About 15 miles north-east of Adelaide, the Houghton Inlier composed of older Precambrian schist, gneiss and pegmatite is overlain unconformably by younger Precambrian sedimentary rocks of the Adelaide System. The inlier forms the core of an anticline which is overturned to the west; north- trending faults are common, and many form the junction between Adelaide System and older Precambrian rocks. The lowest Adelaidean formation, of Torrensian age, is the Aldgate Sandstone, a conglomeratic sandstone containing sporadic carbonate lenses. It is overlain by the Castambul Dolomite phyllites and quartzites, then the Montacute Dolomite. These three units are considered to be equivalent to the Skillogalee Dolomite of the Riverton - Burra region to the north, and are best seen in the Torrens Gorge. The dolomites are of a lenticular nature, and appear to be absent near Williamstown where some small lenses of dolomite are thought to be equivalent to the Beaumont Dolomite, higher in the Torrensian sequence. The sediments have been subject to slight metamorphism, producing phyllites of chlorite grade.