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the very small amount of débris at the base the Blue Mountains, and at its eastern of the cliffs, and even, at a few points, its border is about a thousand feet above entire absence. We see that fragments of the sea-level; but going westward it rock are loosened by rain, frost, and other rises about one hundred feet in a mile, natural causes, along the walls, and prob- so that at its further side, at a distance ably not a winter elapses that some great of twenty-five miles, it is thirty-four mass of detritus does not come thundering hundred feet above the sea. This down from above, adding no inconsiderable amount to the talus. Several of these great slightly undulating, monotonous rock-avalanches have taken place since the face is, however, deeply intersected by valley was inhabited. One, which fell near widely branching ravines which inCathedral Rock, is said to have shaken the crease in depth as we proceed westvalley like an earthquake. This abrasion ward, and which everywhere present of the edges of the valley has unquestion- perpendicular crags and cliffs of a very ably been going on during a vast period of remarkable character. The ravines time ; what has become of the detrital ma- which discharge their waters into the terial? Some masses of granite now lying little river Cox occupy an area of 1,212 in the valley are as large as houses. Such
square miles. The whole forms the masses as these could never have been removed from the valley by currents of water. basin of this mountain stream, and is It appears to us that there is no way
bounded by cliffs increasing from about of disposing of the vast mass of detritus, one thousand feet near its outlet to which must have fallen from the walls of about twenty-five hundred feet near its the Yosemite since the formation of the western limits, the valley bottom being valley, except by assuming that it has gone not much above the sea-level, and the down to fill the abyss which was opened by only outlet being through a gorge about the subsidence which our theory supposes a third of a mile wide. to have taken place.
Further to the north is the smaller This extraordinary theory, put forth valley of the Grose, whose diverging by an experienced geologist in 1874, ravines interlock, as it were, with those will probably not be accepted now; but of the Cox, forming a great obstacle to it serves to show that the Yosemite the early explorers in their attempts has always been considered a remark- to cross the plateau. The Grose valley able and exceptional valley which could has still grander precipices than that of only have been produced by some ex- the Cox, rising at the upper end to ceptional causes. A visit to the valley three thousand fcet in vertical height. a few years since satisfied the present The best account of these valleys is writer that the modern and now gener- that given in Darwin's work on “ Volally accepted theory of valley formation canic Islands,” the last chapter of is quite sufficient to account for the which is devoted to Australia and other Yosemite, though its features have been places visited on the homeward voyage. rendered almost unique by the peculiar He says : character of the rocks out of which it
It is not easy to conceive a more magnifihas been hollowed, combined with the cent spectacle than is presented to a person meteorological and physical conditions walking on the summit-plains, when withof the locality, both now and during the out any notice he arrives at the brink of one latter part of the tertiary epoch. After of these cliffs, which are so perpendicular having described the Australian valleys that he can strike with a stone (as I have referred to at the commencement of tried) the trees growing at a depth of fifteen this article, an attempt will be made to hundred feet below him; on both hands he show that both are true valleys of denu- sees headland beyond headland of the re
ceding line of cliff, and on the opposite side dation.
of the valley, often at a distance of several In some respects the valleys carved
miles, he beholds another line, rising up to out of the great sandstone plateau of the same height with that on which he New South Wales are even more re- stands, and formed of the same horizontal markable than the Yosemite itself. strata of pale sandstone. The bottoms of This plateau forms the eastern side of these valleys are moderately level, and the
fall of the rivers flowing in them, according | irregularly branching arms.
The reto Sir T. Mitchell, very gentle. The main semblance of the cliffs to those of a valleys often send into the platform great bold seacoast suggests marine action, bay-like arms, which expand at their upper “ but then,” he remarks,
occurs the ends ; and, on the other hand, the platform startling difficulty, why has the sea often sends promontories into the valleys,
worn out these great, though circumand even leaves in them great, almosto in
scribed, depressions on a wide platform, 'sulated, masses. So continuous are the bounding lines of cliff, that to descend into and left mere gorges, through which some of these valleys it is necessary to go
the whole vast amount of triturated round twenty miles ; and into others the matter must have been carried
away surveyors have only lately penetrated, and Finally, he suggests, that marine curthe colonists have not yet been able to drive rents often form banks of most irregular in their cattle. But the most remarkable form, and so steep that a small amount point of structure of these valleys is that, of subsequent erosion during elevation although several miles wide at their upper might form them into cliffs. We must parts, they generally contract towards their consider, however, that this plateau has mouths to such a degree as to become im- certainly been elevated since the latter passable. The Surveyor-General, Sir T. Mitchell
, in vain endeavored, first on foot part of the secondary period, leaving and then by crawling between the great ample time for any amount of denudafallen fragments of sandstone, to ascend tion; and Mr. Beete-Jukes, in his through the gorge by which the river Grose
“Sketch of the Physical Structure of joins the Nepean; yet the valley of the Australia," informs us that similar valGrose, in its upper part, as I saw, forms a leys abound throughout the great sandmagnificent basin some miles across, and is stone formation, both at high and low on all sides surrounded by cliffs, the sum- levels ; and they have so exactly the
; mits of which are nowhere less than three character, in the distribution of their thousand feet above the sea-level. When diverging branches, of ordinary streams cattle are driven into the valley of the Wolgan by a path partly cut by the colonists, inclined surface, that no exceptional
carrying off the drainage of a slightly they cannot escape ; for this valley is in
This every other part surrounded by perpendic- origin for them seems needful. ular cliffs, and eight miles lower down it will be more clear when we have discontracts from an average width of half a
cussed the modern theory of valleymile to a mere chasm, impassable to man formation and the special characteristics or beast.
of the rocks in which these remarkable
valleys have been excavated. The origin of these valleys appears to One of the most common ideas, when have been as great a puzzle to the early a person sees a deep gorge or ravine explorers as was that of the Yosemite. bounded by lofty precipices, is, that the Sir Thomas Mitchell estimates that one rocks have been torn asunder by some hundred and thirty-four cubic miles of earthquake or other subterranean moverock must have been removed from ment. A “convulsion of nature is the valley of the Grose alone ; and he almost always referred to in popular deremarks on the absence of indication of scriptions of such scenes. Till recent the agency by which these vast masses years even geologists considered that of stone have been carried away, there many valleys were so formed. The being no accumulations of sand, though article on the “Geology of the Alps," there are many huge blocks of rock, by M. Desor, in “ Ball's Alpine Guide," scarcely worn by attrition, in the bed of published in 1870, gives valleys of the stream, while in the valleys below, disruption" as one of the forms of Alinstead of sandy deposits, there is a pine valleys, and cites the defile of the rich alluvium. Even Darwin was stag- Via Mala on the Hinter Rhein, and the gered at the idea of these enclosed val- valley of the Rhone, between Bex and leys being hollowed out by aqueous Martigny, as examples. He defines erosion. Neither does he accept sub-them as “evidently produced by rents sidence, on account of the numerous that have torn asunder ranges once con
tinuous." Professor Whitney, also, in | mass of fallen fragments, filled in with his “Yosemite Guide Book," speaks of pebbles and sand ; but this considerarents or fissures as one of the recog- tion seems never to have occurred to nized modes of valley-formation. the upholders of the apparently obvi
Now, however, it is held by most, if ous and easy theory of violent disrupnot all, geologists that valleys are never tion. formed in this way. It is to the late It remains, however, to account for J. Beete-Jukes, director of the Geo- the very common phenomenon of rivers logical Survey of Ireland, that we owe apparently going out of their way to the full establishment of the principle cut a narrow passage through a hill,
valleys of all kinds, from the instead of following lower ground to a most open to the most narrow and pro- main valley or to the sea.
Such in our found, are hollows worn by erosion.” 1 own country are the small rivers Ouse He was struck by the fact of many of and Cuckmere, which cut through the the rivers of the south of Ireland, after South Downs between Brighton and running for miles over low plains open Beachy Head, instead of following the to the sea, suddenly turning at right low ground and reaching the sea beangles, cutting through the hills by tween Eastbourne and St. Leonards; deep, narrow ravines, and so reaching while the Avon, which flows through the sea beyond them. Sometimes even the gorge of St. Vincent's rocks at Clifthe hills the river cut through were ton, might apparently have found a isolated, so that the river might, appar- much easier way to the sea by a more ently, have passed round them in either northerly or a more southerly course. direction. The explanation usually Mr. Jukes explains all these cases on offered of these phenomena was that the principle that the courses of almost the hills had been fissured by subter- all the rivers of a country were deterranean forces, and that the rivers had mined by the contour of the land when taken advantage of them to change it first rose above the sea, the surface their course. But close examination water seeking always the easiest course showed that these ravines were not along the hollows and gentle slopes, fissures, but channels eroded in the without any regard to the nature of rock, since the solid rock could often be the rocks beneath. When once these traced unbroken across the very bed streams had formed definite channels, of the stream. And, after examining it was almost impossible to alter them many ravines in different parts of the (except when diverted by lava streams world, he came to what then seemed or glaciers) because movements of elethe very startling conclusion that, ex- vation are so slow that the rivers can cept, perhaps, in districts recently con- cut their way down as fast as the land vulsed by great earthquakes, there is rises up. Thus, the American geolono such thing as a glen, ravine, or gists have proved that the Uintah valley occupying the upper portion of Mountains were upheaved across the an open-mouthed fissure. On the con- valley of the Green River after the trary, in every case the whole space course of that river was established, between the two sides of the valleys and that, as fast as they rose, the river was once filled by rock, which has been cut through them, and now flows in a gradually worn down and carried away. tremendous gorge or cañon. Another The very frequent presence of cascades illustration of the permanence of river and waterfalls in such ravines, formed channels is afforded by the Moselle, by a continuous bed of hard rock cross- which, although it flows at the bottom ing the stream, is itself sufficient to of a deep, narrow valley sunk in a disprove the theory of fissures, in which nearly level plateau, winds about in case the whole bed would present a great curves and deep horseshoe bends
1 Student's Manual of Geology. By J. Beete- exactly like a stream flowing over a flat Jukes, 3rd ed. (edited by Archibald Geikie, F.R.S.), alluvial plain. No explanation of this
can be given except that the river be
gan its existence on a nearly level sur- softer rocks would be denuded by frost face, and after it had established its and rain, so that extensive areas would course in the characteristic winding be lowered as fast as the stream cut its fashion of such streams, it has, in the narrow channel through the hard rock, course of long ages, cut its way deep and was able to carry away the denuded down through the rock, and thus formed material. Hence, in the course of ages its present valley.
we should have the stream flowing over Now, every considerable area of con- a wide lowland, perhaps on one side tinental land is made up of rocks and open to the sea, and then cutting deposits of very unequal hardness and straight across a mountain ridge, or resisting power, from clays and sands to even across an isolated hill entirely surthe various kinds of rock. Some of rounded by lowlands. these can be dissolved and carried away Not very much time, geologically by running water much more quickly speaking, is required for such operathan others ; while rain, frost, and wind tions. Sir Charles Lyell describes a also act upon their exposed edges very channel, cut by the river Simeto across unequally. Hence arise the peculiar a lava stream from Etna, which is over forms assumed by hills of different com- fifty feet wide and in some parts forty position, and hence the reason why to fifty feet deep. The lava is not valleys are in some parts very narrow porous, but is a homogeneous mass of and precipitous, in others wide and hard blue rock. Yet the date of the open. It is an invariable rule that hills eruption which produced this lava and mountains are composed of the stream is known to be 1603.1 But the harder or less soluble rocks, the adja- most wonderful example of the power of cent lowlands and valleys of the softer water to denude and erode the hardest and more soluble. Hence, we see all rocks is afforded by the great cañon of great mountain ranges mainly composed the Colorado River. This has been cut of the older, hard, or crystalline rocks, for about four hundred miles to a depth while the lowlands, plains, and valleys of from four thousand to seven thouare occupied by the newer and softer sand feet, mainly through masses of formations. In our own country the hard paleozoic rocks down to the tertiary or secondary clays and sands archæan, and the whole of this vast are found in the lowland districts, while operation has been performed in the the more ancient and much harder latter half of the tertiary period. The rocks form the hills of Devonshire, formation of the river began, it is true, Wales, the Lake District, and Scotland. in very early tertiary times, but at that
Keeping in mind the extreme ine-epoch the present surface was buried quality of the rate of denudation of about nine thousand feet deep in secdifferent rocks, we are able easily to ondary rocks, which have all been since explain the apparently erratic course of denuded away, so that Captain Dutton so many rivers. When the streams estimates that the river has cut its originated they took their course along channel on the whole through from ten lines of least resistance, depending on thousand to sixteen thousand feet of the form of the surface, not on the mes
esozoic, carboniferous, and other annature of the rocks beneath the sur-cient rocks, all during the tertiary peface. Sometimes this course passed riod.2 over ridges or bosses of very hard rock, buried perhaps hundreds, per- Keeping in mind these remarkable haps thousands, of feet deep. But instances of denudation, let us turn to the channels once fixed could not be consider the probable origin of the altered, and when the bed of the stream remarkable valleys which have seemed reached this rock it cut down into it. to eminent geologists so peculiar as to Then, owing to the hardness of the
1 Principles of Geology, vol. i. 353. rock, the river channel would be a
2 The Tertiary History of the Grand Cañon Disgorge or ravine, while all' around the 'trict, U.S. Geological Survey, 1882.
need some special mode of origin ; and | duced would widen the valleys till those we will take first the great rock-walled of many of the smaller tributaries bevalleys of New South Wales, as being came united together. Thus age after the most simple in their main features. age the valley would widen and deepen,
These are all excavated in sandstones always preserving its precipitous rockand shales of the carboniferous system, walls due to the alternation of hard and though perhaps of mesozoic age. The soft layers. strata are nearly horizontal, and, what The deepening of these great valley's is especially important, they are of very would probably be aided by subterraunequal degrees of hardness. The nean denudation due to the presence of upper beds are usually conglomerates, salt and alum, which Mr. Clarke states and are so comparatively indestructible are found at several places in these that isolated summits often imitate strata. The solution of these salts by ruined castles. In places these beds percolating water would form cavities are so hard that boring-tools will not and water channels, and the subterrapenetrate them, while in other parts nean streams would eat away the softer the rock is so incoherent that large beds, forming caverns, the roofs of blocks will break in pieces by falling which would in time fall in, and the over an embankment. We have here débris be gradually disintegrated by the essential conditions for the forma- atmospheric agencies and then carried tion of vertical escarpments, since by away by floods. This mode of denudathe weathering away of the softer bedstion was seen actually at work by Capthe harder strata above them remain tain (now Sir George) Grey, during his unsupported and break off, and thus exploration of the Glenelg River in the vertical or sometimes overhanging north-west Australia. He describes a character of the precipices is kept up. nearly level table-land covered with
If we look at a large scale map of this numbers of sandstone pillars of various part of Australia, we see that the rivers grotesque shapes and some of them Grose, Cox, and other tributaries of the forty feet high. Hearing the sound of Nepean which drain the sandstone running water at a fissure among some plateau, have great numbers of diverg- of the rocks, he descended, and found ing branches which almost interlace a cavern supported by pillars of the with each other, as so often occurs same character as those above, with a among the streams of a nearly level, small stream, which in the rainy season well-watered district. Now, bearing in would become a torrent. Here, then, mind what has been said of the perma- are ample causes to explain the formanence of water-courses once formed, we tion of these great rock-walled branch
see that these many - branching ing valleys in the sandstone plateau ; streams must have flowed on the sur- the remaining feature - that the rivers face of the plateau at the epoch of all escape through deep gorges often so its first elevation; that surface itself narrow or so blocked up with rock-fragbeing perhaps a long way above the ments as to be impassable — evidently present surface, which has certainly depends on the fact that the outer esbeen lowered by denudation during its carpment of the plateau is formed of a long existence as dry land, probably series of harder rocks, and thus does during the whole of the tertiary period. not wear away laterally. In this reFrom the time that these streams began spect they resemble those numerous to penetrate the sandstone plateau as gorges in the Alps which form the far as the first hard bed, miniature cliffs only outlet for considerable high valwould be formed by the wasting away leys, such as those of the Trient, the of the softer beds beneath it, and the Reuss,
others. continual movement backward thus pro- The difficulty as to whither the de
nuded material has gone, does not seem i Remarks on the Sedimentary Formations of
By the Rev. W. B. Clarke, a great one, when we remember the F.R.S., F.G.S. Fourth ed. 1878, p. 72.
many millions of years the process of
New South Wales.