Heaps of broken rock can hide permafrost in surprisingly mild climates
The loose rubble piled beneath cliffs looks like nothing more than debris. Yet cold air sinking through the gaps can keep the inside of a scree slope several degrees colder than its surroundings, sometimes cold enough for patches of permafrost where the average air temperature is around 7 °C. Arctic plants and insects shelter there.
Scree is the jumble of rock fragments that collects at the foot of a cliff after repeated rockfalls; the word comes from the Old Norse skriða, a landslide, while its near synonym talus is French for a slope. Rockfall is what sets it apart from colluvium, which is carried down gentler hillsides by rainwash or slow creep. Climbers draw a rougher line: scree is fist-sized stuff you can boot-ski down, talus the bigger blocks you hop between. Piles of large boulders may even leave human-sized gaps called talus caves.
Frost is usually blamed for making it. Water seeping into cracks expands by about 9 percent when it freezes, which can split rock or lever blocks loose, especially in spring and autumn when temperatures bounce around freezing. How effective this really is remains disputed, since water may simply squeeze out of wide fractures instead. Living things join in: lichens push their threads into tiny cracks and widen them, and the acids they release dissolve minerals. Within a slope the fragments sort themselves, larger ones tumbling to the bottom.
Contrary to a common assumption, most scree slopes sit below the angle of repose, the steepest a heap can stand; only slopes gaining or losing material quickly approach it. Many are concave, gentler at the foot than at the top.
Scree also shapes ice. A thin dusting on a glacier, under about 2 cm, darkens the surface and speeds melting, but a thicker layer insulates the ice beneath. The chilled interiors of some slopes behave like ice caves, with temperatures 6.8 to 9.0 °C below the outside air. A Czech team studying 66 slopes in 2012 found such spots sheltering northern mosses, ferns and arthropods far from their usual ranges. Scree running, riding the moving stones downhill, has stripped some slopes bare.
Source: Scree
