Pera Grossa, Glacial erratic in Anfiteatro morenico di Rivoli-Avigliana, Italy

Pera Grossa is a glacial erratic in the morainic landscape between Rosta and Reano that rises about 10 meters above ground. The boulder displays the typical features of rock transported by ancient glaciers over considerable distances.

Pera Grossa is a block of serpentinite carried to the ridge between Rosta and Reano by the glacier that ran down the Susa Valley and spread over the Turin plain, forming the Rivoli-Avigliana morainic amphitheatre. In the late nineteenth century part of it was quarried for rubble used in the Fréjus tunnel monument in Turin, and on 15 June 1927 a ministerial decree declared it of notable public interest.

The name Pera Grossa comes from Piedmontese, in which pera means stone. The boulder is also known as Masso di Rosta. It lies on the ridge between Rosta and Reano, near the border of the two municipalities.

The erratic is reachable through several marked hiking trails and cycling routes within the morainic landscape, which suit different fitness levels. Visitors should wear sturdy footwear and bring water, especially on warmer days.

The reddish colour of the undisturbed surface comes from iron oxidation, similar to desert varnish, while the greenish-blue rock exposed by the late-19th-century blasting reveals the serpentinite interior. The mine holes and radiating fractures from that demolition are still visible on the boulder.