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.