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 Iam 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 950and 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, 160feet 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 OFAMERICA, 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: