Darwin's Experience on the Santa Cruz River [1] (Sidebar 1)

The Beagle anchored in the mouth of the Santa Cruz River on April 13, 1834. A previous expedition sent an exploration party thirty miles up the river. Captain Fitz Roy wanted to go even further on this occasion, and so he planned on being out about three weeks. Three whaleboats and twenty-five crewmen started out on the 18^th for the Andean Cordillera (mountain chain system). Initially, the river was about a thousand feet across and about 17 feet at its deepest. Darwin wrote in his diary that,

"The water is of a fine blue colour, but with a slight milky tinge, and not so transparent as at first sight would have been expected. … This valley varies from five to ten miles in breadth; it is bounded by step-formed terraces, which rise in most parts, one above the other, to the height of five hundred feet, and have on the opposite sides a remarkable correspondence."

The boats were tied in a line and towed by the men against the current for about 10 miles per day as the crow flies (15 miles actual). Within two days the party passed the previous excursion point along the coastal plains and headed for an inland basaltic platform. Over the next twenty-eight miles the basalt-layer (a lava flow) expanded from 120 to 320 feet in thickness on both sides of the river valley. It was at this point (about 80 to 100 miles inland) that Darwin passed one of his famous milestones in interpreting earth history,

"At the first glance of the basaltic cliffs on the opposite sides of the valley, it was evident that the strata once were united. What power, then, has removed along a whole line of country, a solid mass of very hard rock, which had an average thickness of nearly three hundred feet, and a breadth varying from rather less than two miles to four miles? This river, though it has so little power in transporting even inconsiderable fragments, yet in the lapse of ages might produce by its gradual erosion an effect, of which it is difficult to judge the amount. … Geologists formerly would have brought into play, the violent action of some overwhelming debacle; but in this case such a supposition would have been quite inadmissible; because the same step-like plains with existing sea shells lying on their surface, which front the long line of the Patagonian coast, sweep up on each side of the valley of Santa Cruz."

Darwin attributed the basaltic channelization to a long period of gradual erosion—one sand grain at a time. Since he only went a maximum of 140 miles inland on this occasion, he knew little of the glaciers, ice dams, and intermittent torrential floods in and from the highlands. All or combinations of these forces in a colder glaciation period might have been the cause, directly or indirectly, of the massive, short-term wasting of the basalt to form the channels and gorge at Condor Cliffs. Even the milky water that he noted at the outset could have been a clue to him of glacial waters from the Andes. In actuality, the theory of an ice age was not accepted widely before the studies of Jean de Charpentier and Louis Agassiz in the late 1830’s and 1840’s. [2]Darwin’s rationalization materialized from reading Lyell's explanation for the presence of erratic boulders in the streambed; he attributed their transport to floating icebergs.

By the 5^th of May the Captain ended the ascent and started back to the coast in the whaleboats. They arrived at the Beagle on the 8^th having enjoyed the down stream current of several knots per hour.

1. Darwin, Charles. The Voyage of the Beagle (New York, Mentor Books, 1972), pp 152-161. Return to Text

2. Flint, Richard Foster. Glacial and Quaternary Geology (New York, John Wiley and Sons, Inc., 1971), pp 11-15. Return to Text

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