The photo below is from Cerro Cristales, looking northeast across the valley now occupied by Lago Argentina (lake in the distance) and its narrow western arms (lakes in the foreground). Twenty thousand years ago this valley was filled with glaciers. El Calafate, the popular tourist town that sits on the south shore of Lago Argentina, and that is the base for exploring this area, would have been under a thick sheet of ice at that time.
What is the evidence for this incursion of ice out onto the flatlands away from the cordillera? There are many types of evidence; the photos below show some of the more obvious features. The valleys themselves are evidence – after being carved by advancing glaciers, the valleys filled with water as the glaciers retreated. Moraines, which are linear ridges formed where glaciers dumped their load of sediment carried from the mountains, mark the edges of the ancient glaciers. Terminal moraines form at the front edge of the glacier and mark the farthest advance of the glacier. Terminal moraines are found at the eastern end of Lago Argentina where they help provide a dam for the water. Easier to see on the ground are the lateral moraines that mark the side edges of the glacier and give a sense of the glacier’s height. The photo below shows a lateral moraine along the valley’s edge. The moraine is the middle ridge between flat ground in the foreground and mountain bedrock in the background.
What are the moraines made of? The sediment, called glacial till, is distinctively poorly sorted, with a very large variety of clast sizes ranging from mud to huge boulders. Only ice, which is highly viscous, can carry such large clasts. Imagine the ice eroding rock in the high mountains, conveying the resulting sediments downslope within the ice, then unceremoniously dumping the sediments at the edges. Below is a photo of glacial till, with a large boulder (Anita for scale) along the highway. As in all countries, locals promote their local sports team with their rock grattifi!
Sometimes the large boulders are found isolated on the landscape and not enclosed in a till deposit. These rocks are called erratics – they form when single hunks of rock are carried in the middle of the glacier and simply dropped to the ground when the ice melts. Here is an erratic in the middle of the valley, looking west (up valley) to where the glacier has retreated.
Yesterday we went on a boat up the western arms of Lago Argentina seeking views of the fast-retreating glaciers. A comparison of satellite images of Upsala glacier between the years 1986 and 2006 shows that the glacier has retreated an average of 200 km (>300 feet) each year! As it retreats, icebergs break off and choke the lake. We were therefore not able to get too close, but could only gaze upon the icebergs and get a glimpse of the glacier flowing lake ward in the distance (photo below).
Other glaciers, and evidence of their retreat, were more visible along the lake edges. The retreat in the photo below is obvious from the parts along the ice edges where vegetation has not yet had time to get established.
The mountains themselves are strong evidence for larger glaciers in the recent past. The mountains are not smooth shapes sculpted by water; rather, they have the jagged edges that are the hallmark of glacial erosion. Sharp peaks (horns), knife-edged rIdges (arêtes), and bowl-shaped depressions beneath the ridges (cirques) on mountains now far above the ice show that the ice once filled the adjacent valleys.
The Patagonian steppe, the relatively dry, sparsely vegetated area located east of the Patagonian Ice Field, is an important location for another ubiquitous landscape feature – the estancia (sheep ranch). Most of this area is too arid for cattle, and the sheep provide both meat and wool. Estancias dot the landscape almost everywhere, even within the national park. They take advantage of tourism by providing lodging, meals, sheep-shearing demonstrations, horseback riding, and views of mate-sipping gauchos! The estancia below is located in a field of glacially-deposited boulders.
Footnote: Today is both happy and a little sad. Sadly, Anita, who has been an awesome traveling companion and a patient landscape scale, is now returning to Buenos Aires and then to San Francisco. Happily, Jay, my well-tested traveling companion, is now on his way from Buenos Aires. We’ll meet in the El Calafate airport and then travel together to Ushuaia! Anita, thanks for your cheerful friendship and easy-going travel attitude. Exploring Patagonia with you was fabulous!