An ice age in the Apennines
High in the Monti Sibillini, a range in the central Apennines, the landscape contains clear evidence of a much colder past. Armchair-shaped hollows, or glacial cirques, are carved into the limestone flanks of the highest peaks. These features were sculpted by mountain glaciers during the Pleistocene ice ages. The most intense period of this glaciation, known as the Last Glacial Maximum (LGM), peaked here about 20,000 years ago. At that time, glaciers filled these valleys with ice up to 200 meters thick.
The evidence for this ancient ice is recorded in the rocks. As glaciers grow and flow, they pluck rocks from the mountainside and deposit them in ridges called moraines. Scientists can determine how long these moraine boulders have been exposed to the open air by using a technique called cosmogenic nuclide dating. High-energy cosmic rays from space strike atoms in the rock, creating rare isotopes like Beryllium-10. By measuring the concentration of these isotopes, geologists can calculate the moment the ice receded and exposed the boulder. This method dates the end of the last glaciation in this part of Italy.
The geology of the range is primarily limestone, formed in the warm, shallow Tethys Ocean during the Jurassic period. Tectonic forces later uplifted these carbonate platforms to form the Apennine mountains. During the Quaternary period, repeated glacial cycles carved the distinctive cirques and U-shaped valleys seen today.
The valley of Pilate's Lake
One of the most distinct glacial landforms in the Monti Sibillini is the valley holding Lago di Pilato, situated at an altitude of 1,941 meters below the summit of Monte Vettore. The lake itself occupies a cirque dammed by a complex moraine system, a direct deposit from the Quaternary glacier that once filled this basin. The lake's shape and size fluctuate seasonally with rainfall and snowmelt. In late spring, high water levels submerge parts of the moraine, giving the lake a distinctive shape that resembles a pair of spectacles. As water levels drop in summer, the moraine emerges, splitting the lake into two separate pools.
This glacially-formed, isolated environment is the exclusive habitat for a unique species: the fairy shrimp Chirocephalus marchesonii. This small, red-colored crustacean, which swims on its back, is endemic to this lake and found nowhere else on Earth. It survives the harsh, freezing winters and summer dry periods in a dormant state. The shrimp lay eggs that can withstand being frozen or dried out, hatching only when conditions are favorable again in the spring. This remarkable adaptation is a consequence of the ephemeral habitat created by the ancient glacier.