XXVII.--On the Distribution of the Erratic Boulders and on the Contemporaneous Unstratified Deposits of South America

1842 Transactions of the Geological Society of London  
Boulder Formation in the Valley of the Santa Cruz. DURING the survey of the shores of South America, southward of the Rio Plato, by Capt. FitzRoy in H.M.S. Beagle, I did not meet with any boulders on the eastern plains of the continent until we arrived on the banks of the river Santa Cruz, in lat. 50° lC S. Nor did they occur there near the coast, but were first noticed in ascending the river at the distance of about 100 geographical miles from the Atlantic, and 67 from the nearest slope of the
more » ... earest slope of the Cordillera. Twelve miles further west, in Ion. 70° 50' W., that is, fifty-five miles from the mountains, they were extraordinarily numerous; consisting of compact clay-slate, feldspathic rock, a quartzose chloritic schist, and basaltic lava; and they were generally of an angular form, and many of them resembled fragments of rock at the foot of a precipice. The size of some was immense : I measured a square one of chloritic schist, which Was five yards on each side and projected five feet above the ground; a second, which was more rounded, was sixty feet in circumference, and stood six feet above the ground; how much of each was buried beneath the surface I could not ascertain. There were innumerable other fragments from two to four feet square. The vast open plain on which they lay scattered, is 1400 feet above the level of the 3 H 2 Downloaded from 418 Mr. DARWIN on the Boulders nian formation, fringed by deposits of much more recent origin, the height of which varies from about 100 to 250 feet. These lower, irregular plains have been elevated within the post-pliocene period. They consist of fine-grained, earthy or argillaceous sandstone, in very thin, horizontal, but sometimes inclined laminse, and often associated with curved layers of gravel. On the borders, however, 6f the eastern parts of the Strait of Magellan, this fine-grained formation often passes into, and alternates with, great unstratified beds, either of an earthy consistence and whitish colour, or of a dark colour and of a consistence like hardened coarse-grained mud, with the particles not separated according to their size. These beds contain angular and rounded fragments of various kinds of rock, together with great boulders. At Elizabeth Island, within the Strait, there are good sections of this deposit in cliffs 150 feet high, and composed chiefly of whitish earth, with fragments of syenite, greenstone, feldspathic rocks, clay and hornblendic slates and quartz, most of which do not occur, in situ, in the neighbourhood. These frag* ments are generally arranged without the slightest trace of order,-large and small, angular and rounded being close together; but in some parts of the cliff, the mass is divided by beds of stratified shingle, and these are most frequent in the upper part,-a fact which I observed in other places. Few of the fragments much exceed in size a man's head, but there are numerous large boulders on the beach. In the cliff at Cape Negro, which is close to Elizabeth Island, and is of the same height and of nearly the same nature, I saw a great boulder imbedded. This deposit at Nuestra Senora de Gracia is rather finer grained, and contains fewer fragments; some of which are perfectly rounded, some quite angular; and a single one, of considerable size, is often imbedded by itself in fine-grained and fine-laminated matter. I here, also, observed a boulder at least four feet in diameter, projecting from the face of the cliff. In a neighbouring cliff, a whitish mass fills up hollows in an underlying fine-grained bed. North of Cape Virgins, close outside the mouth of the Strait, the cliffs are between 200 and 300 feet in height; and they consist of an argillaceous sandstone in horizontal laminae, as fine as roofing-slate, which in several places is interstratified with two or three beds of the coarse nature just described, each stratum being from five feet to twenty thick. These beds often thin out and become curvilinear at each end. The imbedded fragments are of the same nature and shape as before mentioned; and their parent rock cannot be less, and probably is considerably more, than 120 geographical miles distant. In the other cases above described, the distance must,be at least sixty miles. The mountains, from which they all probably came, lie west and south-west. The numerous boulders before noticed on the beach at the foot of the cliffs on Elizabeth Island, consist of the same varieties as the smaller imbedded fragments,
doi:10.1144/transgslb.6.2.415 fatcat:snclcszwnnfilgwj6u36s6ais4