The Werribee Plain is an expansive low-lying flat land located between the northwestern shore of

Port Phillip Bay Port Phillip ( Kulin: ''Narm-Narm'') or Port Phillip Bay is a horsehead-shaped enclosed bay on the central coast of southern Victoria, Australia. The bay opens into the Bass Strait via a short, narrow channel known as The Rip, and is comple ...

and the

Grampians The Grampian Mountains () is one of the three major mountain ranges in Scotland, that together occupy about half of Scotland. The other two ranges are the Northwest Highlands and the Southern Uplands. The Grampian range extends northeast to so ...

Central Highlands of

Victoria Victoria most commonly refers to: * Queen Victoria (1819–1901), Queen of the United Kingdom and Empress of India * Victoria (state), a state of Australia * Victoria, British Columbia, Canada, a provincial capital * Victoria, Seychelles, the capi ...

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, shared between

Melbourne Melbourne ( , ; Boonwurrung language, Boonwurrung/ or ) is the List of Australian capital cities, capital and List of cities in Australia by population, most populous city of the States and territories of Australia, Australian state of Victori ...

and

Geelong Geelong ( ) (Wathawurrung language, Wathawurrung: ''Djilang''/''Djalang'') is a port city in Victoria, Australia, located at the eastern end of Corio Bay (the smaller western portion of Port Phillip Bay) and the left bank of Barwon River (Victo ...

. Named after the

Werribee River The Werribee River is a perennial river of the Port Phillip catchment that is located on the expansive lowland plain southwest of Melbourne, Victoria, Australia. The headwaters of a tributary, the Lerderderg River, are north of Ballan near ...

, the plains are notable for their extensive, flat and rather featureless spans, interspersed by the occasional vertical feature, such as the

You Yangs The You Yangs are a series of granite ridges that rise up to above the flat and low-lying Werribee Plain in southern Victoria, Australia, approximately due west of the rural town of Little River, southwest of Melbourne CBD and north o ...

, a series of

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

ridge A ridge is a long, narrow, elevated geomorphologic landform, structural feature, or a combination of both separated from the surrounding terrain by steep sides. The sides of a ridge slope away from a narrow top, the crest or ridgecrest, wi ...

s and several small

extinct volcano A volcano is commonly defined as a vent or fissure in the Crust (geology), crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface. On Earth ...

es. The Werribee Plain forms the eastern bayside portion of the vast Western Volcanic Plains between the

and the

Otway Ranges The Great Otway National Park is a national park located in the Barwon South West region of Victoria, Australia. The national park is situated approximately southwest of Melbourne, in the Otway Ranges, a low coastal mountain range. It conta ...

, which extend from the western suburbs of Melbourne to the

South Australia South Australia (commonly abbreviated as SA) is a States and territories of Australia, state in the southern central part of Australia. With a total land area of , it is the fourth-largest of Australia's states and territories by area, which in ...

n border.

Geology

Volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

es 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 new eruptions are possible at any time. The granite that forms the

was originally a mass of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

that worked its way up into the surrounding

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s during a period of geological time known as the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

. The

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. The nearest volcanoes to the You Yangs are the Anakies, which appear as three low hills on the western horizon. These all have summit craters and provided lava flows for the plains south of the You Yangs. There were also flows from the low volcanoes of Bald Hill and Spring Hill to the north of the You Yangs. 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.

References

{{coord missing, Victoria (state) Landforms of Victoria (state) Plains of Australia Werribee, Victoria