Stone run

A stone run (called also stone river, stone stream or stone sea) is a rock landform resulting from the erosion of particular rock varieties caused by freezing-thawing cycles in periglacial conditions during the last Ice Age.

The actual formation of stone runs involved five processes: weathering, solifluction, frost heaving, frost sorting, and washing. The stone runs are essentially different from moraines, rock glaciers, and rock flows or other rock phenomena involving the actual flow of rock blocks under stress that is sufficient to break down the cement or to cause crushing of the angularities and points of the boulders. By contrast, the stone run boulders are fixed quite stably, providing for safer climbing and crossing of the run.

Stone runs are accumulations of boulders with no finer material between them. In the Falklands, they occur on slopes of between 1 and 10 degrees, and are the product of mass-movement and stone sorting during past periods of cold climate. They everywhere occur in association with poorly sorted, clay-rich solifluction deposits.

The Falkland Islands and Vitosha Mountain (Bulgaria), both have an abundance of large stone runs. The highly specific combination of particular climatic conditions and rock varieties that existed there during the Quaternary explains both the formation of stone runs in those two territories and their absence in areas with otherwise comparable nature conditions.

For instance, while the present Falklands climate is quite similar to that of Scotland, the latter was completely glacial rather than periglacial during the relevant period, which would not allow for the formation of stone runs. On the other hand, due to geological and other specifics of the southern temperate and sub-Antarctic territories with climatic history similar to that of the Falklands (Prince Edward Islands, Crozet Islands, Kerguelen Islands, Macquarie Island, Campbell Islands, or nearby Tierra del Fuego and Patagonia), none of them features landforms comparable to the Falklands stone runs.

Likewise, the specific geology of Vitosha accounts for the fairly restricted examples of similar landforms in other Bulgarian or indeed Balkan mountains with comparable climatic record among which Vitosha is one of the smallest, extending just 15 km (9.3 mi) by 10 km (6.2 mi). However, even on that small territory the stone runs exist along with screes and other rock landforms, suggesting that the right periglacial conditions and rock composition are necessary but not sufficient conditions for the formation of stone runs.

Other examples of stone runs occur in England, including at the Stiperstones, Shropshire. They are also known in Pennsylvania. Small examples are probably very widespread where solifluction deposits contain large concentrations of frost-resistant rock blocks.

The Falklands stone runs are made up of hard quartzite blocks. They are more widespread and larger on East Falkland, especially in the Wickham Heights area where the largest of them extend over 5 km in length. Those on West Falkland and the minor islands are fewer in number and of smaller dimensions. Darwin's "great valley of fragments", subsequently renamed Princes Street Stone Run after Edinburgh's Princes Street that was cobbled at the time, occupies a 4 km long and 400 m wide shallow valley trending east-west. The feature is situated off the road to Port Louis, some 20 km northwest of Stanley.