While the Beagle was laid up for three weeks at the mouth of the Rio Santa Cruz (for repairs to its hull), Captain Fitzroy took 25 men, including Darwin, in three whale boats, on an expedition up-river. Darwin described how “a wide, turbid and very rapid river rushes through a confined opening into the ocean ... and is opposed and driven back by the flood-tide" making progress up-river by oar or sail impossible; each boat had to be pulled up the river by ropes.
When not pulling on the ropes, Darwin observed the local flora and fauna, but one of his main objectives was to continue his study of the elevation of the gravel-capped plains of Patagonia in this valley. Further upstream, he found that the plains were covered with beds of lava up to 300 feet thick. These lava beds created springs and ravines, including the Basalt Glen painted by Conrad, ideal conditions for large numbers of guanacos - some of which provided the necessary fresh meat.
From 29th April, the snowy summits of the Cordilleras were visible, but by 4th May, with provisions exhausted, the party returned to the Beagle. The journey took but three days with the aid of the rapid current, in contrast to the 15-day struggle up-stream.
Although the party felt that little had been achieved, Darwin was pleased to have completed his geological work and added to his knowledge of the Patagonian plains.