Quaternary materials make up much of the Western Plains and the landscape itself has largely developed during the Quaternary. During the late Tertiary, about 5 to 6 Ma ago, a shallow sea reached up to the southern margin of the Western Uplands, the east-west spine of Western Victoria which extends from around Kilmore, north of Melbourne, to the Grampians Range and beyond. The retreat of this sea exposed for the first time a marine floor of clays and marls, with many parallel ridges marking successive shorelines. Most of these shorelines ridges have only just been recognised on new radiometric and magnetic imagery. As the sea retreated rivers in the Uplands gradually extended southwards, reworking and burying the marine surface with thin alluvial deposits.
About the same time both the Uplands and the Plains gave birth to a new volcanic province, and over the last 5 Ma nearly 400 scoria cones and lava shields have been built up, with fluid basalt flows spreading laterally around vents, and often southwards for many kilometres down stream valleys. Where the lava blocked drainage lakes and swamps formed, and on the plateau-like flow surfaces irregular ridges and collapse depressions also produced lakes and swamps. Some lava flows have been dated by K/Ar and radiocarbon, and changes in landforms, drainage, and soil and regolith development, can be used to build up a detailed chronosequence through the Quaternary.
Quaternary volcanism has left well-preserved cones, craters and crater lakes, scoria with iridescence, and stony rise lava flows with ropy and glassy surface textures - in fact such an obvious youthful appearance overall that the explorer Major Mitchell, the first person to recognise the area as volcanic in 1836, suggested that eruption had been "within the memory of man". The youngest dated eruption is that of Mt Gambier in nearby southeastern South Australia, at 4000-4300 B.P. by radiocarbon, and perhaps a dozen volcanoes may eventually be found to have erupted within the last 20,000 to 30,000 years. Writers on volcanism on the Western Plains after Mitchell, 1838 have included Brough-Smyth 1858, Bonwick 1858 & 1866, Gregory 1903, Grayson & Mahony 1910, Skeats & James 1937, Gill 1953, and Ollier & Joyce 1964.
Along a line from Port Fairy to Colac, near the southern limit of volcanic activity, groundwater has interacted with rising magma to cause phreatomagmatic explosions, reaming out some 40 or so deep maar craters and building up rims of ash or tuff. Rain and groundwater has filled many of these craters, and in the lakes sediment, pollen and microfauna have accumulated, recording changing climate during the latter part of the Quaternary. Many hundreds of small lakes and swamps can also be found on the flat to undulating clay plains with duplex soils and gilgai developed on lava flows which erupted between 3 and 1 Ma ago. Also on the clay plains larger lakes have built up lunette complexes by deflation; in what is possibly a tectonic depression Lake Corangamite and nearby lakes make up a lake-lunette complex which is also a RAMSAR heritage site. Other evidence of neotectonics is found in faults and monoclines affecting flows and underlying Tertiary sediments, and uplifted blocks are associated with volcanic activity as at the Staughtons Hill volcanic complex. In a number of maar crater and lunette-bounded swamps megafaunal remains were first discovered last century.
Where the Plains have no cover of lava, tuff or lake and river deposits, the Tertiary sediments are near the surface, with distinctive sandy loam topsoils and mottled clay subsoils with much "buckshot" or iron pisolites, and sometimes still retaining the form of old shoreline ridges. Nearer the coast and south of the volcanic areas, karst is often well developed on the limestone coastal plains, with impressive sinkholes and caves, and spectacular coastal cliffs and rock stacks in the Port Campbell area. Westwards around Warrnambool and Port Fairy dunes and lagoons mark modern and interglacial shorelines. Older now partly cemented dunes may show several palaeosols, and syngenetic karst and cave development are of major significance.
Humans have lived in this landscape for probably 50,000 years, experiencing climate and sea level change, and changes in vegetation and in lake level; somewhat more than 20,000 years ago Tower Hill volcano was active, and the sea lay far to the south across the dry Bassian Plain. As the sea level rose, aboriginal occupants of that greater plain would have been forced back until only the modern Western Plains remained. Here they achieved a close form of settlement, building stone houses and setting-up fish and eel traps to provide regular food supplies. The characteristic grasslands of much of the modern Plains may be related in part to aboriginal occupation and fire-farming, so that in a sense a last-glacial treeless landscape has survived. 150 years of European settlement have maintained this grassland through cultivation and grazing, while also marking it with bluestone mansions, farm and town buildings, and stone fences, and many shelter belts of cypress pine and sugargum. The smaller settlers built cottages and grew potatoes on the Tower Hill ash deposits, or ran dairy cows in the stony rises.
The future post-Quaternary landscape may be little different to that of the immediate past. Although crater lake levels are falling dramatically, and salinity can be a major problem in some areas, climate change may not be the predominant factor; perhaps the Plains will be most affected by human activity such as bluegum agroforestry and other changes in land use such as grazing and cropping. Nature however may still have a dramatic role to play if, as many geologists now believe possible, volcanic activity were to return again to the Western Plains.