RECORD: Darwin, C. R. Geological diary: Valleys - Cordilleras - Chili. (2-6.1835) CUL-DAR36.452-461 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)

REVISION HISTORY: Transcribed by Kees Rookmaaker, corrected and edited by John van Wyhe, corrections by Gordon Chancellor 10.2011. RN2

NOTE: This document, part of the largest scientific document composed by Darwin during the voyage of the Beagle, is written mostly in ink. Where pencil was used instead this is noted in the textual notes. Marginal notes are here integrated into the text. See the Beagle Diary pp. 553-596.

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Reproduced with the permission of the Syndics of Cambridge University Library and William Huxley Darwin.

See the introduction to the Geological Diary by Gordon Chancellor.


452

1835 Valleys. — Cordilleras. Chili 1

From what I have seen in all parts of the Cordilleras, the valleys bear an evident relation to the stratification. a — In few places the strata on each hand exactly correspond. — the most frequent is a small anticlinal dip, the valley having been excavated in the axis. — This anticlinal line may often in part be only of a subordinate one in a general dip. — I met with some synclinal vallies, but they appear rare. — Another very common form cause, is a change in [sketch] the angle of inclination, which must have produced a fissure. — There are certainly very many lines of great disturbance & faults which have never been formed lines of drainage; the reverse of this however is not true common. —That the removal by aqueous causes has been immense is undeniable.

Perhaps this has been greatest where the vallies are nearly straight E & W. — I may give as an example the descent on East side of Portillo, where corresponding patches of mica slate (& lava) show how deep the excavation has been. — I have stated that the exterior lines of elevation are generally oblique to the main central axis. —

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(a) In the mouths of the broard valleys at base of central range at Cauquenes — I thought otherwise. But I believe the removal there has been so great that small differences in stratification on the opposite sides could hardly be preserved. —

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1835 Valleys. Cordilleras 2

Inspection of any map will show that these lines are much more abundant on the side of the Pacifick than the Atlantic; hence the road, which follows some principal valley, is generally zig-zag on that side. (c) — Where there are external N & S lines they are cut through at the point of least angular (& probably positive) elevation. The central grand ridges are unbroken passed at a great elevation. —

All the vallies in the Cordilleras, excepting the ravines in the lofty central parts, are have a thick bottom of shingle which forms a level terrace on each side of the torrent. — I say all advisedly, having seen many vallies in whole central & North Chili & some on the side of the Pampas. — This occurs not only in the main line of any valley, but branches arms of the shingle plain (a) extend for some distance in nearly all the side ravines. (b) — These terraces join in one continuous, but most gentle slope, to the plains, of similar constitution which bound the Cordilleras on both sides. —

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(a) It is particularly conspicuous where two large streams unite in the mountains. — Here there generally is a large clear space (Viz. (Volcan & Yeso.) (Tupungato & Mendoza) A tongue of level plain extends from the ridge, which divides their [souvels] & exactly corresponds to the plain on each side. —

[sketch]

(b) Where a very precipitous lateral ravine, during its floods, brings more matter than water; this is deposited on the side terrace, in place of a channel being cut. — The outline of this is furrowed, & the form is a convex fan. — Some of the diverging heaps are large. —

[sketch] Section of [illeg]
Red porphyry

Beyond plain of Uspallata, & ravine descended from bright red porphyry hills. — the cliffs of the river showed a section of the form, where the color as well as form showed pointed out that the processes of the present day are very different from those which produced the shingle of terraces. — In this latter there was no predominant proportion of red porphyry pebbles. —

(c) I frequently observed at Copiapo & northern Chili that a zig zag line joining the lowest points of upheaval in the ridges or subsequently lowered deepened, here formed the valleys. —

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1835 Valleys. Cordilleras. 3

The shingle as seen by the section of rivers, near the mouth of valley is often from 2 — even to 400 ft deep. — from this it gradually lessens till in the ascent. — There are however some exceptions to this. — The superior surface however almost invariably presents one uniform slope. —

Such terraces reach up to any elevation of 6 7 & even 8000 ft. — Above this they become so thin, narrow & irregular (a), as not to be conspicuous.

Lower down they form a very remarkable character in the landscape, & where irrigated for make a most picturesque base to the bare & sterile mountains. — As thus we everywhere see the rivers in the main & side valleys, removing eating a channel & removing masses of this shingle, no one will say that once they all deposited this matter: if I had ever seen this process in action, I could believe it — but otherwise I utterly reject this point of view. — The rivers viewed solely as agents of removal perform & have performed no contemptible work: the bed of the river, is bounded,

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(a) The alluvium on each side of the uppermost torrents is irregular in outline. — the pebbles are only partially & to a different degree, rounded & are mingled with earth. — This will be seen to be essentially different from the constitution of the shingle terraces: & properly derive the name of alluvium. —

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1835 Cordilleras Chili 4

as I have said, by vertical cliffs, some hundred ft deep, it is moreover generally broard; hence the mass must be very considerable which has been removed. — It is not very common, in the lower parts of the valleys, for the river to reach any point of the hard rock on either hand. — & still less to excavate the hard solid strata at the bottom. — Speaking generally sides & bottom are coarse shingle.

Besides the grand channel, which has been removed through the terraces, it must be remembered that all the fresh matter & large fragments which are annually brought down by the hard extreme sources uppermost torrents are either at once carried down, or in the course of years ground into powder.

In proof of this, the water is of the color of mud & the noise of the valley stones, rattling one over the other, can distinctly be heard, amongst the roar of the passing water.

The effect of these concussions night & day, must be indeed be great. fragments as large as a church, when they fall within the river course, must be thus removed.

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1835 Cordilleras — Chili 5

diminished, till at last rolling onwards, become the agents, instead of the objects of destruction.

Taking it then for granted, that the terraces of shingle have not been deposited by the rivers. —

They must either have been deposited by debacles (a) or the quiet residence of the sea;

The shingle is all perfectly round, it is stratified lines of sand sometimes parting the beds. — the pebbles are often coated by or whitewashed, with a aluminous or calcareous matter, are sometimes cemented by cryst. carb of lime. (Cauquenes & Maypo). It may be well doubted if any debacle could produce these effects. — Again, a debacle would heap up piles of alluvium, where the valley bends, which is not the case; neither would the surface be left be the smooth, not could it join on continuing with plains, such as the Pampas. — Again I invariably observed (b), that where on (or in) the terraces large fragments of any rock, abounded, such occurred in situ, at no great distance higher up. — (In same manner as in S. Cruz)

how would this happen if a debacle had passed

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(b) If this only happened in bed of river, present floods would produce this effect. —

(a) I do not take into account lakes, it is ridiculous to imagine every expanding valley has once been closed up by a most lofty broard barrier. —

(d) What a contrast with the Alps. —

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1835 Cordilleras Chili 6

down the valley; would not the alternate bands of granite, porphyry, granite porphyry blocks, be mingled in utter confusion. — All the above facts, which could not be produced by debacles, assuredly would happen from the residence of [illeg] sea, which has been quietly retiring. during the progressive rise of the land. — The only positive kind of evidence I have for this view, is to be found. in the union & gradual slope of these terraces with those plains, which are modeled into successive steps. — No one will now dispute that this is the work of the retiring sea: if such evidence is wanting, it will be found in the marine remains now lying on their surfaces & in one instance (a) imbedded in their strata (Coquimbo).

I confess I at first hesitated in attributing small strips of plains, at an elevation of many thousand ft to the quiet deposition of the sea. — But from the above reasons such now in my conviction & that after an attentive examinations from the above reasons of all the circumstances. — If then indeed it is allowed, that the shingle terraces were left by the gradually retiring sea; that sea, must have retired gradually: the analogy in known manner

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(a) The only demonstrative evidence would be in finding marine organic remains, either on the surface, or in the coarse strata. — V. Lima geology for reasons why we cannot expect the former, & in the case of the latter is but slightly different, owing to the facility of percolation as in large round stones. —

(a) A part of this hesitation will, I think be removed by inspecting the charts of Southern part of the Cordilleras. — for instance Moralledas exact plan of the sea coast behind Chiloe & Chonos. — The winding creeks will there be seen to penetrate to beyond the line of highest peaks & volcanos. — No one will doubt of what the bottom of these creeks are is composed. — If this land was raised, these would form the principal valleys. —

N.B. An objection to curved valleys being formed by this sea is often raised — no doubt valley [sketch] such as this. — But when considering this objection I shall not forget the numerous but narrow & much twisting creeks in very centre of Falklands Isds.

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1835 Cordilleras Valleys Chili (7

of elevation of the last 1000 ft. must be continued nearly to the highest points. — otherwise the terrace could not possess one slope. — an upheaval of 1000 ft at one blow would entirely break through such a continuity. — I may here remark, that the sea in these sounds, would not probably leave ridges as successive beaches: there would be no surf & the tide in an almost open ocean is small. —

Another evident conclusion, is that the whole of what has been excavated in the valleys above the terraces has been the work of the sea. — To me this appears a thoroughly efficient cause. it must be remembered that the most central & highest ridge once stood as an exposed row of islands, & then as a slip narrow band of hills &c &c:

Moreover I believe, the lower parts of the central ridges, which if a subsidence took place, would now be protected from the outer sea, were formerly more generally exposed, because the exterior chains have been elevated subsequently. — The transverse gorges in the latter are well explained by the sawing action of the tides belonging to the ocean & the inland sea, between the ranges. —

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(a) In the earliest stages, the stratification would determine the presence of a creek, & from that moment the head of a valley would be formed. —

459

1835 Cordilleras. Valleys Chili (8)

I have before stated, that the constant action of the rivers is to remove the shingle, & where it encounters the solid strata to cut through them into a gorge. V Cauquenes P — We may then look forward to a period, doubtless far distant, where the terraces will be removed & the valleys excavated into acute wedges. — this to a certain extent has happened in the upper ravines, & would probably have done so more quickly, under a wetter climate. — May not, in all cases, such forms of valleys roughly indicate or a recent that no long time has elapsed since the mountain rose from beneath the waters? — Will not the arguments of De Luc &c about the quantity of detritus in the a mountain talus &c &c apply in most districts to measure ascertain the remoteness of that epoch in each case place of an universal deluge? —

To the above descriptions of the valleys there is one remarkable exception. It occurs valle however at a very considerable elevation. first Before entering the Basin of the Valle del Yeso, the valley is very broard. In the centre there is a broard nearly level mass of alluvium or shingle. — On the South side the [sketch]

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460

1835 Valleys. Cordilleras Chili (9)

main stream of the R. Yeso flows: its channel bounded on the North by shingle cliffs must be 800 ft deep on the S. by the mountains. — On the North side its side there is a small plain or terrace.

On the North side of the central mass there is another channel (nearly of course parallel) with lesser stream 400 ft deep. As far as can be seen the whole intermediate mass is shingle.

There is a case where it will be evident the rivers could never have deposited this mass. — The central mass higher up is bounded by low hills on each hand or has central channel. — [sketch] These lateral hills increase in height & become a confused hilly mass of rounded & angular fragments many of very large size, which must be 1200 ft above bed of South stream. —

The road passes over these (Cuesta del India) & then suddenly descends into the basin of Yeso. — The hill as far as can be seen is all alluvium. there may be central but small nucleus of hard rock. — The basin of Yeso is several miles long, 2 or 3 wide smooth & very little inclined, the river (a) has cut a narrow channel on South side through mountain of alluvium & descends on the South side, as described. This basin

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(a) It is so narrow that the road cannot follow this line, but has to ascend & descent the alluvium hill

[sketch]
mountain of alluvium
Line of road
central mass surrounded by a channel
course of river
Valle del Yesu
xx Line of great fault??

Section South side of Valle del Yeso, hill of alluvium & central mass. — &c &c —

xx This purely hypothetical fault coincides with a real line of grand disturbance, where there is pointed section of granite underlying porph. breccia. —

This view has the advantage of explaining the pile of diluvium which otherwise I see no way of explaining. — If the lake had now existed. — the question might have been asked where has all the alluvium from the upper valleys which enter the lake disappeared? why has not the lake long since been filled up? The explanation of fault appears to me so simple: that if in any large valley there was much alluvium high up on the lower side of lake. I should suspect the case was parallel. —

The idea of a fault crossing a river course is quite borne out by the Lima fact of sloping of bed of river, terminated by descent of 40 ft. — Probably not anticlinal. — [sketch] V. Private Journal1

1 Beagle diary.

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1835 Cordilleras. Valleys. Chili 10

has the appearance of a bed of a lake. — at the upper extremely smooth valleys slope up continuously. this however is almost beyond the usual limits of the terraces. — The water from this great hollow probably once flowed in central part in the channel (z) in middle of shingle mass: subsequently a lateral gorge was excavated. — The size of this brown basin may be partly owing to the facility with which the neighbouring gypseous formations could be removed. — But the whole appearance must I think be explained by some great subterranean movement shortly before the sea returned & subsequently to the accumulation of great mass of shingle or alluvium. — V. Section

Pampas

With respect to the margin of the Pampas, which joins on to East foot of the Cordilleras, I have a few words to say. Beyond the extreme water worn hillocks, (which are not very numerous the subordinate lines of elevation being on the East few) the plain is level, composed of shingle & slopes at a considerable angle for what truly deserves the name of a plain, — it looks [sentence breaks off in this document]

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