"Beagle" now sailed to southern Patagonia, and on 19 April, an expedition including FitzRoy and Darwin set off to take boats as far as possible up the Santa Cruz river, with all involved taking turns in teams dragging the boats upstream. The river cut through a series of rises, then through plateaux forming wide plains covered with shells and shingle. Darwin discussed with FitzRoy his interpretation that these terraces had been shores that had gradually raised per Lyell's theories. Several of the smaller rheas were seen in the distance but were too elusive to catch. The expedition approached the Andes but had to turn back.