Igneous activity between 350 and 400 million years ago, coupled with faulting, led to the creation of tough basalt volcanic plugs, which predominate over much of the area.: One such example is the Castle Rock which forced the advancing ice sheet to divide, sheltering the softer rock and forming a tail of material to the east, thus creating a distinctive crag and tail formation.: Glacial erosion on the north side of the crag gouged a deep valley later filled by the now drained Nor Loch. These features, along with another hollow on the rock's south side, formed an ideal natural strongpoint upon which Edinburgh Castle was built.: Similarly, Arthur's Seat is the remains of a volcano dating from the Carboniferous period, which was eroded by a glacier moving west to east during the ice age.: Erosive action such as plucking and abrasion exposed the rocky crags to the west before leaving a tail of deposited glacial material swept to the east. This process formed the distinctive Salisbury Crags, a series of teschenite cliffs between Arthur's Seat and the location of the early burgh. The residential areas of Marchmont and Bruntsfield are built along a series of drumlin ridges south of the city centre, which were deposited as the glacier receded.: