Uruguay: a small country with many beaches and very old rocks

In the Austral summer, people throughout South America flock to Uruguay to enjoy the sun and sand at the country’s many beaches. They do not go to Uruguay for the mountains, since the country’s highest elevation is a hill 513.66 m (1,685.2 ft) high, and most of the country is quite flat.

Uruguay is underlain by ancient rocks that in most places are billions of years old. Thin layers of younger sediments and volcanic rocks cover much of the interior, but the older rocks are well exposed along the coast.

We spent the last 2 weeks of our 6-week-long February–March trip in Uruguay. This post is delayed because I needed time to learn more about Uruguay’s geologic history. In this post, I show some of the rocks we saw along different parts of the coast. Although their complicated history makes them difficult to decipher, some are quite beautiful. But first, a little background.

Uruguay’s location in South America

Uruguay has a long border with Brazil on its northeast side and a long border with Argentina on its west side. To the south it is separated from Argentina by the Río de la Plata estuary. Its long coastline transitions from estuarine to oceanic near the capital city of Montevideo.

We spent most of our two weeks in Uruguay along the coastline. We traveled via a large ferry from Buenos Aires to Montevideo, where we explored the city and the Canelones wine area just north of the city. We then rented a car and traveled east to Maldonado and vicinity—best known for the Miami-style city of Punta del Este. After returning to Montevideo, we took a bus westward to Colonia del Sacramento. We then returned to Buenos Aires via another large ferry. In general, flights to the U.S. are better from Buenos Aires than from Montevideo.

Brief geologic history

All of Uruguay is underlain by ancient rocks of the Río de la Plata craton. Cratons are made of the oldest rocks on Earth. They have been subjected to many tectonic events and now form the rigid basement of igneous and metamorphic rocks that are the foundation upon which younger rocks have been deposited.

The Río de la Plata cratonic rocks are covered by younger rocks in the central and northern parts of the country (yellow color in map above), but they are well exposed in the southern part, including the southern coastline where we spent most of our time.

These Precambrian rocks went through many episodes of continental collisions and separations, most recently the collision that united the southern continents of Gondwana with the northern continents of North America and Eurasia to form Pangea (“all lands”) about 250 million years ago, followed by the separation of North and South America from Africa and Europe to form the Atlantic Ocean. The South Atlantic began to form about 135 million years ago, causing extensive basaltic lava flows in the northern part of Uruguay as the continents began to rift apart.

The Cuchilla (knife) Dionisio Terrane is a piece of the African continent—similar rocks are found in Namibia—that got left behind when the South Atlantic began to open. All of these terranes have a complex history that is still being diciphered.

Montevideo and Canelones

We spent our time in Montevideo exploring its cultural elements. The city has a long and indented coastline that provides a series of beaches for its residents and visitors to enjoy.

Uruguay has a thriving wine industry. Probably its most famous region is Canelones, which is located in the Cuenca de Santa Lucia (see geology map above). The younger sedimentary rocks in this basin (cuenca) are more porous, and can hold more water, than the crystalline Precambrian basement rocks. They form an aquifer that provides water for the city of Montevideo and for agriculture, including many wineries.

All three of the wineries we visited had a similar story. They are being run by the fourth generation of Italian immigrants from the Piedmont region. Apparently, someone had success and others followed! The quality and quantity of the produced wines has certainly improved over the years. Uruguay is most known for a red wine from the Tannat grape, but they also produce many common varietals such as merlot and albariño.

The beaches between Montevideo and Punta del Este

I’ll start at Punta del Este (East Point) and move westward toward Montevideo. This coastline consists of a series of promontories and bays. The promontories are formed where the ancient rocks are more resistant to erosion whereas the bays form where the rock types are less resistant. Punta del Este is adjacent to Maldonado, which is shown on the maps above.

Punta Ballena (Whale Point)

Punta Negra (Black Point)

Piriápolis

Colonia del Sacramento

Colonia is a popular tourist destination because of its old town that is a UNESCO World Heritage Site (see location on maps above). It is one of Uruguay’s oldest towns and is built on some of Uruguay’s oldest rocks. We stayed in a colonial building that had been converted into a charming B+B. We enjoyed walking around the old town that was mostly built with stones quarried from the ancient local rocks.

The town was a site where Portugal and Spain fought for dominance in South America, and the town’s structures reflect the styles of Portugal first, and then of Spain next, once they achieved final control.

Final Note

Most people I’ve talked with know very little about Uruguay, so I’ll do one more post about the culture, with lots of photos.

Reference

Demarco N., et al., 2018, Historia de la Geología Precámbrica de Uruguay: https://www.researchgate.net/publication/330888413_HISTORIA_DE_LA_GEOLOGIA_PRECAMBRICA_DE_URUGUAY_Revision_de_las_divisiones_estructurales_tectoestratigraficas_sus_limites_y_nomenclaturas.