"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 turn in teams dragging the boats upstream.The river cut through a series of rises then plateaux forming wide plains covered with shells and shingle, and Darwin discussed with FitzRoy his interpretation that these terraces had been shores that had gradually raised in accordance with Lyell's theories.