This hill, Lonehill, represents one of the most prominent of a number of Archaean granite tors ('Castle Koppies') developed in the central and south-western parts of the 3200 million-year-old Johannesburg granite dome. The tors vary in composition and texture, but generally consist of medium to coarse-grained, homogenous granodiorites and ademellites, as well as banded gneisses and migmatites. In some tors the granite rocks contain large feldspar crystals in a finer-grained matrix and are referred to as porphyritic granites. Tors similar to Lonehill are extensively developed in the Archaean granites of Zimbabwe, the Eastern Transvaal and Swaziland, and formed mainly by differential weathering of the faulted, fractured and jointed granites.
Geological Society of South Africa