Hallival and Askival are formed from an extraordinary series of layered igneous rocks created as olivine and feldspar crystals accumulated at the base of a magma chamber. The chamber eventually collapsed, forming a caldera. There are swarms of near-vertical dykes of basalt on the northwest coast between Kilmory and Guirdil, created by basaltic magma forcing its way into fissures in the pre-existing rock. The western hills, although less elevated than the Cuillin, exhibit a superb collection of periglacial landforms including boulder sheets and lobes, turf-banked terraces, ploughing boulders and patterned ground. On Orval and Ard Nev the weathered basalt and granophyre has been sorted by frost heaving into circles 50 centimetres in diameter and weathering on Barkeval has produced unusual rock sculptures. On Sròn an t-Saighdeir there are large sorted granite boulder circles 2-3 metres across on the flat summit and sorted stripes on the slopes. Lava flowing away from the volcanic centre formed Bloodstone Hill, gas bubbles leaving holes in the structure that were then filled with green agate flecked with red. There are some outcrops of the pre-volcanic Lewisian gneiss near Dibidil in the southeast corner of the island, and more extensive deposits of sandstone in the north and east.