Geology
Contrary to popular belief, the You Yangs are not the remains of a volcano. The granite that forms them was originally a mass of magma that had worked its way up into the surrounding sedimentary rocks during the Devonian period, when the land surface in Victoria was several kilometres higher than today.
The magma crystallised before it reached the surface, so it did not produce any volcanic activity. Instead, a very slow cooling rate allowed many large white crystals of feldspar to form. These can be seen in many of the granite outcrops throughout the ranges. In places the crystals appear to be lined up, probably because the gooey magma was still moving around when they were growing. 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. In many places in the granite there are dark grey clots and lumps. These are called xenoliths and are foreign pieces of sedimentary rock that have been incorporated into the magma as it moved to the location at which it solidified.
The land surface has eroded over the millions of years since the granite solidified, leaving it exposed. Because granite is a hard rock, it has resisted erosion better than the 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 has formed a blanket of sandy soil that covers any contacts with surrounding rocks.
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