The Pampean formation consists almost entirely of loose deposits, sand and clays of various sorts. There is no gravel.[80] Even in the vicinity of the sierras the beds of gravel, with round or angular pebbles, are almost always covered by clay, and are exposed only in the banks of the streams. Olascoaga mentions the surprise of the _gauchos_ of General Roca's army when they found Patagonian pebbles on the ground during their stay at Choele Choel on the Colorado, in the course of the campaign on the Rio Negro. Officers and soldiers dismounted to pick them up. Sand and clay form a thick bed of continental alluvia. The Tertiary maritime transgressions, which have left their mark in the clays and limestones of the left bank of the lower Parana, and the layers of shells at San Pedro on the right bank, never penetrated far into the interior of the Pampean region, and one finds no trace of them when one leaves the coast or the river. [80] Roth claims to have found gravel in the San Nicolas _barranca_ on the Parana. I have myself found small rounded flints in the clay of the Chaco at Tartagal. But these deposits probably come from the left bank of the Parana, where the beds of river gravel are considerable. The source of the elements which compose the Pampean alluvia is very uncertain. Their composition does not clearly show their origin. The clays are comparatively rich in calcareous matter, which seems to indicate that they do not come from tropical America or the upper basin of the Parana. Wright and Fenner insist upon the high proportion of siliceous glass of volcanic origin which they contain, which points to an intense eruptive activity during or before their formation.[81] Doering had already noticed in the Cordoba region the prevalence of beds of volcanic ash, which become thicker as one approaches the sierra. It is certain that the Pampean sierras have had their share in the formation of the Pampean beds. But the main mass is probably of Andean origin. However that may be, as soon as one gets away from the fringe of the mountains, the only variety noticeable in the lands of the Pampa is that which they owe to the conditions in which they have been deposited. [81] In Ales Hrdlicka, _Early Man in South America_ (Smithsonian Instit. Bull., 52, Washington, 1912). River deposits strictly so called, estuary deposits, lagoon deposits, aeolian deposits, aeolian deposits redistributed by water, river deposits redistributed by wind--all these different types are represented in the Pampean formation, but their relative importance is still disputed.[82]