The geological constitution is in accordance with the orographic physiognomy
. The Tertiary plateau, flat on the east, gradually rising on the west, shows Upper Cretaceous caps at its base. First come Lower Cretaceous hills, raised by granite and dioritic
rocks, undoubtedly of Tertiary origin, as in some cases these rocks have broken across the Tertiary beds, so rich in mammal
remains; then follow, on the west, metamorphic
schists of uncertain age; then quartzites
appear, resting directly on the primitive granite and gneiss
which form the axis of the Cordillera
rocks occur between the schists and the quartzites. The Tertiary deposits are greatly varied in character, and there is considerable difference of opinion concerning the succession and correlation of the beds. They are divided by Wilckensi into the following series (in ascending order):
- Pyrotherium-Notostylops beds. Of terrestrial origin, containing remains of mammalia . Eocene and Oligocene .
- Patagonian Molasse. Partly marine, partly terrestrial. Lower Miocene .
- Santa Cruz series. Containing remains of mammals. Middle and Upper Miocene.
- Paranfl series. Sandstones and conglomerates with marine fossils . Pliocene . Confined to the eastern part of the region.
The Upper Cretaceous and Tertiary deposits have revealed a most interesting vertebrate fauna. This, together with the discovery of the perfect cranium of a chelonian of the genus Myolania , which may be said to be almost identical with Myolania oweni of the Pleistocene age in Queensland , forms an evident proof of the connection between the Australian and South American continents. The Patagonian Myolania belongs to the Upper Chalk, having been found associated with remains of Dinosauria . Other specimens of the interesting fauna of Patagonia, belonging to the Middle Tertiary, are the gigantic wingless birds, exceeding in size any hitherto known, and the singular mammal Pyrotherium , also of very large dimensions. In the Tertiary marine formation a considerable number of cetaceans has been discovered. In deposits of much later date, formed when the physiognomy of the country did not differ materially from that of the present time, there have been discovered remains of pampean mammals, such as Glyptodon and Macrauchenia , and in a cave near Last Hope Inlet, a gigantic ground sloth ( Grypoiherium listai ), an animal which lived contemporaneously with man, and whose skin, well preserved, showed that its extermination was undoubtedly very recent. With the remains of Grypotherium have been found those of the horse ( Onoshippidium ), which are known only from the lower pampas mud, and of the Arciotherium , which is found, although not in abundance, in even the most modern Pleistocene deposits in the pampas of Buenos Aires . It would not be surprising if this latter animal were still in existence, for footprints, which may be attributed to it, have been observed on the borders of the rivers Tamangoand Pista, affluents of the Las Hefas, which run through the eastern foot-hills of the Cordillera in 47°S.
Aerial view of the Perito Moreno Glacier (Santa Cruz Province) and the Andean ice-sheet Glaciers occupy the valleys of the main chain and some of the lateral ridges of the Cordillera, and descend to lakes San Martín Lake , Viedma Lake , Argentino Lake and others in the same locality, strewing them with icebergs . In Patagonia an immense ice-sheet extended to the east of the present Atlantic coast during the first ice age, at the close of the Tertiary epoch, while, during the second glacial age in modern times, the terminal moraines have generally stopped, 30 miles (50 km) in the north and 50 miles (80 km) in the south, east of the summit of the Cordillera. These ice-sheets, which scooped out the greater part of the longitudinal depressions, and appear to have rapidly retreated to the point where the glaciers now exist, did not, however, in their retirement fill up with their detritus the fjords of the Cordillera, for these are now occupied by deep lakes on the east, and on the west by the Pacific channels, some of which are as much as 250 fathoms (460 m) in depth, and soundings taken in them show that the fjords are as usual deeper in the vicinity of the mountains than to the west of the islands. Several of the high peaks are still active volcanoes. In so far as its main characteristics are concerned, Patagonia seems to be a portion of the Antarctic continent, the permanence of which dates from very recent times, as is evidenced by the apparent recent emergence of the islets around Chiloé , and by the general character of the pampean formation. Some of the promontories of Chiloé are still called huapi , the Araucanian equivalent for " islands "; and this may perhaps be accepted as perpetuating the recollection of the time when they actually were islands. They are composed of caps of shingle, with great, more or less rounded boulders, sand and volcanic ashes, precisely of the same form as occurs on the Patagonian plateau. From an examination of the pampean formation it is evident that in recent times the land of the province of Buenos Aires extended farther to the east, and that the advance of the sea, and the salt-water deposits left by it when it retired, forming some of the lowlands which occur on the littoral and in the interior of the pampas , are much more recent phenomena; and certain caps of shingle, derived from rocks of a different class from those of the neighboring hills, which are observed on the Atlantic coasts of the same province, and increase in quantity and size towards the south, seem to indicate that the caps of shingle which now cover such a great part of the Patagonian territory recently extended farther to the east, over land which has now disappeared beneath the sea, while other marine deposits along the same coasts became converted into bays during the subsequent advance of the sea. There are besides, in the neighborhood of the present coast, deposits of volcanic ashes, and the ocean throws up on its shores blocks of basaltic lava , which in all probability proceed from eruptions of submerged volcanoes now extinct. One fact, however, which apparently demonstrates with greater certainty the existence in recent times of land that is now lost, is the presence of remains of pampean mammals in Pleistocene deposits in the bay of Puerto San Julian and in Santa Cruz . The animals undoubtedly reached these localities from the east; it is not at all probable that they advanced from the north southwards across the plateau intersected at that time by great rivers and covered by the ice-sheet. With the exception of the discoveries at the inlet of Ultima Esperanza , which is in close communication with the Atlantic valley of Río Gallegos , none of these remains have been discovered in the Andean regions.