During the time that the Beagle spent surveying the coast south of Conception, Darwin rode across the Andes, from Santiago to Mendoza and back, in order to examine the geology of the Cordilleras. As his party ascended the flat valley that cut across the narrow shingle plain, vegetation became scarce, with the exception of a few pretty alpine flowers. On the first evening, they arrived at Valle del Yeso - a large basin once filled water. Here they found a great white bed of gypsum, nearly 2000 feet thick.
Following the valley the next day, they reached the watershed between the Pacific and Atlantic Oceans. The road zig-zagged steeply up the Cordillera to 12,000 feet. As they climbed they experienced shortness of breath, known locally as puna. On reaching the crest of the ridge, Darwin looked back on the glorious view declaring that
“the atmosphere so resplendently clear, the sky an intense blue, the profound valleys, the wild broken forms, the heaps of ruins piled up during the lapse of ages, the bright colored rocks, contrasted with the quiet mountains of Snow, together produced a scene I never could have imagined.”
Descending to 10,000 feet, they camped for the night in an area of sparse vegetation and extreme cold. After crossing the Cordillera they descended the steeper, shorter, eastern side through a band of cloud. Darwin commented on the contrast between the vegetation on either side.
Crossing an extensive saline plain, they took 2 days to reach Mendoza, encountering a swarm of locusts en route. The cultivated countryside around Mendoza reminded Darwin of Chilli. Setting out to examine the local rock strata, Darwin found a “small wood of petrified trees in a vertical position” with layers of fine sandstone deposited between. This can still be seen today, at the head of the Uspallata pass beside the main road from Menedoza to Santiago. Darwin then rode on and reached the ruins of Tambillos; the southern limit of the Incas. From here they continued down the valley of the Rio Aconcagua, reaching Santiago on 10th April, after a 24-day journey.