The peaks of the Black Cuillin are mainly composed of gabbro, a very rough igneous rock which provides a superb grip for mountaineers; and basalt, which can be very slippery when wet. The rocks forming the ridge of the Black Cuillin (and outliers such Bla Bheinn) are dark in colour, particularly in the shade, but when in sunlight the Black Cuillin can appear grey to brown in colour. The main ridge forms a narrow crest, with steep cliffs and scree slopes. The ridge is about 14 km long (measured from Gars-bheinn in the south to Sgùrr nan Gillean in the northeast), and curves in an irregular semi-circle around Loch Coruisk, which lies at the heart of the range. The highest point of the Cuillin, and of the Isle of Skye, is Sgùrr Alasdair in the Black Cuillin at 992 m (3,255 ft).