Orea Stone Run: a boulder river you can hear

Neither river nor glacier

Although called a “stone run” or “rock river”, these boulders were not carried by running water or glacial ice. They broke away from nearby quartzite ridges: sharp temperature swings and ice expanding in cracks split the rock, and the fragments fell down the slope of Cerro de San Cristóbal.

How the stones creep

For a stone run to develop, the blocks had to keep moving. The waterlogged sandy soil froze during cold periods, expanding and lifting the stones slightly; when it thawed, they settled a fraction further downhill. Repeated over thousands of years under much colder Ice Age climates, the boulder field crept into place.

Listen closely

Beneath the boulders flows the Arroyo del Enebral. If you stand quietly, you can hear it running beneath your feet.