A Mountain Range on the Plains
The vast, flat Pampas of Argentina are interrupted by a single, striking mountain range: the Sierra de la Ventana. These are not volcanic peaks or recently uplifted blocks, but the deeply eroded remnants of a much larger, ancient mountain system. The range, also known as the Sierras Australes, extends for about 180 kilometers and contains the highest point in Buenos Aires Province, Cerro Tres Picos, at 1,239 meters. The core of these mountains consists of exceptionally hard quartzites that began as sand deposits on a continental shelf around 500 million years ago, during the Cambrian and Ordovician periods.
The original sediments were part of a thick Paleozoic sequence deposited along the passive margin of the supercontinent Gondwana. These layers of sand, accumulating over millions of years in a marine environment, were eventually buried and subjected to intense pressure and heat, which metamorphosed the sandstone into quartzite. This process recrystallized the quartz grains, creating an interlocking structure that makes the rock extremely resistant to erosion. The main rock units that display these features are the Curamalal and Ventana Groups.
A Permian Collision
The dramatic folds visible today were not part of the original rock formation. They are the result of a massive geological event that occurred around 280 million years ago during the Permian period. This event was the Gondwanide orogeny, a mountain-building episode that affected large parts of Gondwana. Specifically, it involved the collision of the Patagonia terrane with the southwestern margin of the Gondwanan continent.
The immense compressional forces generated by this collision buckled and folded the thick, rigid layers of quartzite. The deformation is visible as large-scale asymmetric folds and thrust faults throughout the range. This tectonic activity created a substantial mountain range, geologically linked to the Cape Fold Belt in South Africa, which at the time was adjacent to South America within Gondwana. Over the subsequent 280 million years, erosion has stripped away the softer overlying rocks, exposing the contorted quartzite skeleton of this ancient mountain belt and recording the powerful forces that shaped the continents.