Glacial Clues and Patterns in the Stones
Evidence of glaciation is immediately striking on the Isle of Rum. The landscape provides a superb record of the mighty glacial forces that have carved the rugged rocks into smooth U-shaped glens and deep corries. Rocks embedded in the base of the ice sheet scoured and abraded the valley sides, smoothing any sharp obstacles in their path. Roche moutonnées - ice smoothed bedrock hummocks - are found to the south of Kinloch Glen and indicate that ice once flowed from east to west. Upon closer examination, these streamlined rocks are covered in small scratches or striae - clues that the ice was once at work here.
As the ice sheet melted it deposited huge boulders, some composed of rock types not found anywhere on Rum. These erratics can only have come from the Scottish mainland - yet more clues that a vast ice sheet once covered the island. Later, smaller glaciers deposited coarse gravelly debris mixed with silt and sand. This material forms moraines where the ice margin readvanced over the debris, bulldozing it up to form a ridge in front of the glacier. Good examples are found in Glen Dibidil in the southwest of the island.
Although glaciers once covered almost the entire island, the tops of the highest mountains probably remained ice-free throughout the last glacial period. On these peaks, intense frost shattering in the extreme cold created blockfields of loose, sharp, broken rock. Some of these blockfields tumbled downslope to form screes. Over time the rock fragments have been broken down further into finer, soil-like, material. Freeze-thaw processes churn the soil and sort particles into curious regular patterns. Fine examples of these stone stripes and polygons occur near the summit of Orval. These features are still actively forming - proof that the conditions on Rum are still cold enough for ice to grow in the ground even today!