There are in this neighbourhood some other and lower plains, but they do not abut one at the foot of the other, in the manner hereafter to be described, so characteristic of Patagonia.The plain on which the settlement stands is crossed by many low sand-dunes, abounding with the minute shells of the Paludestrina australis, d'Orbigny, which now lives in the bay.This low plain is bounded to the south, at the Cabeza del Buey, by the cliff-formed margin of a wide plain of the Pampean formation, which Iestimated at sixty feet in height.On the summit of this cliff there is a range of high sand-dunes extending several miles in an east and west line.
Southward of Bahia Blanca, the river Colorado flows between two plains, apparently from thirty to forty feet in height.Of these plains, the southern one slopes up to the foot of the great sandstone plateau of the Rio Negro; and the northern one against an escarpment of the Pampean deposit; so that the Colorado flows in a valley fifty miles in width, between the upper escarpments.I state this, because on the low plain at the foot of the northern escarpment, I crossed an immense accumulation of high sand-dunes, estimated by the Gauchos at no less than eight miles in breadth.These dunes range westward from the coast, which is twenty miles distant, to far inland, in lines parallel to the valley; they are separated from each other by argillaceous flats, precisely like those on the northern shore of Bahia Blanca.At present there is no source whence this immense accumulation of sand could proceed; but if, as I believe, the upper escarpments once formed the shores of an estuary, in that case the sandstone formation of the river Negro would have afforded an inexhaustible supply of sand, which would naturally have accumulated on the northern shore, as on every part of the coast open to the south winds between Bahia Blanca and Buenos Ayres.
At San Blas (40 degrees 40' S.) a little south of the mouth of the Colorado, M.d'Orbigny found fourteen species of existing shells (six of them identical with those from Bahia Blanca), embedded in their natural positions.("Voyage" etc.page 54.) From the zone of depth which these shells are known to inhabit, they must have been uplifted thirty-two feet.
He also found, at from fifteen to twenty feet above this bed, the remains of an ancient beach.
Ten miles southward, but 120 miles to the west, at Port S.Antonio, the Officers employed on the Survey assured me that they saw many old sea-shells strewed on the surface of the ground, similar to those found on other parts of the coast of Patagonia.At San Josef, ninety miles south in nearly the same longitude, I found, above the gravel, which caps an old tertiary formation, an irregular bed and hillock of sand, several feet in thickness, abounding with shells of Patella deaurita, Mytilus Magellanicus, the latter retaining much of its colour; Fusus Magellanicus (and a variety of the same), and a large Balanus (probably B.Tulipa), all now found on this coast: I estimated this bed at from eighty to one hundred feet above the level of the sea.To the westward of this bay, there is a plain estimated at between two hundred and three hundred feet in height: this plain seems, from many measurements, to be a continuation of the sandstone platform of the river Negro.The next place southward, where I landed, was at Port Desire, 340 miles distant; but from the intermediate districts Ireceived, through the kindness of the Officers of the Survey, especially from Lieutenant Stokes and Mr.King, many specimens and sketches, quite sufficient to show the general uniformity of the whole line of coast.I may here state, that the whole of Patagonia consists of a tertiary formation, resting on and sometimes surrounding hills of porphyry and quartz: the surface is worn into many wide valleys and into level step-formed plains, rising one above another, all capped by irregular beds of gravel, chiefly composed of porphyritic rocks.This gravel formation will be separately described at the end of the chapter.
My object in giving the following measurements of the plains, as taken by the Officers of the Survey, is, as will hereafter be seen, to show the remarkable equability of the recent elevatory movements.Round the southern parts of Nuevo Gulf, as far as the River Chupat (seventy miles southward of San Josef), there appear to be several plains, of which the best defined are here represented.
(In the following Diagrams:
1.Baseline is Level of sea.
2.Scale is 1/20 of inch to 100 feet vertical.
3.Height is shown in feet thus:
An.M.always stands for angular or trigonometrical measurement.
Ba.M.always stands for barometrical measurement.
Est.always stands for estimation by the Officers of the Survey.
DIAGRAM 1.SECTION OF STEP-FORMED PLAINS SOUTH OF NUEVO GULF.
>From East (sea level) to West (high):
Terrace 1.80 Est.
Terrace 2.200-220 An.M.
Terrace 3.350 An.M.)
The upper plain is here well defined (called Table Hills); its edge forms a cliff or line of escarpment many miles in length, projecting over a lower plain.The lowest plain corresponds with that at San Josef with the recent shells on its surface.Between this lowest and the uppermost plain, there is probably more than one step-formed terrace: several measurements show the existence of the intermediate one of the height given in Diagram 1.
(DIAGRAM 2.SECTION OF PLAINS IN THE BAY OF ST.GEORGE.
>From East (sea level) to West (high):
Terrace 1.250 An.M.
Terrace 2.330 An.M.
Terrace 3.580 An.M.
Terraces 4, 5 and 6 not measured.
Terrace 7.1,200 Est.)
Near the north headland of the great Bay of St.George (100 miles south of the Chupat), two well-marked plains of 250 and 330 feet were measured: