A landscape of collapse
The Sarca Valley in Trentino presents a dramatic landscape of vertical limestone and dolomite walls. But the seemingly chaotic boulder fields covering the valley floor are not random; they are the remnants of colossal prehistoric landslides, known as rock avalanches. These events fundamentally reshaped the valley, leaving behind some impressive landslide deposits in the Alps. The largest of these deposits are clustered in an area known as the Marocche di Dro.
Geological studies have distinguished several distinct collapse events. The most massive was the Marocca Principale rock avalanche, which occurred approximately 5,300 years ago. This single event involved an estimated 1 billion cubic meters (1000 Mm³) of limestone detaching from the vertical rock walls between Monte Casale and Monte Granzoline. A later, significant event known as the Kas rock avalanche occurred around 1,080 years ago. It added another 300 million cubic meters of debris, burying the southern portion of the earlier deposit. The source of the Kas failure was below the peak of Monte Brento.
Causes and consequences
The primary preparatory factor for these massive failures was the last glacial period. Pleistocene glaciers carved out the Sarca Valley, over-steepening its walls and removing the supporting rock and ice from their base. This process, called glacial debuttressing, leaves the valley sides unstable and prone to collapse long after the glaciers retreat. While the glacial retreat created the conditions, specific triggers for the collapses are debated. The Marocca Principale and the nearby Molveno landslide (600 million m³, dated to 4,800 years ago) may have been triggered by a shift to a cooler, wetter climate at the beginning of the late Holocene, which would have increased water pressure within rock fractures. For the more recent Kas event, scientists suggest a seismic trigger, possibly the "Middle Adige Valley" earthquake of 1046 CE.
Scientists use a technique called cosmogenic nuclide dating to determine the age of these landslides. By measuring the concentration of isotopes like Chlorine-36 (³⁶Cl) in the surface of limestone boulders, they can calculate how long the rock has been exposed to cosmic rays since the landslide occurred. This method has been important in distinguishing and dating the various events that were once thought to have happened immediately after the ice age. The consequences of these collapses were permanent. The debris from the rock avalanches dammed the Sarca River, leading to the formation of Lake Cavedine and other smaller bodies of water. The landscape today results of these ancient, catastrophic events.