While the Beagle was completing her survey of the Patagonian coast, Darwin was landed on the north bank of the Rio Negro to travel by horseback to the camp of General Rosa and the Argentinean army at war with the Araucanian Indians at the River Colorado. Guided by Mr Harris and five Spanish Gauchos, they set out on the 85 mile ride. Darwin remarked to his sister Caroline that
“I am become quite a Gaucho, drink my Mattee & smoke my cigar, & then lie down & sleep as comfortably with the Heavens for a Canopy as in a feather bed”
On meeting General Rosa, Darwin obtained a passport and order for government post horses, which would enable him to travel to Bahia Blanca, and then on to Buenos Aires. He set off on 16th August, traveling from one posta to another along the fertile river plains. At the 2nd and 3rd postas he came across the grand Pampean geological formation, a belt of red sand dunes 8 miles wide, which stretched for 25 miles north of the river.
When Darwin finally reached the Fort of Bahia Blanca, the Beagle had not yet arrived. With time on his hands he hired a guide and visited Punta Alta, where he spent some time marking places where there were bones lying, to be collected later. They returned to Bahia next day in heavy rain. The Beagle was sighted on 24th August but prevented from sending a boat for Darwin until August 26th.