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Geological Observations On South America

C >> Charles Darwin >> Geological Observations On South America




THE VALLEY OF THE R. SANTA CRUZ.

This valley runs in an east and west direction to the Cordillera, a
distance of about one hundred and sixty miles. It cuts through the great
Patagonian tertiary formation, including, in the upper half of the valley,
immense streams of basaltic lava, which as well as the softer beds, are
capped by gravel; and this gravel, high up the river, is associated with a
vast boulder formation. (I have described this formation in a paper in the
"Geological Transactions" volume 6 page 415.) In ascending the valley, the
plain which at the mouth on the southern side is 355 feet high, is seen to
trend towards the corresponding plain on the northern side, so that their
escarpments appear like the shores of a former estuary, larger than the
existing one: the escarpments, also, of the 840 feet summit-plain (with a
corresponding northern one, which is met with some way up the valley),
appear like the shores of a still larger estuary. Farther up the valley,
the sides are bounded throughout its entire length by level, gravel-capped
terraces, rising above each other in steps. The width between the upper
escarpments is on an average between seven and ten miles; in one spot,
however, where cutting through the basaltic lava, it was only one mile and
a half. Between the escarpments of the second highest terrace the average
width is about four or five miles. The bottom of the valley, at the
distance of 110 miles from its mouth, begins sensibly to expand, and soon
forms a considerable plain, 440 feet above the level of the sea, through
which the river flows in a gut from twenty to forty feet in depth. I here
found, at a point 140 miles from the Atlantic, and seventy miles from the
nearest creek of the Pacific, at the height of 410 feet, a very old and
worn shell of Patella deaurita. Lower down the valley, 105 miles from the
Atlantic (longitude 71 degrees W.), and at an elevation of about 300 feet,
I also found, in the bed of the river, two much worn and broken shells of
the Voluta ancilla, still retaining traces of their colours; and one of the
Patella deaurita. It appeared that these shells had been washed from the
banks into the river; considering the distance from the sea, the desert and
absolutely unfrequented character of the country, and the very ancient
appearance of the shells (exactly like those found on the plains nearer the
coast), there is, I think, no cause to suspect that they could have been
brought here by Indians.

The plain at the head of the valley is tolerably level, but water-worn, and
with many sand-dunes on it like those on a sea-coast. At the highest point
to which we ascended, it was sixteen miles wide in a north and south line;
and forty-five miles in length in an east and west line. It is bordered by
the escarpments, one above the other, of two plains, which diverge as they
approach the Cordillera, and consequently resemble, at two levels, the
shores of great bays facing the mountains; and these mountains are breached
in front of the lower plain by a remarkable gap. The valley, therefore, of
the Santa Cruz consists of a straight broad cut, about ninety miles in
length, bordered by gravel-capped terraces and plains, the escarpments of
which at both ends diverge or expand, one over the other, after the manner
of the shores of great bays. Bearing in mind this peculiar form of the
land--the sand-dunes on the plain at the head of the valley--the gap in the
Cordillera, in front of it--the presence in two places of very ancient
shells of existing species--and lastly, the circumstance of the 355-453
feet plain, with the numerous marine remains on its surface, sweeping from
the Atlantic coast, far up the valley, I think we must admit, that within
the recent period, the course of the Santa Cruz formed a sea-strait
intersecting the continent. At this period, the southern part of South
America consisted of an archipelago of islands 360 miles in a north and
south line. We shall presently see, that two other straits also, since
closed, then cut through Tierra del Fuego; I may add, that one of them must
at that time have expanded at the foot of the Cordillera into a great bay
(now Otway Water) like that which formerly covered the 440 feet plain at
the head of the Santa Cruz.

(DIAGRAM 6. NORTH AND SOUTH SECTION ACROSS THE TERRACES BOUNDING THE VALLEY
OF THE RIVER SANTA CRUZ, HIGH UP ITS COURSE.

The height of each terrace, above the level of the river (furthest to
nearest to the river) in feet:

A, north and south: 1,122
B, north and south: 869
C, north and south: 639
D, north: not measured. D, north? (suggest south): 185
E: 20
Bed of River.

Vertical scale 1/20 of inch to 100 feet; but terrace E, being only twenty
feet above the river, has necessarily been raised. The horizontal distances
much contracted; the distance from the edge of A North to A South being on
an average from seven to ten miles.)
I have said that the valley in its whole course is bordered by gravel-
capped plains. The section (Diagram 6), supposed to be drawn in a north and
south line across the valley, can scarcely be considered as more than
illustrative; for during our hurried ascent it was impossible to measure
all the plains at any one place. At a point nearly midway between the
Cordillera and the Atlantic, I found the plain (A north) 1,122 feet above
the river; all the lower plains on this side were here united into one
great broken cliff: at a point sixteen miles lower down the stream, I found
by measurement and estimation that B (north) was 869 above the river: very
near to where A (north) was measured, C (north) was 639 above the same
level: the terrace D (north) was nowhere measured: the lowest E (north) was
in many places about twenty feet above the river. These plains or terraces
were best developed where the valley was widest; the whole five, like
gigantic steps, occurred together only at a few points. The lower terraces
are less continuous than the higher ones, and appear to be entirely lost in
the upper third of the valley. Terrace C (south), however was traced
continuously for a great distance. The terrace B (north), at a point fifty-
five miles from the mouth of the river, was four miles in width; higher up
the valley this terrace (or at least the second highest one, for I could
not always trace it continuously) was about eight miles wide. This second
plain was generally wider than the lower ones--as indeed follows from the
valley from A (north) to A (south) being generally nearly double the width
of from B (north) to B (south). Low down the valley, the summit-plain A
(south) is continuous with the 840 feet plain on the coast, but it is soon
lost or unites with the escarpment of B (south). The corresponding plain A
(north), on the north side of the valley, appears to range continuously
from the Cordillera to the head of the present estuary of the Santa Cruz,
where it trends northward towards Port St. Julian. Near the Cordillera the
summit-plain on both sides of the valley is between 3,200 and 3,300 feet in
height; at 100 miles from the Atlantic, it is 1,416 feet, and on the coast
840 feet, all above the sea-beach; so that in a distance of 100 miles the
plain rises 576 feet, and much more rapidly near to the Cordillera. The
lower terraces B and C also appear to rise as they run up the valley; thus
D (north), measured at two points twenty-four miles apart, was found to
have risen 185 feet. From several reasons I suspect, that this gradual
inclination of the plains up the valley, has been chiefly caused by the
elevation of the continent in mass, having been the greater the nearer to
the Cordillera.

All the terraces are capped with well-rounded gravel, which rests either on
the denuded and sometimes furrowed surface of the soft tertiary deposits,
or on the basaltic lava. The difference in height between some of the lower
steps or terraces seems to be entirely owing to a difference in the
thickness of the capping gravel. Furrows and inequalities in the gravel,
where such occur, are filled up and smoothed over with sandy earth. The
pebbles, especially on the higher plains, are often whitewashed, and even
cemented together by a white aluminous substance, and I occasionally found
this to be the case with the gravel on the terrace D. I could not perceive
any trace of a similar deposition on the pebbles now thrown up by the
river, and therefore I do not think that terrace D was river-formed. As the
terrace E generally stands about twenty feet above the bed of the river, my
first impression was to doubt whether even this lowest one could have been
so formed; but it should always be borne in mind, that the horizontal
upheaval of a district, by increasing the total descent of the streams,
will always tend to increase, first near the sea-coast and then further and
further up the valley, their corroding and deepening powers: so that an
alluvial plain, formed almost on a level with a stream, will, after an
elevation of this kind, in time be cut through, and left standing at a
height never again to be reached by the water. With respect to the three
upper terraces of the Santa Cruz, I think there can be no doubt, that they
were modelled by the sea, when the valley was occupied by a strait, in the
same manner (hereafter to be discussed) as the greater step-formed, shell-
strewed plains along the coast of Patagonia.

To return to the shores of the Atlantic: the 840 feet plain, at the mouth
of the Santa Cruz, is seen extending horizontally far to the south; and I
am informed by the Officers of the Survey, that bending round the head of
Coy Inlet (sixty-five miles southward), it trends inland. Outliers of
apparently the same height are seen forty miles farther south, inland of
the river Gallegos; and a plain comes down to Cape Gregory (thirty-five
miles southward), in the Strait of Magellan, which was estimated at between
eight hundred and one thousand feet in height, and which, rising towards
the interior, is capped by the boulder formation. South of the Strait of
Magellan, there are large outlying masses of apparently the same great
tableland, extending at intervals along the eastern coast of Tierra del
Fuego: at two places here, 110 miles a part, this plain was found to be 950
and 970 feet in height.

From Coy Inlet, where the high summit-plain trends inland, a plain
estimated at 350 feet in height, extends for forty miles to the river
Gallegos. From this point to the Strait of Magellan, and on each side of
that Strait, the country has been much denuded and is less level. It
consists chiefly of the boulder formation, which rises to a height of
between one hundred and fifty and two hundred and fifty feet, and is often
capped by beds of gravel. At N.S. Gracia, on the north side of the Inner
Narrows of the Strait of Magellan, I found on the summit of a cliff, 160
feet in height, shells of existing Patellae and Mytili, scattered on the
surface and partially embedded in earth. On the eastern coast, also, of
Tierra del Fuego, in latitude 53 degrees 20' south, I found many Mytili on
some level land, estimated at 200 feet in height. Anterior to the elevation
attested by these shells, it is evident by the present form of the land,
and by the distribution of the great erratic boulders on the surface, that
two sea-channels connected the Strait of Magellan both with Sebastian Bay
and with Otway Water. ("Geological Transactions" volume 6 page 419.)

CONCLUDING REMARKS ON THE RECENT ELEVATION OF THE SOUTH-EASTERN COASTS OF
AMERICA, AND ON THE ACTION OF THE SEA ON THE LAND.

Upraised shells of species, still existing as the commonest kinds in the
adjoining sea, occur, as we have seen, at heights of between a few feet and
410 feet, at intervals from latitude 33 degrees 40' to 53 degrees 20'
south. This is a distance of 1,180 geographical miles--about equal from
London to the North Cape of Sweden. As the boulder formation extends with
nearly the same height 150 miles south of 53 degrees 20', the most southern
point where I landed and found upraised shells; and as the level Pampas
ranges many hundred miles northward of the point, where M. d'Orbigny found
at the height of 100 feet beds of the Azara, the space in a north and south
line, which has been uplifted within the recent period, must have been much
above the 1,180 miles. By the term "recent," I refer only to that period
within which the now living mollusca were called into existence; for it
will be seen in the Fourth Chapter, that both at Bahia Blanca and P. S.
Julian, the mammiferous quadrupeds which co-existed with these shells
belong to extinct species. I have said that the upraised shells were found
only at intervals on this line of coast, but this in all probability may be
attributed to my not having landed at the intermediate points; for wherever
I did land, with the exception of the river Negro, shells were found:
moreover, the shells are strewed on plains or terraces, which, as we shall
immediately see, extend for great distances with a uniform height. I
ascended the higher plains only in a few places, owing to the distance at
which their escarpments generally range from the coast, so that I am far
from knowing that 410 feet is the maximum of elevation of these upraised
remains. The shells are those now most abundant in a living state in the
adjoining sea. (Captain King "Voyages of 'Adventure' and 'Beagle'" volume 1
pages 6 and 133.) All of them have an ancient appearance; but some,
especially the mussels, although lying fully exposed to the weather, retain
to a considerable extent their colours: this circumstance appears at first
surprising, but it is now known that the colouring principle of the Mytilus
is so enduring, that it is preserved when the shell itself is completely
disintegrated. (See Mr. Lyell "Proofs of a Gradual Rising in Sweden" in the
"Philosophical Transactions" 1835 page 1. See also Mr. Smith of Jordan Hill
in the "Edinburgh New Philosophical Journal" volume 25 page 393.) Most of
the shells are broken; I nowhere found two valves united; the fragments are
not rounded, at least in none of the specimens which I brought home.

With respect to the breadth of the upraised area in an east and west line,
we know from the shells found at the Inner Narrows of the Strait of
Magellan, that the entire width of the plain, although there very narrow,
has been elevated. It is probable that in this southernmost part of the
continent, the movement has extended under the sea far eastward; for at the
Falkland Islands, though I could not find any shells, the bones of whales
have been noticed by several competent observers, lying on the land at a
considerable distance from the sea, and at the height of some hundred feet
above it. ("Voyages of the 'Adventure' and 'Beagle'" volume 2 page 227. And
Bougainville's "Voyage" tome 1 page 112.) Moreover, we know that in Tierra
del Fuego the boulder formation has been uplifted within the recent period,
and a similar formation occurs on the north-western shores (Byron Sound) of
these islands. (I owe this fact to the kindness of Captain Sulivan, R.N., a
highly competent observer. I mention it more especially, as in my Paper
(page 427) on the Boulder Formation, I have, after having examined the
northern and middle parts of the eastern island, said that the formation
was here wholly absent.) The distance from this point to the Cordillera of
Tierra del Fuego, is 360 miles, which we may take as the probable width of
the recently upraised area. In the latitude of the R. Santa Cruz, we know
from the shells found at the mouth and head, and in the middle of the
valley, that the entire width (about 160 miles) of the surface eastward of
the Cordillera has been upraised. From the slope of the plains, as shown by
the course of the rivers, for several degrees northward of the Santa Cruz,
it is probable that the elevation attested by the shells on the coast has
likewise extended to the Cordillera. When, however, we look as far
northward as the provinces of La Plata, this conclusion would be very
hazardous; not only is the distance from Maldonado (where I found upraised
shells) to the Cordillera great, namely, 760 miles, but at the head of the
estuary of the Plata, a N.N.E. and S.S.W. range of tertiary volcanic rocks
has been observed (This volcanic formation will be described in Chapter IV.
It is not improbable that the height of the upraised shells at the head of
the estuary of the Plata, being greater than at Bahia Blanca or at San
Blas, may be owing to the upheaval of these latter places having been
connected with the distant line of the Cordillera, whilst that of the
provinces of La Plata was in connection with the adjoining tertiary
volcanic axis.), which may well indicate an axis of elevation quite
distinct from that of the Andes. Moreover, in the centre of the Pampas in
the chain of Cordova, severe earthquakes have been felt (See Sir W.
Parish's work on "La Plata" page 242. For a notice of an earthquake which
drained a lake near Cordova, see also Temple's "Travels in Peru." Sir W.
Parish informs me, that a town between Salta and Tucuman (north of Cordova)
was formerly utterly overthrown by an earthquake.); whereas at Mendoza, at
the eastern foot of the Cordillera, only gentle oscillations, transmitted
from the shores of the Pacific, have ever been experienced. Hence the
elevation of the Pampas may be due to several distinct axes of movement;
and we cannot judge, from the upraised shells round the estuary of the
Plata, of the breadth of the area uplifted within the recent period.

Not only has the above specified long range of coast been elevated within
the recent period, but I think it may be safely inferred from the
similarity in height of the gravel-capped plains at distant points, that
there has been a remarkable degree of equability in the elevatory process.
I may premise, that when I measured the plains, it was simply to ascertain
the heights at which shells occurred; afterwards, comparing these
measurements with some of those made during the Survey, I was struck with
their uniformity, and accordingly tabulated all those which represented the
summit-edges of plains. The extension of the 330 to 355 feet plain is very
striking, being found over a space of 500 geographical miles in a north and
south line. A table (Table 1) of the measurements is given below. The
angular measurements and all the estimations (in feet) are by the Officers
of the Survey; the barometrical ones by myself:--

TABLE 1.

Gallegos River to Coy Inlet (partly angular partly estimation) 350
South Side of Santa Cruz (angular and barometric) 355
North Side of Santa Cruz (angular and barometric) 330
Bird Island, plain opposite to (angular) 350
Port Desire, plain extending far along coast (barometric) 330
St. George's Bay, north promontory (angular) 330
Table Land, south of New Bay (angular) 350

A plain, varying from 245 to 255 feet, seems to extend with much uniformity
from Port Desire to the north of St. George's Bay, a distance of 170 miles;
and some approximate measurements (in feet), also given in Table 2 below,
indicate the much greater extension of 780 miles:--

TABLE 2.

Coy Inlet, south of (partly angular and partly estimation) 200 to 300
Port Desire (barometric) 245 to 255
C. Blanco (angular) 250
North Promontory of St. George's Bay (angular) 250
South of New Bay (angular) 200 to 220
North of S. Josef (estimation) 200 to 300
Plain of Rio Negro (angular) 200 to 220
Bahia Blanca (estimation) 200 to 300

The extension, moreover, of the 560 to 580, and of the 80 to 100 feet,
plains is remarkable, though somewhat less obvious than in the former
cases. Bearing in mind that I have not picked these measurements out of a
series, but have used all those which represented the edges of plains, I
think it scarcely possible that these coincidences in height should be
accidental. We must therefore conclude that the action, whatever it may
have been, by which these plains have been modelled into their present
forms, has been singularly uniform.

These plains or great terraces, of which three and four often rise like
steps one behind the other, are formed by the denudation of the old
Patagonian tertiary beds, and by the deposition on their surfaces of a mass
of well-rounded gravel, varying, near the coast, from ten to thirty-five
feet in thickness, but increasing in thickness towards the interior. The
gravel is often capped by a thin irregular bed of sandy earth. The plains
slope up, though seldom sensibly to the eye, from the summit edge of one
escarpment to the foot of the next highest one. Within a distance of 150
miles, between Santa Cruz to Port Desire, where the plains are particularly
well developed, there are at least seven stages or steps, one above the
other. On the three lower ones, namely, those of 100 feet, 250 feet, and
350 feet in height, existing littoral shells are abundantly strewed, either
on the surface, or partially embedded in the superficial sandy earth. By
whatever action these three lower plains have been modelled, so undoubtedly
have all the higher ones, up to a height of 950 feet at S. Julian, and of
1,200 feet (by estimation) along St. George's Bay. I think it will not be
disputed, considering the presence of the upraised marine shells, that the
sea has been the active power during stages of some kind in the elevatory
process.

We will now briefly consider this subject: if we look at the existing
coast-line, the evidence of the great denuding power of the sea is very
distinct; for, from Cape St. Diego, in latitude 54 degrees 30' to the mouth
of the Rio Negro, in latitude 31 degrees (a length of more than eight
hundred miles), the shore is formed, with singularly few exceptions, of
bold and naked cliffs: in many places the cliffs are high; thus, south of
the Santa Cruz, they are between eight and nine hundred feet in height,
with their horizontal strata abruptly cut off, showing the immense mass of
matter which has been removed. Nearly this whole line of coast consists of
a series of greater or lesser curves, the horns of which, and likewise
certain straight projecting portions, are formed of hard rocks; hence the
concave parts are evidently the effect and the measure of the denuding
action on the softer strata. At the foot of all the cliffs, the sea shoals
very gradually far outwards; and the bottom, for a space of some miles,
everywhere consists of gravel. I carefully examined the bed of the sea off
the Santa Cruz, and found that its inclination was exactly the same, both
in amount and in its peculiar curvature, with that of the 355 feet plain at
this same place. If, therefore, the coast, with the bed of the adjoining
sea, were now suddenly elevated one or two hundred feet, an inland line of
cliffs, that is an escarpment, would be formed, with a gravel-capped plain
at its foot gently sloping to the sea, and having an inclination like that
of the existing 355 feet plain. From the denuding tendency of the sea, this
newly formed plain would in time be eaten back into a cliff: and
repetitions of this elevatory and denuding process would produce a series
of gravel-capped sloping terraces, rising one above another, like those
fronting the shores of Patagonia.

The chief difficulty (for there are other inconsiderable ones) on this
view, is the fact,--as far as I can trust two continuous lines of soundings
carefully taken between Santa Cruz and the Falkland Islands, and several
scattered observations on this and other coasts,--that the pebbles at the
bottom of the sea QUICKLY and REGULARLY decrease in size with the
increasing depth and distance from the shore, whereas in the gravel on the
sloping plains, no such decrease in size was perceptible.

Table 3 below gives the average result of many soundings off the Santa
Cruz:--
TABLE 3.

Under two miles from the shore, many of the pebbles were of large size,
mingled with some small ones.

Column 1. Distance in miles from the shore.

Column 2. Depth in fathoms.

Column 3. Size of Pebbles.

1. 2. 3.

3 to 4 11 to 12 As large as walnuts; mingled in every case with
some smaller ones.

6 to 7 17 to 19 As large as hazel-nuts.

10 to 11 23 to 25 From three- to four-tenths of an inch in diameter.

12 30 to 40 Two-tenths of an inch.

22 to 150 45 to 65 One-tenth of an inch, to the finest sand.

I particularly attended to the size of the pebbles on the 355 feet Santa
Cruz plain, and I noticed that on the summit-edge of the present sea cliffs
many were as large as half a man's head; and in crossing from these cliffs
to the foot of the next highest escarpment, a distance of six miles, I
could not observe any increase in their size. We shall presently see that
the theory of a slow and almost insensible rise of the land, will explain
all the facts connected with the gravel-capped terraces, better than the
theory of sudden elevations of from one to two hundred feet.

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