104

Valley of S. Cruz (2)

was guessed to be 330 ft. — ([letter deleted] Where Dn En¹ (including Dn) comes close to river being 1168 ft.: Cn was 639 40 miles west of noon cliff. — Where Dn was 743. cn was conjectured 500 ft. (28^th) When I at first merely considered the form of these plains I supposed a river cutting down its course, as land arose was the cause. — Examination of the individual phenomenon convinced me quite a different explanation is necessary. — The upper third of the valley is perhaps a little narrower being about 5 miles (from D to Dn). & in one place where passing cutting through a field of lava is no more than 1 & 1/2 mile. — About Long 71°: 20', the valley begins widening, at the point furthest so & so increases as to form a great basin, bounded by cliffs like the valley, this at the furthest point, we reached (140 miles from coast) was 16 miles wide from North to South. & extended up to the foot of the Andes, about 25 miles distant. —

Standing on the plain Dn & viewing the broken line of escarpement of plain En first brought the idea into my head this must be the work of the sea. — The plain Dn is most perfectly smooth, & level: — how could the river form a plain at least 8 miles wide & so level.

the low land occasionally seen on the its margins has a very different aspect. — the gravel is moreover often white-washed with white aluminous matter, & cemented together by abundance of same substance. — Now these characters are those exactly of the shingle beds which I have known

¹ The letters in this discussion refer to sections of a sketch DAR34.99.

104 verso

(a) the way the pebbles were settled down & the extent of this plain, brought thorough almost conviction in my mind, that the sea deposited it. —

105

Valley of S. Cruz (3)

to be found beneath the sea. — Indeed I doubt whither a rapid river could thus coat & partly cement the gravel. —

The above characters In one of these plain the lava was unequal, these the hummocks were smoothed over with gravel in a continuous line or slope or curve with the plain. which could not have happened in a river. — The two above characters apply with equal truth to the plain to plains C & Cn. & even in the one or two lower plains these appearance were almost as strong. —

The river occasionally is certainly fringed by a low irregular plain, the origin of which may be attributed to the river; but far more generally (the lowest plain being about 20 feet high) & the river at it runs in a gut between two cliffs of the Patagonian formation, & is manifestly (at present) exerting no other action than slightly deepening its gravel bed. — The river being so little subject to floods is much against its forming plains alluvial plains. — These plains consisting of coarse gravel does not favor the idea. — In fact I never saw a river where the evidences of its power were so little to be recognized. — I may mention another difficulty, I have proved (V Geol of S Cruz) how little or no power the river has in transporting blocks, yet we see the valley has removed masses beds of Hard lava 200 to 300 ft thick from a width of 1 1/2 miles to 4 miles. — (a)

But My strongest argument rests in having found in two places near bed of river 4 recent marine shells. — Two Volutas &

105 verso

(a). It is very curious, how even the sea even could remove, or break the immense number of fragments, which must have resulted from the breaking through so great a field of lava. — As the lava always is at a considerable elevation, it must have been cut through in the [illeg] early stages of the formation of the valley, so that very many blocks may be buried in & Also the debris must have been carried away before the for deposition of the lower plains within the valley, otherwise they would be contained in them & be exposed on banks of river.

Worn into fine sediment.

106

Valley of S. Cruz (4)

& two patellas. They were broken & appeared very old. The Volutas however yet partially retained their color. —

3 of them were found at Long. 71°. & 294 ft above present sea: the other fourth at our furthest point 400 feet. elevation. —

Finally no one would imagine the great basin, into which the valley expands, was formed by the river. — its bottom is too level, about 50 ft above river, it is composed of sandy earth. — it is very remarkable that there were many sand dunes in the mouth of the valley. —

Taking all these facts into consideration there cannot be any doubt, but that the sea has excavated this great valley & that by the successive elevation of the the step-like plains were deposited in its bosom & the escarpements being its former coasts. —

The manner in which this has happened, I will try to explain. — I require as the foundation of my hypothesis that if when the land was depressed from 1000 to 2000 feet water an arm inlet of the sea would cross the Andes. Now in itself, this is not so very improbable, because further to the South the Andes are at the present day thus traversed at Obstruction Sound. And, due west of the Basin, there is a remarkable gap in the mountains, which was noticed by everyone as quite breaking the continuity of the chain. (x) It must be remembered I have endeavoured to show (P 10a) that the deepening

106 verso

(x) neither are the cordilleras here very lofty, the very highest peak being only 6400 ft. —

107

Valley of S. Cruz (5)

of the former sea, in a distance of 160 miles (from Andes to present coast) would perhaps be about almost [illeg] 100 fathoms. —

Allude to immense removal

The Andes being then a chain of hills, standing in the sea, with a break, through which currents would pass, had, on its East side a shoaling ocean. the bed of which was composed of shingle. As we see at mouth of St of Magellan, it is probable the part of sea in front of the channel would be deeper from the currents removing the depositions. — Durin Subsequent then to the very first elevations, a bay would be formed, the action of the sea on this, would make it larger. — (it contracts afterwards probably owing to the hard lava). —

Suppose the elevations to continue (I have shown if the elevations are fragment & small in degree, the relative height of diff. points in new land would correspond, to those on the former to the inclination of strata or sea's [swarm]. V10 Height of Plains) when the 840 ft plain at the coast was just elevated reaching above level of sea, the land near the Andes would be 100 fathoms or 600 ft high. — As Whilst the land thus gradually increased in horizontal section there can be no doubt the action of currents would prolong & deepen the channel from the break in the mountain, in a similar degree. — At this time the East mouth probably had a large embouchure, the inland higest plain (En) of nearly about 1000 ft (at 69°: 59'), being probably directly connected, with the

Allude to immense removal] written over the following paragraph.

107v [blank]

108

Valley of S. Cruz (6)

950 ft. plain of St. Julians. — It is clear that in this state, the straits are bounded by the highest cliffs E & En & that they are 6 to 10 miles broard. — One or more elevations having taken place, so as at the coast to produce the 710 plain, a fringe or patches of this might be continued up side of whole channel & basin. — We will suppose the straits were 300 ft (or any other number) deep & the basin shoaler 200 ft. —

Also that at this period (or any other) that the passage in the in the mountain by elevation of hard rock was closed. — We should now have a shoal inland sea, like Otway or Skyring water,¹ connected with the Atlantic by a long channel. During the time, which silting & successive elevations would require to elevate the basin into dry land, another plain might be formed marking some step in the upheaval. —

I have hitherto supposed the elevations were concentric with earths surface (the probability of this is discussed in Height of Plains) & hence the bottom of the straits horizontal. If we assume a dip from the line of the Andes, each elevation would affect the nearer, much more than the distant parts, hence the channel would have an inclination to sea-ward. So that in place of the whole creek being of uniform depth & liable to be

¹ Skyring in Tierra del Fuego: Beagle Diary, p. 419. KR

108v [blank]

109

Valley of S. Cruz. (7)

dried up at some sudden period, the sea would gradually retire & we may well imagine would deposit the lower plains. — On this presumption we have also the advantage of a longer communication between Atlantic & pacific oceans, which better accounts for excavation of valley.

The Basin is at present 450 ft above sea; it might happen that, during the elevation of (for instance) only the 200, last feet on the coast, this whole 450 ft could be elevated & therefore untill that late period the communication would be open & consequent degradation from currents so much the more rapid. — on the contrary supposition of concentric elevation, there would be much difficulty in explaining the slope of valley & great current of the river of S. Cruz. —

In this manner I believe all the plains were deposited; they the lower ones would be modified by the river & the elevations having attained their present extent, we have the valley of S. Cruz, as it now exists. — The whole hypothesis was slowly formed from the above facts, & these facts were collected before I could at all understand the figure & origin of the valley. — It appears to me satisfactory. — This class of valley may very likely be of common occurrence; in any old formation, most of the above evidence would be quite obliterated. Whenever a chain of hills passes through the sea, & is traversed by a channel, currents may be expected to exist: whenever the strata at the bottom of the ocean, on either side are elevated, these currents would keep open

109v [blank]

110

Valley of S. Cruz. (8)

a passage or straits through them. Whenever by upheaval of hard rock the channel in mountains is stopped a dry (or sooner by silting) a dry valley would be formed, into by which all not near rivers & streams disembogue themselves. — In the coarse of time the above conditions must often take place: I feel pretty sure, from examination of S. Cruz, that the result will be as described. —

M^r Martens has made two drawings¹ (in possession of Capt Fitz Roy) very illustrative of the geology of this valley: one is a view of the Andes in which the Gap may be seen, it shows the form of Basin & is drawn in the neck of gully just where it begins to expand. —

The other is a view taken from one of the sand dunes near our last station, in the Basin it looks to the Southward. — 3 sets of plains. — the lowest is that of the basin (450 ft above sea). The next is perhaps a plain which 20 miles to the East holds the same relative position & is 639 ft. above sea. The highest from the side sides of valley & basin. —

¹ See the engraved and published versions of the drawings referred to in Narrative 2: Basalt Glen and Santa Cruz Valley.

110 verso

Port Desire Valley of same original formation as S. Cruz. —

؟ Is it necessary to suppose a tilt? — So many considerations render a concentric elevation more probable. = Might not retiring sea excavate the eastern entrance of channel.

NB When I say concentric, I mean not truly so. — but an enlargement of the curve of the world: is the most bent part (A) the axis of emption & upheaval of mounted strata — which however will give a tilt over whole curve [sketch]

111

Notes.
S. Cruz

Data for Slope of the
Bottom
of the Atlantic