Unable to link up with the Beagle, Darwin set off on his 400 mile ride to Buenos Aires complete with guide and passport for government horses. His main objective was the Sierra de la Ventana mountain, estimated by Fitzroy to be 3,500 feet high.
“It is very steep, rough & broken. — It is so completely destitute of all trees, that we were unable to find even a stick to stretch out the meat for roasting, our fire being made of dry thistle stalks. — The strangeness of its appearance chiefly is caused by its abrupt rise from the sea-like plain. ... The climbing up such very rough rocks was fatiguing; the sides are so indented ... At last when I reached the summit of the ridge, my disappointment was great to find a precipitous valley, as deep as the plain, separating me from the four peaks”
Darwin returned to his bivouac, and the ride north continued without incident. After passing General Rosa’s large cattle estancia, at the 12th posta, they travelled on, crossing a deep river, and reached Guardia del Monte:
“ [a] nice scattered little town, with many gardens full of peaches and quinces ... I first noticed here two plants, which Botanists say have been introduced by the Spaniards. — Fennel which grows in the greatest abundance in all the hedge rows. — & a thistle looking plant which especially in Banda Oriental forms immense beds leagues in extent, & quite impenetrable by man or beast."
From there they rode through uniform country abundant with cattle, horses and sheep, arriving at Buenos Aires on 20th September, where Darwin stayed with Edward Lumb, who helped him to obtain supplies and arrange shipment of his specimens to England.