 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 <unk> , 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 <unk> 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 @-@ <unk> 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 <unk> in the southeast corner of the island , and more extensive deposits of sandstone in the north and east . 
