the upper outlet also, but in almost every case there is some break in the rock walls on one side or the other with easy slopes for the entrance of men and animals.
The only considerable valleys that can be classed as originally inaccessible--though of course no valley, any more than any mountain, is absolutely so--seem to be, the Yosemite in California, and the valleys of the Grose and Cox rivers in New South Wales. It may, therefore, be interesting to describe these valleys, which are in many ways very remarkable. The theories that have been suggested to account for them may then be considered; and we shall thus be led to discuss the general theory of valley-formation and the peculiar combination of conditions which in these two very dissimilar cases have led to a somewhat similar result.
The Yosemite valley is a portion of the upper course of the Merced River, which rises near the summit of the Sierra Nevada about 170 miles almost due east of San Francisco. This great mountain range, forming the western edge of the lofty table-land of which the Rocky Mountains form the eastern border, has a very gradual slope from the central valley of California, the distance from the foothills to the summit varying from sixty to eighty miles, while the height is from 8,000 to nearly 15,000 feet. This average slope of from 100 to 250 feet in a mile is rendered exceedingly irregular by numerous large winding valleys, some with easy slopes, some more precipitous, and all more or less covered with forest so as to render the journey from one point to another both circuitous and difficult. The higher portion of the Sierra Nevada is usually of granitic rock, lower down are metamorphic slates, followed by enormous beds of late tertiary gravels, which are often covered with great sheets of lava and ashes, bearing witness to the numerous volcanoes on the summit of the range at a period geologically very recent. The Yosemite valley is situated a little above the middle of the slope and entirely in the granite region, which is here very wide. It is about seven miles long and from half a mile to a mile wide, the bottom nearly level but rising slightly to the base of the cliffs on either side. These precipices are among the grandest in the world, some of them absolutely perpendicular from base to summit, others with alternate slopes and rock-cliffs, but everywhere equally inaccessible to the ordinary traveller, except in a few places by narrow shelves and steep gullies originally discovered by the Indians and since made into practicable paths or roads. At the lower end the valley becomes narrowed into a deep ravine or cañon for a considerable distance, while at the upper end it branches out into three equally rock-walled valleys with grand waterfalls, leading up to the crest of the mountain range.
This remarkable valley may be said to average about half a mile in vertical depth, but some of the precipices that give it so impressive [[p. 393]] a character are considerably more than this height, El Capitan at the lower end of the valley being a smooth vertical wall of granite 3,300 feet high with no visible crack or ledge upon it from top to bottom. Cathedral rock, nearly opposite, is 2,600 feet; the Sentinel Rock, nearly the middle of the south side of the valley, is over 3,000 feet; while the Half Dome at the upper end of the valley is no less than 4,737 feet high, the upper 1,500 feet of which is quite vertical, while the lower part slopes at an angle of 60° or 70° and is partly concealed by fallen fragments. The great dome-shaped masses of granite are a characteristic feature of the Sierra Nevada, as they are of some other granitic regions. Nearly opposite the Half Dome is the North Dome, 3,568 feet high, its summit beautifully rounded, but broken lower down so as to show the concentric layers of which it is formed. The Sentinel Dome on the south side is of similar character. The Half Dome is exactly like the other domes in character, but appears as if cut off vertically, leaving the southern half quite perfect and of a fine spherical contour.
Professor J. D. Whitney, formerly State Geologist of California, thus characterises the valley in his Yosemite Guide Book:
The principal features of the Yosemite, and those by which it is distinguished from all other known valleys, are: first, the near approach to verticality of its walls; second, their great height, not only absolutely, but as compared with the width of the valley itself; and, finally, the very small amount of talus or débris at the base of these gigantic cliffs. These are the great characteristic features of the Yosemite throughout its whole length; but, besides these, there are many other striking peculiarities and features, both of sublimity and beauty, which can hardly be surpassed, if equalled, by those of any mountain valleys in the world. Either the domes, or the waterfalls of the Yosemite, or any single one of them even, would be sufficient in any European country to attract travellers from far and wide.
The origin of this wonderful valley has been a puzzle even to geologists. After describing the formation of most of the valleys of the Sierra Nevada as being due to denudation, Professor Whitney says:
The eroded cañons of the Sierra, however, whose formation is due to the action of water, never have vertical walls, nor do their sides present the peculiar angular forms which are seen in the Yosemite, as, for instance, in El Capitan, where two perpendicular surfaces of smooth granite, more than 3,000 feet high, meet each other at a right angle. These squarely-cut, re-entering angles, like those below El Capitan, and between Cathedral Rock and the Sentinel, or in the Illilouette cañon, were never produced by ordinary erosion. Much less could any such cause be called in to account for the peculiar formation of the Half Dome, the vertical portion of which is all above the ordinary level of the walls of the valley, rising 2,000 feet, in sublime isolation, above any point which could have been reached by denuding agencies, even supposing the current of water to have filled the whole valley.
He then goes on to discuss the possible agency of ice, which he dismisses as quite inadequate. Valleys formed by fissures of the earth's crust are then discussed, and it is shown that the Yosemite [[p. 394]] cannot have been formed in this way, partly because it is too wide, and also because there is no correspondence between the opposite sides.
In default of any of the usually accepted theories of valley-formation, Professor Whitney has been led to adopt one which has hardly yet been recognised by geologists, as probable or even possible, and which he describes as follows:
We conceive that, during the process of upheaval of the Sierra, or, possibly, at some time after that had taken place, there was at the Yosemite a subsidence of a limited area, marked by lines of fault or fissures crossing each other nearly at right angles. In other and more simple words, the bottom of the valley sank down to an unknown depth, owing to its support being withdrawn from underneath during some of those convulsive movements which must have attended the upheaval of so extensive and elevated a chain, no matter how slow we imagine the process to have been.
After showing that subsidence is a well-ascertained fact, the only difficulty in this place being the great vertical displacement of such a small area, he adds:
By the adoption of the subsidence theory for the formation of the Yosemite we are able to get over one difficulty which appears insurmountable with any other. This is, the very small amount of débris at the base of the cliffs, and even, at a few points, its entire absence. We see that fragments of rock are loosened by rain, frost, and other natural causes, along the walls, and probably not a winter elapses that some great mass of detritus does not come thundering down from above, adding no inconsiderable amount to the talus. Several of these great rock-avalanches have taken place since the valley was inhabited. One, which fell near Cathedral Rock, is said to have shaken the valley like an earthquake. This abrasion of the edges of the valley has unquestionably been going on during a vast period of time; what has become of the detrital material? Some masses of granite now lying in the valley are as large as houses. Such masses as these could never have been removed from the valley by currents of water. . . . It appears to us that there is no way of disposing of the vast mass of detritus, which must have fallen from the walls of the Yosemite since the formation of the valley, except by assuming that it has gone down to fill the abyss which was opened by the subsidence which our theory supposes to have taken place.
This extraordinary theory, put forth by an experienced geologist in 1874, will probably not be accepted now; but it serves to show that the Yosemite has always been considered a remarkable and exceptional valley which could only have been produced by some exceptional causes. A visit to the valley a few years since satisfied the present writer that the modern and now generally accepted theory of valley-formation is quite sufficient to account for the Yosemite, though its features have been rendered almost unique by the peculiar character of the rocks out of which it has been hollowed, combined with the meteorological and physical conditions of the locality, both now and during the latter part of the tertiary epoch. After having described the Australian valleys referred to at the commencement of this article, an attempt will be made to show that both are true valleys of denudation.
[[p. 395]] In some respects the valleys carved out of the great sandstone plateau of New South Wales are even more remarkable than the Yosemite itself. This plateau forms the eastern side of the Blue Mountains, and at its eastern border is about a thousand feet above the sea level; but going westward it rises about 100 feet in a mile, so that at its further side, at a distance of twenty-five miles, it is 3,400 feet above the sea. This slightly undulating monotonous surface is, however, deeply intersected by widely branching ravines which increase in depth as we proceed westward, and which everywhere present perpendicular crags and cliffs of a very remarkable character. The ravines which discharge their waters into the little river Cox occupy an area of 1,212 square miles. The whole forms the basin of this mountain stream, and is bounded by cliffs increasing from about 1,000 feet near its outlet to about 2,500 feet near its western limits, the valley bottom being not much above the sea level, and the only outlet being through a gorge about a third of a mile wide.
Further to the north is the smaller valley of the Grose, whose diverging ravines interlock, as it were, with those of the Cox, forming a great obstacle to the early explorers in their attempts to cross the plateau. The Grose valley has still grander precipices than that of the Cox, rising at the upper end to 3,000 feet in vertical height. The best account of these valleys is that given in Darwin's work on Volcanic Islands, the last chapter of which is devoted to Australia and other places visited on the homeward voyage. He says:--
It is not easy to conceive a more magnificent spectacle than is presented to a person walking on the summit-plains, when without any notice he arrives at the brink of one of these cliffs, which are so perpendicular that he can strike with a stone (as I have tried) the trees growing at a depth of 1,500 feet below him; on both hands he sees headland beyond headland of the receding line of cliff, and on the opposite side of the valley, often at a distance of several miles, he beholds another line, rising up to the same height with that on which be stands, and formed of the same horizontal strata of pale sandstone. The bottoms of these valleys are moderately level, and the fall of the rivers flowing in them, according to Sir T. Mitchell, very gentle. The main valleys often send into the platform great bay-like arms, which expand at their upper ends; and, on the other hand, the platform often sends promontories into the valleys, and even leaves in them great, almost insulated, masses. So continuous are the bounding lines of cliff, that to descend into some of these valleys it is necessary to go round twenty miles; and into others the surveyors have only lately penetrated, and the colonists have not yet been able to drive in their cattle. But the most remarkable point of structure of these valleys is that, although several miles wide at their upper parts, they generally contract towards their mouths to such a degree as to become impassable. The Surveyor-General, Sir T. Mitchell, in vain endeavoured, first on foot and then by crawling between the great fallen fragments of sandstone, to ascend through the gorge by which the river Grose joins the Nepean; yet the valley of the Grose, in its upper part, as I saw, forms a magnificent basin some miles across, and is on all sides surrounded by cliffs, the summits of which are nowhere less than 3,000 feet above the sea-level. When cattle are driven into the valley of the Wolgan by a path partly cut by the colonists, they cannot escape; for this valley is in [[p. 396]] every other part surrounded by perpendicular cliffs, and eight miles lower down it contracts from an average width of half a mile to a mere chasm, impassable to man or beast.
The origin of these valleys appears to have been as great a puzzle to the early explorers as was that of the Yosemite. Sir Thomas Mitchell estimates that 134 cubic miles of rock must have been removed from the valley of the Grose alone; and he remarks on the absence of indication of the agency by which these vast masses of stone have been carried away, there being no accumulations of sand, though there are many huge blocks of rock, scarcely worn by attrition, in the bed of the stream, while in the valleys below, instead of sandy deposits, there is a rich alluvium. Even Darwin was staggered at the idea of these enclosed valleys being hollowed out by aqueous erosion. Neither does he accept subsidence, on account of the numerous irregularly branching arms. The resemblance of the cliffs to those of a bold sea coast suggests marine action, 'but then,' he remarks, 'occurs the startling difficulty, why has the sea worn out these great, though circumscribed, depressions on a wide platform, and left mere gorges, through which the whole vast amount of triturated matter must have been carried away?' Finally, he suggests, that marine currents often form banks of most irregular form, and so steep that a small amount of subsequent erosion during elevation might form them into cliffs. We must consider, however, that this plateau has certainly been elevated since the latter part of the secondary period, leaving ample time for any amount of denudation; and Mr. Beete-Jukes, in his Sketch of the Physical Structure of Australia, informs us that similar valleys abound throughout the great sandstone formation, both at high and low levels; and they have so exactly the character, in the distribution of their diverging branches, of ordinary streams carrying off the drainage of a slightly inclined surface, that no exceptional origin for them seems needful. This will be more clear when we have discussed the modern theory of valley-formation and the special characteristics of the rocks in which these remarkable valleys have been excavated.
One of the most common ideas, when a person sees a deep gorge or ravine bounded by lofty precipices, is, that the rocks have been torn asunder by some earthquake or other subterranean movement. A 'convulsion of nature' is almost always referred to in popular descriptions of such scenes. Till recent years even geologists considered that many valleys were so formed. The article on the 'Geology of the Alps,' by M. Desor, in Ball's Alpine Guide, published in 1870, gives 'valleys of disruption' as one of the forms of Alpine valleys, and cites the defile of the Via Mala on the Hinter Rhein, and the valley of the Rhone, between Bex and Martigny, as examples. He defines them as 'evidently produced by rents that have torn asunder ranges once continuous.' Professor Whitney, also, in his [[p. 397]] Yosemite Guide-Book, speaks of rents or fissures as one of the recognised modes of valley-formation.
Now, however, it is held by most, if not all, geologists that valleys are never formed in this way. It is to the late J. Beete-Jukes, Director of the Geological Survey of Ireland, that we owe the full establishment of the principle that 'valleys of all kinds, from the most open to the most narrow and profound, are hollows worn by erosion.'1 He was struck by the fact of many of the rivers of the south of Ireland, after running for miles over low plains open to the sea, suddenly turning at right angles, cutting through the hills by deep narrow ravines, and so reaching the sea beyond them. Sometimes even the hills the river cut through were isolated, so that the river might, apparently, have passed round them in either direction. The explanation usually offered of these phenomena was that the hills had been fissured by subterranean forces, and that the rivers had taken advantage of them to change their course. But close examination showed that these ravines were not fissures, but channels eroded in the rock, since the solid rock could often be traced unbroken across the very bed of the stream. And, after examining many ravines in different parts of the world, he came to what then seemed the very startling conclusion that, except, perhaps, in districts recently convulsed by great earthquakes, there is no such thing as a glen, ravine, or valley occupying the upper portion of an open-mouthed fissure. On the contrary, in every case the whole space between the two sides of the valleys was once filled by rock, which has been gradually worn down and carried away. The very frequent presence of cascades and waterfalls in such ravines, formed by a continuous bed of hard rock crossing the stream, is itself sufficient to disprove the theory of fissures, in which case the whole bed would present a mass of fallen fragments, filled in with pebbles and sand; but this consideration seems never to have occurred to the upholders of the apparently obvious and easy theory of violent disruption.
It remains, however, to account for the very common phenomenon of rivers apparently going out of their way to cut a narrow passage through a hill, instead of following lower ground to a main valley or to the sea. Such in our own country are the small rivers Ouse and Cuckmere, which cut through the South Downs between Brighton and Beachy Head, instead of following the low ground and reaching the sea between Eastbourne and St. Leonards; while the Avon, which flows through the gorge of St. Vincent's rocks at Clifton, might apparently have found a much easier way to the sea by a more northerly or a more southerly course. Mr. Jukes explains all these cases on the principle that the courses of almost all the rivers of a country were determined by the contour of the land when it first rose above the sea, the surface [[p. 398]] water seeking always the easiest course along the hollows and gentle slopes, without any regard to the nature of the rocks beneath. When once these streams had formed definite channels, it was almost impossible to alter them (except when diverted by lava streams or glaciers) because movements of elevation are so slow that the rivers can cut their way down as fast as the land rises up. Thus, the American geologists have proved that the Uintah Mountains were upheaved across the valley of the Green River after the course of that river was established, and that, as fast as they rose, the river cut through them, and now flows in a tremendous gorge or cañon. Another illustration of the permanence of river channels is afforded by the Moselle, which, although it flows at the bottom of a deep, narrow valley sunk in a nearly level plateau, winds about in great curves and deep horseshoe bends exactly like a stream flowing over a flat alluvial plain. No explanation of this can be given except that the river began its existence on a nearly level surface, and after it had established its course in the characteristic winding fashion of such streams, it has, in the course of long ages, cut its way deep down through the rock, and thus formed its present valley.
Now, every considerable area of continental land is made up of rocks and deposits of very unequal hardness and resisting power, from clays and sands to the various kinds of rock. Some of these can be dissolved and carried away by running water much more quickly than others; while rain, frost, and wind, also act upon their exposed edges very unequally. Hence arise the peculiar forms assumed by hills of different composition, and hence the reason why valleys are in some parts very narrow and precipitous, in others wide and open. It is an invariable rule that hills and mountains are composed of the harder or less soluble rocks, the adjacent lowlands and valleys of the softer and more soluble. Hence, we see all great mountain ranges mainly composed of the older, hard, or crystalline rocks, while the lowlands, plains, and valleys are occupied by the newer and softer formations. In our own country the tertiary or secondary clays and sands are found in the lowland districts, while the more ancient and much harder rocks form the hills of Devonshire, Wales, the Lake District, and Scotland.
Keeping in mind the extreme inequality of the rate of denudation of different rocks, we are able easily to explain the apparently erratic course of so many rivers. When the streams originated they took their course along lines of least resistance, depending on the form of the surface, not on the nature of the rocks beneath the surface. Sometimes this course passed over ridges or bosses of very hard rock, buried perhaps hundreds, perhaps thousands, of feet deep. But the channels once fixed could not be altered, and when the bed of the stream reached this rock it cut down into it. Then, owing to the hardness of the rock, the river channel would be a gorge or ravine, [[p. 399]] while all around the softer rocks would be denuded by frost and rain, so that extensive areas would be lowered as fast as the stream cut its narrow channel through the hard rock, and was able to carry away the denuded material. Hence, in the course of ages we should have the stream flowing over a wide lowland, perhaps on one side open to the sea, and then cutting straight across a mountain ridge, or even across an isolated hill entirely surrounded by lowlands.
Not very much time, geologically speaking, is required for such operations. Sir Charles Lyell describes a channel, cut by the river Simeto across a lava stream from Etna, which is over fifty feet wide and in some parts forty to fifty feet deep. The lava is not porous, but is a homogeneous mass of hard blue rock. Yet the date of the eruption which produced this lava stream is known to be 1603.2 But the most wonderful example of the power of water to denude and erode the hardest rocks is afforded by the great cañon of the Colorado river. This has been cut for about 400 miles to a depth of from 4,000 to 7,000 feet, mainly through masses of hard palæozoic rocks down to the archæan, and the whole of this vast operation has been performed in the latter half of the tertiary period. The formation of the river began, it is true, in very early tertiary times, but at that epoch the present surface was buried about 9,000 feet deep in secondary rocks, which have all been since denuded away, so that Captain Dutton estimates that the river has cut its channel on the whole through from 10,000 to 16,000 feet of mesozoic, carboniferous, and other ancient rocks, all during the tertiary period.3
Keeping in mind these remarkable instances of denudation, let us turn to consider the probable origin of the remarkable valleys which have seemed to eminent geologists so peculiar as to need some special mode of origin; and we will take first the great rock-walled valleys of New South Wales, as being the most simple in their main features.
These are all excavated in sandstones and shales of the carboniferous system, though perhaps of mesozoic age. The strata are nearly horizontal, and, what is especially important, they are of very unequal degrees of hardness. The upper beds are usually conglomerates, and are so comparatively indestructible that isolated summits often imitate ruined castles. In places these beds are so hard that boring-tools will not penetrate them, while in other parts the rock is so incoherent that large blocks will break in pieces by falling over an embankment.4 We have here the essential conditions for the formation of vertical escarpments, since by the weathering away [[p. 400]] of the softer beds the harder strata above them remain unsupported and break off, and thus the vertical or sometimes overhanging character of the precipices is kept up.
If we look at a large-scale map of this part of Australia, we see that the rivers Grose, Cox, and other tributaries of the Nepean which drain the sandstone plateau, have great numbers of diverging branches which almost interlace with each other, as so often occurs among the streams of a nearly level well-watered district. Now, bearing in mind what has been said of the permanence of watercourses once formed, we can see that these many-branching streams must have flowed on the surface of the plateau at the epoch of its first elevation; that surface itself being perhaps a long way above the present surface, which has certainly been lowered by denudation during its long existence as dry land, probably during the whole of the tertiary period. From the time that these streams began to penetrate the sandstone plateau as far as the first hard bed, miniature cliffs would be formed by the wasting away of the softer beds beneath it, and the continual movement backward thus produced would widen the valleys till those of many of the smaller tributaries became united together. Thus age after age the valley would widen and deepen, always preserving its precipitous rock-walls due to the alternation of hard and soft layers.
The deepening of these great valleys would probably be aided by subterranean denudation due to the presence of salt and alum, which Mr. Clarke states are found at several places in these strata. The solution of these salts by percolating water would form cavities and water channels, and the subterranean streams would eat away the softer beds, forming caverns, the roofs of which would in time fall in, and the débris be gradually disintegrated by atmospheric agencies and then carried away by floods. This mode of denudation was seen actually at work by Captain (now Sir George) Grey, during his exploration of the Glenelg River in North-West Australia. He describes a nearly level table-land covered with numbers of sandstone pillars of various grotesque shapes and some of them forty feet high. Hearing the sound of running water at a fissure among some of the rocks, he descended, and found a cavern supported by pillars of the same character as those above, with a small stream, which in the rainy season would become a torrent. Here, then, are ample causes to explain the formation of these great rock-walled branching valleys in the sandstone plateau; the remaining feature--that the rivers all escape through deep gorges often so narrow or so blocked up with rock-fragments as to be impassable--evidently depends on the fact that the outer escarpment of the plateau is formed of a series of harder rocks, and thus does not wear away laterally. In this respect they resemble those numerous gorges in the Alps which form the only outlet for [[p. 401]] considerable high valleys, such as those of the Trient, the Reuss, and many others.
The difficulty as to whither the denuded material has gone, does not seem a great one, when we remember the many millions of years the process of denudation has been going on, with alternating epochs of greater rainfall producing more rapid-flowing streams and greater floods, by which the bulk of the sandy material would be carried out to sea, while the finer suspended matter would be deposited during wide-spreading floods on the valley bottoms and alluvial plains. The absence of great quantities of rock in the valleys themselves merely indicates that the degradation of the cliffs is now so comparatively slow that the fallen masses are worn down by atmospheric agency at about the same rate as they are reproduced.
Let us now see how the same general principles and the same denuding agencies will apply under the very different conditions which have prevailed in the district of the Yosemite. These differences are, mainly, the much loftier mountains and the very much greater extremes of climate; the recent occurrence both of glacial and of volcanic action on a large scale; and, lastly, the whole valley being excavated in granite instead of in sandstone rock.
The granite of the central and highest parts of the Sierra Nevada is flanked near the Yosemite with Silurian slates, lower by some triassic or jurassic beds followed by enormous deposits of late tertiary gravels, which have been largely preserved from denudation by extensive flows of lava, the remnants of which form the numerous table-mountains so characteristic of the lower slopes of the Sierra. As granite can only be formed deep down in the crust of the earth, it is certain that, when first elevated to form the mass of the Sierra Nevada, it was everywhere deeply buried under Silurian and other palæozoic rocks, and not improbably under a further deposit of mesozoic age. These various beds, of an unknown thickness, must all have been denuded away before the granitic core was exposed, and during that process the main lines of the valleys must have been fixed, and the streams might have begun to cut their way into the granite substratum.
Although granite appears to be, and sometimes is, a very durable rock, it varies greatly in its power of resisting denudation, owing perhaps, in part, to the nature and thickness of the overlying rocks, beneath or among which it was forced up, and which in some cases determined the characteristic forms it assumes when exposed to atmospheric agencies. These forms are either rude cubical masses, as seen on some of our Dartmoor tors; peaks and pinnacles, as in some of the Alps of Dauphiné and in the cathedral spires of the Yosemite; but more commonly rounded forms, culminating in cones or almost perfect domes or hemispheres, as in the great domes of the Yosemite. It is an interesting fact that all these forms occur also in the granite region of the Upper Rio Negro in Brazil. The Cocoi Mountain [[p. 402]] forming the boundary between Brazil and Venezuela is a quadrangular or cubical mass of granite, about a thousand feet high, rising abruptly out of a great undulating plateau of the same rock. Others in the same region are conical or dome-shaped; and on the southern bank of the river Uaupes, about sixty miles from its mouth, is an isolated dome-shaped mountain about a thousand feet high, of so regular an outline as to look like a gigantic half-globe. Now, it is evident that these cubical, hemispherical, and conical hills, rising out of a nearly level plateau which extends for several hundred miles around them in every direction, must owe their present position to the slow degradation by atmospheric agency of the vast masses of rock in which they were once buried, but whose destruction they have survived owing to their superior hardness or tenacity. It is true the rocks here have been subject to tropical rain and heat and to the powerful aid of tropical vegetation; but, on the other hand, the rocks of the Yosemite have been exposed to the even more powerful agencies of alternations of intense frost and great sun-heat, as well as of torrents formed by melting snows, and probably of occasional débâcles caused by bursting glacier lakes.
It is well known that granite often weathers very rapidly, sometimes becoming completely decomposed to a depth of twenty or thirty feet, so that it can be dug out with pick and spade. This process of decomposition is greatly facilitated by the action of carbonic acid either in air or water. Now, during the latter part of the tertiary epoch, there was a long period of volcanic action in the Sierra Nevada, and as both carbonic acid and many other powerful gases are emitted during eruptions, and also permeate the earth and are absorbed by the water, we should have all the conditions for the decomposition and denudation of the granite rocks. The alternations of temperature on the higher parts of the Sierra Nevada are very great. During the long bright Californian summer the action of direct sun-heat on the exposed rocks must be considerable, the air temperature in the Yosemite valley being usually over 80°, while at a height of 8,700 feet ice an inch thick was formed at night in June and July. In winter at such elevations--that of the present summit of some of the domes--the temperature must fall below zero of Fahrenheit every night. The alternate expansion and contraction produced by such changes of temperature are among the most powerful agencies in the splitting up and decomposition of rocks. Small cracks thus produced receive water which freezes at night, and the crack is widened by the irresistible force of the ice wedge. It is by this agency that the final touches have been given to the Yosemite scenery, after all the softer and more decomposable portions of the rock had been removed by the ordinary modes of weathering. The huge domes and spires, and the subquadrangular mass of El Capitan, must be looked upon as intensely hard and compact cores of rock that remain after all the more [[p. 403]] friable masses that inclosed them have been removed. They show us the natural forms into which granite weathers, due probably to the mode in which it has originally cooled from the molten or plastic state. In the case of the dome form the mass consists of concentric layers, probably of different density, which peel off successively like the coat of a gigantic onion. On some of the domes we can see one of these coats partially removed, and the same thing was observed by myself in the dome-shaped mountains as well as in the smaller sub-globular masses of granite in the Rio Negro.
The fact that the process of denudation, continued perhaps throughout the greater part of the tertiary period, has now eaten away all the more friable and soluble portions of the rocks which once occupied the site of the valley, leaving only those compact central masses which are hardly affected by ordinary atmospheric action, will account for what seemed such a great difficulty to Professor Whitney--the small amount of rock débris under the great precipices or in the valley generally. For the last few thousand years, probably, the amount of rock-falls has been comparatively small, so that it barely equals the rate at which atmospheric agencies, aided by vegetation, break up and decompose the fallen masses, which then, in the form of the coarse granitic sand that forms the surface soil in all the drier portions of the valley, is gradually carried by wind, rain, and melting snow into the river, and ultimately into the great bay of San Francisco. That some considerable amount of decay is still going on in these giant cliffs is evident, not only from the rock-falls that actually occur every year, but from the numerous places where great flakes or jutting blocks can be observed in every stage of detachment from the parent rock. These fallen masses, however large, are at once subject to fresh causes of decay. Almost all their surfaces are exposed to atmospheric action or to expansion and contraction by heat and cold. Every crack and cranny is seized upon by vegetation--first the lowly herb, then the shrub, later the tree, whose roots penetrate the minutest fissure, eat away the surface, or even split off portions by the power of growth. And though in the life of a man a block may seem unchanged, in a few thousand years it may have entirely disappeared; and such a lapse of time probably bears a less proportion to the period occupied by the valley's formation, than does a single hour to the life of a man.
It has now, I think, been shown that the valleys here described do not owe their exceptional physical features to any catastrophic or unusual mode of origin. Every characteristic they possess is fully explained by that simple theory of earth sculpture by atmospheric agency which has been found applicable to the solution of similar problems in all other parts of the world. This theory does not, of course, imply that subterranean movements have no part in determining the direction of some valleys, but only that they have [[p. 404]] in no case produced the valleys themselves. Many examples can be pointed out in which valleys follow for a certain distance lines of fault, of the junction of different strata, or of the fractured summit of an anticlinal; but the explanation of these cases is, probably, that during elevation above the sea, wave-action produced slight hollows along these several lines of weakness, and that the hollows thus formed were occupied by the primitive rivulets as their line of least resistance when flowing towards the ocean. But these cases are very few as compared with those of valleys which pay no regard whatever to the geological features of the undercrust, but which cross over faults and outcrops, and break through transverse hills and mountain ranges, as if the causes which determined their direction of flow were of an altogether different nature. And as regards what used to be considered the most striking cases of 'valleys of disruption'--the narrow defiles and gorges like those of the Trient and the Reuss--it may now be affirmed, that in no single instance which has been carefully examined has any evidence of an open fissure been discovered, while in most cases there is the clearest proof that the gorges in question have been wholly excavated by the action of running water.
It was for the purpose of bringing clearly before non-geologic readers the total inaccuracy of the popular view--that every rock-walled valley or deep Alpine gorge has had its origin in some 'convulsion of nature'--and to impress upon such readers the grand but simple theory, which we owe mainly to the late Sir Charles Lyell, of the efficiency of causes now in action in producing the varied contours of the earth's surface, that this account of some of the most remarkable of known valleys has been written.