Darwin marveled at the terraces and gravel, and so do we
We are now at 50 degrees latitude, at the lovely little town of Puerto Santa Cruz, located on the south bank of Rio Santa Cruz not far west of where it empties into the Atlantic Ocean. In 1834, during his voyage on the Beagle, Darwin entered this river and traveled up it until he could see the cordillera (the Andes). [Note that we will get to the other end of this river on Thursday when we travel to El Calafate on the shores of Lago Argentina, a lake that is fed by glacial melt waters from the Andes that then flow to the ocean via Rio Santa Cruz.]
Since we first began exploring the Atlantic coast of Patagonia at Peninsula Valdez, we have been impressed by the vast quantities of gravel we see everywhere. The gravel is found on top of very flat landscape features called terraces. But the gravel is also found mantling hill slopes and on just about every beach we’ve visited. Here is a photo of the beach at Puerto Santa Cruz, which is typical of the beaches we’ve seen.
The gravel is on the beaches because the clasts are large and much energy is required to move them. They fall off of the eroding sea cliffs where they overlie sediments of mud and fine sand that are much softer and that take very little energy to move. So the sand and mud is carried into the sea and the gravel remains behind. The next photo shows the gravel in place, where it overlies sediment of fine sand and mud that was deposited on the continental shelf in ocean water about 10-100 meters deep (it is full of fossil clams, snails, and oysters).
The thing that’s really amazing is that this gravel extends all the way from the coast to the Andes and it varies from a meter to many meters thick (it is getting even thicker to the west, toward the Andes that are the source of the gravel). Like Darwin, we have been trying to imagine the volume of this immense amount of gravel; in 1834 he said “If all this gravel were piled in a mound, it would make a great mountain chain”. And indeed it would – we need to picture the immense amount of Andean erosion and the rivers that have carried these pebbles and cobbles across the plains toward the sea.
How did that happen? It’s all about the glaciers, with remnants still in the Patagonian Ice Field (stay tuned for that next!). During the last several million years, glaciers have continually waxed and waned, and the gravels were deposited when large quantities of ice were melting and rivers were carrying the sediments eroded by glaciers to lower elevations. Here is a close-up photo of the gravel, which shows the channelized patterns typical of braided river deposits (the white colors are from calcium carbonate deposited afterward in the arid-climate soils).
There is also the terrace landscape feature. For those like me who study marine terraces, these are features of immense interest. But they are difficult to describe without animations and hand gestures. Suffice it to say that the land here is being uplifted to higher elevations all the time. This is surprising, because the coast here is a passive continental margin; that is, it is located far from a tectonic plate boundary. In California these features are common because active faults are moving and uplifting the land. They can also be found in areas that were covered with ice – when the ice melts the land rises, like an ice cube would rise when you put your finger on it and then remove it. But the ice never reached this far east.
Darwin marveled at the many steps of terraces (looking like a flight of stairs) and at the horizontal extent of these terraces. He did not understand how they formed and even now it’s uncertain. But recent studies suggest that it may be a result of tectonic plate movements on the other side of the continent, where oceanic plates are descending beneath the continent of South America along the west coast of Chile. Below is a photo of Rio Chico, which connects with Rio Santa Cruz. In the distance, you may be able to see three levels of terraces where the river has cut down as the land was rising.
For you penguin lovers, we visited another colony today. To see these critters, we had to rent bicycles and bike 15 km, then walk another 2.5 km along the beach. It seemed a more natural situation, though, as the penguins scurried away when they saw us (we were the only there, as the journey was more arduous). At the place where we last visited a penguin colony, wooden paths had been constructed across their paths to the sea and although they didn’t seem worried to be walking near us, we worried that the effect on them might not be good. Here is a photo of the penguins on the beach as we were walking along.
This view cost us! On the return, we were assaulted by fierce winds that made it nearly impossible to walk or bicycle. Anita said that she will never again complain about her summertime commute to the university, when she is biking into the wind coming off the fog – compared to today, that will feel like a gentle breeze! Here is a photo of me “gearing up” for the walk back into the wind, with sand blasting as an added attraction (if you look closely, you’ll also see a penguin on the right side).
Finally, here’s a photo along the coast on the way to the penguin colony, in the morning before the wind completely changed this landscape. The colors of blue in the sky and ithe water are intensely beautiful and unlike anything I’ve seen before. It’s a magical landscape, but harsh. And lastly, in my favorite words from Darwin: “What a history of geologic changes does the simply-constructed coast of Patagonia imply!”.