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!”.




The flood with unexpected consequences (it fossilized plants!)

West of Comodoro Rivadavia, a prosperous but mostly unattractive town devoted to the petroleum business, is the village of Sarmiento. Most of Patagonia is a desert with low-growing bushes and long vistas. Between C. Rivadavia and Sarmiento, one is reminded of Texas, with oil wells (here referred to as penguinos – penguins!) on a relatively featureless landscape (affectionately referred to as vast by Texans!). There are oases in this desert. Places like Sarmiento are located in broad river valleys that flow mostly from west to east, from the cordillera (the Andes) to the sea. These valleys benefit from additional water and are green, agriculturally-productive areas. We are staying at Estancia Labrador (estancia=ranch) that has sheep, horses, and cherry orchards. They sell honey and sweet products such as alfajores, an Argentinian specialty whose main ingredient is dulce de leche. It’s a great place to sample the estancia culture as well as delicious homemade food.

The main attraction for this trip is the Bosque Petrificado, a protected area with rangers to provide information and keep visitors from slipping hunks of petrified wood into their pockets (it’s very tempting, as my husband Jay can tell you!). The landscape has a badlands-like appearance. Although they are 65 million years old, the sediments are not well consolidated and they erode into soft mound shapes. Adding to the allure of the place is the multiple colors of the sedimentary layers, shown in the photo below.


In the time when these sediments were deposited (near the Meozoic-Cenozoic boundary when the dinosaurs went extinct) this area was located on a broad coastal plain with estuaries and marshes. Landward of the coast were forests with palm trees and other tropical plants. One day a large flood carried vast amounts of plant material, including tree trunks up to a meter in diameter and many meters long, into the coastal zone where they were quickly buried by sediments, thus preserving them from rotting, the fate of most plant material. After burial, silica-rich fluids, apparently provided by volcanic activity in the region, flowed through the porous sand and turned the wood into rock made of quartz. Most of the tree trunks are surrounded by a thin veneer of sand that is also cemented with silica. In the photo below, there is a log sticking out of the sediment within which it was deposited; most of the fossil plants are no longer in their original positions but are located on the landscape where they have collected as the softer sediment around them has eroded away.


Unlike the petrified forest you may have seen in Arizona, this event preserved even the smallest splinter, as well as seeds. A sign from along the path explains how this occurred.


Another photo may give a sense of how real these tree pieces appear – I found myself continually checking the pieces to make sure they were really rock and not wood, because the smallest details of the wood have been preserved.


Our weather yesterday was not ideal, as we had wind that nearly blew us away, and we were hoping for sun to better show the brilliant sediment colors. But we were not disappointed with the petrified forest. (Note: to the south there is another petrified forest that was buried and silicified as a result of a volcanic eruption.) Another benefit of the outing was some cultural education. Our driver brought with her several friends and, in true Argentinian style, it quickly turned into a moving party. Below you can see Anita drinking her first mate, as we received instructions on proper form when invited to partake of this ubiquitous national beverage.


Our favorite critters and don’t forget that plants make fossils too

Almost a million penguins – even a geologist has to be impressed by the quantity, not to mention the silly antics, of these crazy flightless birds. We went to Punta Tombo where at least 250,000 mating pairs have made burrows in the sediment – they return each year to give birth and raise their young (called pichons) before they all return to the sea for some months of feeding and frolicking. The part on land looked difficult to us. They have to hike a long way to the beach to reach the sea and go fishing. But worse of all, they have these adolescent pichons who are constantly whining for food. Neither Anita or I have been parents, but we would not have put up with this behavior! Here is a poor Mom (or Dad?) putting up with the two kids relentlessly perusing their quest for food. Too bad I can’t include the video for sound effects.


We were amazed by penguins passing us on the path. One looked like it was going to lick Anita’s feet! We just stood still and let them do what they wanted. Another photo shows their determined march to and from the sea.



Finally, a classic pose with hundreds of nests and the sea in the background.


Some of the critters went extinct (fossils!)

This province (Provincia Chubut) is super rich in life forms. But even more diverse than the organisms living today are those that have gone extinct. From east (near the coast) to west (near the Andes), sedimentary deposits are progressively older and have fossils from the Cenozoic (age of mammals), Mesozoic (age of reptiles, including dinosaurs), and Paleozoic (old life, including trilobites). Paleontologists from around the world flock here to help with excavations and study the specimens.

We are staying near the town of Trelew, which is known for its Museo Paleontológico Egidio Feruglio (fossil museum – if you google the name you’ll find their excellent web site). We were fortunate to have two scientists from the museum guide us on a field trip to Parque Bryn Gwyn, where we could see fossils in sediments ranging in age from 40 – 10 million years ago. Here you can see Anita and our awesome guides: Pedro, an astronomer who leads the educational programs for the museum, and Amalia, a biologist/paleontologist who explained what we were seeing along the way.


For a sedimentologist (me!) there were lots of cool features to see – strata formed by moving ripples, ancient soils, beds of volcanic ash. But the features most interesting to nongeologists are usually the fossils – our record of how life has evolved through time. There were land animals – anteaters, several types of marsupials and other life forms not living today. Then the ocean flooded the land and the sediments contained sharks, whales, and smaller life forms such as oysters. The most impressive of these was a thick layer of shells that were preserved in their life position (see picture below). In Anita’s hand you can see their large size – imagine having these on the half shell!



Then it was off to the museum. You have probably read about some of the fossils from the area, because they include the largest dinosaur yet found. It is a vegetarian sauropod that is named Argentinosaurus! It was 80-100 tons and 10-12 meters long. The photo shows its legs with Anita for scale. Are we glad they don’t live now? But they eat plants; more frightening would have been the tyrannosaurus who was even bigger than the Rex. The museum has a large number of fossils from all of the time periods found in Chubut province and is well worth a visit should you find yourself in Trelew (meanwhile, check out their website).




Peninsula Valdéz is at 42 degrees (Buenos Aires is at 34 degrees) so the days are longer – it’s not dark until about 9). For those of you who know the Point Reyes Peninsula, where I’ve done geologic research, there are some surprising similarities. A narrow strip of land extends seaward and ends at coastal cliffs that are uplifted marine terraces (surprising because this coast is not at a plate boundary like the marine terraces I’ve studied at Point Reyes in California – but more about that in a future post). The coastal cliffs are made of a sedimentary formation that looks like the Purisima Formation on the south cliffs of Point Reyes – a similar age (about 10 million years old) and a similar environment (fine sand and mud with lots of marine fossils deposited on the continental shelf). The image below, of cliffs behind Puerto Pirámides where we are staying on the south coast of Peninsula Valdéz, sure looks a lot like the cliffs near Drakes Beach on the south coast of Point Reyes Peninsula.


But I have promised stuff about critters! This peninsula isn’t called “Reserva Faunística” (animal reserve) for nothing. It’s also been named a World Heritage Site by UNESCO. During one day we saw an amazing variety of interesting and unusual critters. First off was a large group of Southern Sea Lions (lobos marinos) that were toward the end of their birthing season. You can see many babies in the photo below. You will also notice that the males are larger than the sea lions we have in California.


Next, it was a colony of Magellanic Penguins, whose young had hatched a month or more ago. These youngsters, like the one in photo below, were in the process of loosing their fuzzy coats and were starting to acquire a more adult look. Their nests are burrows in the soil, where they come to rest and warm up after going to the ocean to feed. Other marine critters viewed were orcas and dolphins, and some far-away, inert elephant seals. On land we saw guanacos (related to llamas), Ńandu (related to ostriches), foxes, an armadillo, giant rabbits, and various birds. Another similarity with Point Reyes is the cultural history of ranching. On Point Reyes, cows continue to graze on historically-designated ranches; on Peninsula Valdes, sheep continue to graze on historic ranches called estancias. Somehow, it’s comforting to see familiar faces, be they sediments or sea lions, when traveling in distant lands!


Nature in Buenos Aires?

Buenos Aires located on southwest shore of Rio de la Plata estuary, the boundary between Argentina and Uruguay.

To get to Patagonia, it’s first necessary to go through Buenos Aires, a large bustling city where more than 1/3 of the country’s 40 million people live. To get to Buenos Aires, we passed through Washington, D.C., now in the Northern Hemisphere’s winter. Here, in the Southern Hemisphere, it feels like D.C. in the  summer – super hot and humid! Also like D.C., Buenos Aires is situated next to a river. But unlike the Potomac River, which flows into the Chesapeake Bay estuary (where fresh water from the river mixes with salt water from the ocean), the Rio de la Plata is its own estuary – it empties directly into the sea and is slightly salty from the intrusion of ocean water.

The old port has been redeveloped and cranes, formerly used to load ships, are retained only as remnants of the past. Note brown water from suspended sediment in river estuary.

Buenos Aires does not readily acknowledge its coastal position, and various commercial/industrial interests block residents from access to their river estuary – this  made me appreciate San Francisco’s super open waterfront! But my current traveling partner, Anita (helpfully, an intrepid oceanographer) and I were determined to find the coast and some natural greenery. It wasn’t easy! Happily, along the city’s eastern side – east of the famous Plaza de Mayo – the district of Puerto Madero – the old commercial port that’s now inactive – has been converted into a walkable area with condo highrises, businesses, and restaurants. The muddy estuarine water is visible in the old canals where an ancient sailing ship can be boarded.

But nature remained illusive. Continuing east we finally encountered a large street lined with food vendors. Beyond, a strip of green enticed us to enter la Reserva Ecological Costanero Sur – the south coast ecological preserve. Fences restricted our entry but after walking 1.5 km, we found an access point and bicycles for rent. What a welcome site for our weary legs! We hopped on the bikes and peddled along dirt paths until finally reaching the  coast – admittedly with artificial embankment and a beach with large chunks from past human constructions – but with waves and the smell of salt air. Like in San Francisco Bay, the river was being traversed by tanker ships and tug boats going to meet them. Also like in SF Bay, very little of the original tidal wetland area is preserved, but in this one place, a strip of marshland provides habitat for birds and humans eager to escape the concrete jungle of the city. We felt satisfied to have found this nugget of nature!

A view west across a remnant of preserved wetlands, with city buildings in the background

Postnote: Yes, we’ve also enjoyed the cultural offerings of this very cosmopolitan city! We’ve sampled excellent food and viewed interesting sites, perhaps the most memorable a densely packed cemetery with amazingly huge and elaborate monuments over the remains cittern city’s prominent deceased. The next step is a long overnight bus ride to Puerto Madryn, 10 degrees of latitude to the south. Stay tuned!

View eastward toward the Atlantic Ocean. The muddy water of Rio de la Plata flows into the ocean. Buenos Aires is whitish sprawl on lower right bank of the river.

Join me for a trip to the end of the world!

El fin del mundo—the end of the world! Only the southern tip of South America dares poke its head south into the 40-55° latitudes where the planet’s strongest winds—the West Wind Drift—drive the planet’s strongest current—the Circum-Antarctic Current. In this wind-swept part of the world, the stark yet stunning landscapes take their character from a base of amalgamated crustal blocks that have been covered with sediments, intruded by magma, and carved by wind, water and, most spectacularly, ice. Today, 20,000 years after the Last Glacial Maximum, a continental ice sheet continues to shove more than 40 glaciers out onto the flat lands of Chile and Argentina. Active volcanoes continue to spew their ash into the air, and marine critters ply the nutrient-rich coastal waters.

My goal is to explore these fantastic landscapes and to provide you, the reader, with a taste of their beauty and the geologic underpinnings that have created them. The time will be during the Austral summer, from 18 January to 7 March 2012. After leaving Buenos Aires, the journey will extend south along the Atlantic coast of Patagonia, from Peninsula Valdéz to Santa Cruz, where in 1834 Darwin navigated an inland river and observed a terraced landscape recently uplifted from the sea.  After a week in Ushuaia (Tierra del Fuego) Patagonian explorations will continue northward along the eastern side of the Andes, from El Calafate and Parque Nacional Los Glaciares (including famous climbing mountains Cerro Torre and Cerro Fitz Roy) to Bariloche in the Lake District.

I welcome you to sign up to follow my blog. I also welcome your comments, particularly to share insights that will help further reveal the wonders of this magnificent place on Earth.

Southern Chile and Argentina, between 40–55° S latitude. White=ice and snow of the Patagonia Ice Field; orange=active volcanoes; yellow push pins=localities to be explored.