The rock on Mount Stromlo is ignimbrite from a volcanic eruption during the upper Silurian period. This is covering rhyodacite from a much earlier period, which can be seen on the surface on the lower slopes in the east and southeast sides. The northern lower slopes are covered with a calcareous shale also from the Silurian period. This is cut off on the northwest side by the Winslade Fault, which heads northeast to Cook and Kaleen. A spur fault heads off east from the Winslade Fault to under the Scrivener Dam. On the other side (northwest and north) of these faults are middle Silurian period rhyodacite volcanic deposits. The northern side has been uplifted compared with the southern side.