History and Geology
The You Yangs
The You Yangs ranges are a series of low granite ridges, with Flinders Peak at 347m above sea level, which dominant the Werribee Plain. Mt. Rothwell is on the northernmost tip of the main ridge, which runs roughly N-S for about 9 km with a lower extension running 15 km to the west.
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, when Australia was then 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 rate 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. In many places in the granite there are dark grey clots and lumps. These are called ‘xenoliths’ and 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 it 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.
The young volcanoes
The countryside surrounding the granite ridges is a lava plain. Known as the Werribee Plain, it forms part of the vast Western District Volcanic Plains that extend from Melbourne to the South Australian border. Volcanoes began erupting lava flows about 4.5 million years ago, and the youngest eruptions are only about 10,000 years old. There are over 400 mapped craters and vents on the plains. While all these individual volcanoes are extinct, the volcanic field itself is only dormant, so that a new eruption is possible at any time.
The nearest volcanoes are the Anakies, the three low hills on the western horizon. These all have summit craters and provided lava flows to the plains south of the You Yangs, but are now scoria quarries. There were also flows from the low volcanoes of Bald Hill and Spring Hill to the north. When these volcanoes were active, probably between 2 and 3 million years ago, the You Yangs would have been granite islands in a sea of lava. Flows of lava determined where water drainage patterns would form, creating the present course of the Little River and Hovell’s Creek.
Other geological features The Brisbane Ranges, the wooded scarp along the western skyline, contains the Rowsley Fault that has been active intermittently for millions of years. This fault is the western edge of a geological structure known as the Port Phillip Basin. Over millions of years, this basin has experienced periodic flooding by the sea, which is now represented by Port Phillip Bay. At a time of high sea level, perhaps a million years or so prior to the volcanic eruptions, the You Yangs would have been granite islands in a sparkling sea. Sands and gravels containing marine fossils occur along the southern edge of the granite ridges and mark the beaches and shorelines that formed at that time.
Using the rocks
The settlers made good use of local stone for building and fences. The Mt. Rothwell homestead is constructed mainly of blocks of lava, known as basalt or bluestone, which was supposedly quarried on the property. Some granite blocks were used in the extension at the rear of the homestead.
The You Yangs region has been described as having a moderate to dry climate, but has also experienced extremes in weather with floods, drought, fire, snow and windstorm events all occurring in the last 150 years. As one of the oldest and most consistent rainfall records in Victoria, the rainfall records for Mt. Rothwell date back to 1882, with an average annual rainfall of 470mm, and ranging from a record low of 227mm in 1967 to a record high of 711mm in 1978. Mt. Rothwell’s annual rainfall average is lower than the adjacent areas due rain shadow effects caused by the You Yangs and Brisbane Ranges, as well as the larger effect caused by the Otway Ranges over the Geelong region.