The Lammermuir Hills are formed from a succession of Silurian and Ordovician age marine sediments known as greywackes together with siltstones, shales and mudstones. They are assigned to the Gala Group. Unconformably overlying these highly faulted and folded strata are outcrops of the early Devonian age Great Conglomerate Formation which forms a part of the Reston Group of Old Red Sandstone rocks. These coarse red-purple conglomerates underlie a band of country in the east between Longformacus and Oldhamstocks and also occur in an isolated outcrop east of Soutra Hill and beneath the Dun Law Wind Farm on the western margin of the hills. The same strata extend down Lauderdale on the western margin of the hills.