Contrary to popular belief, the You Yangs are not the remains of a volcano. The granite that forms them was originally a mass of molten magma that had worked its way up into the surrounding sedimentary rocks during a period of geological time known as the Devonian. At this time, Australia was found in the Antarctic Circle linked with Africa and Antarctica, forming Gondwanaland. The land surface in Victoria would have been several kilometres higher than today. The molten magma crystallised before it reached the surface, so it did not produce any volcanic activity. Instead, a very slow cooling process allowed many large white crystals of feldspar to form. The rock enclosing the big feldspar crystals mainly contains crystals of greyish quartz and two black minerals (hornblende and a variety of mica known as biotite). There are also some tiny crystals of two minerals, allanite and titanite, that contain radioactive elements such as uranium and thorium. Titanite crystals have been used to calculate that the You Yangs granite solidified 365 million years ago. Dark grey clots and lumps called 'xenoliths' occur in many places these are pieces of sedimentary rock that have been picked up and baked by the magma.
Over millions of years since the granite solidified, the land surface has been lowered by erosion, which has gradually exposed the granite. Because granite is a hard rock, it has resisted erosion better than the softer rocks that surrounded it. The size and shape of the rounded tors are controlled by fractures in the granite that resulted from slight shrinkage during cooling. Weathering and erosion of the granite over millions of years has formed a sandy granitic soil outwash that covers the surrounding rocks, and is one of the main soil types found at Mt Rothwell. This soil type is very hydrophobic or water repellent once it dries out, but becomes completely water logged and turns to slurry when there’s been heavy rain.