As plate tectonics moved land masses around, raised mountains, and filled basins, the continents we know today have merged and split a number of times. When an oceanic crustal plate is forced (subducted) under a continental plate, friction melts rocks to form magma which then erupts as volcanic lava. The oceanic crust is basaltic and high in calcium, iron, and magnesium while the rocks of the continental crust are rich in silica and poor in those minerals (eg rhyolite, granite). This influences the soils they form.
During the Late Devonian-Early Carboniferous (370 – 335 million years ago) Australia was part of the super-continent Gondwana. Life was abundant in the oceans and primitive plants had appeared on land. Land animals were restricted to simple amphibians. At this time, the east coast of the continent was further inland than the present coastline. Erosion dropped large volumes of sediments into a deep water trench which was then lying east of the continental shelf.
Beginning about 335mya Gondwana merged with Euroamerica to form Pangaea. From about 300 million years ago (mya), the crumpled and recrystalised rocks (metamorphic) from the oceanic trench formed the underlying basement rocks (part of the Brisbane Metamorphics) of this area. You can see these basement rocks outcropping at Cape Byron and Broken Head.
Some 250mya, the Clarence-Moreton Basin was formed. Into this spilled lavas and volcanic ash from the Chillingham Volcanics (229mya) which were subsequently overlain by eroded sediments.
Pangea split again round 175mya to form what became known as Laurasia and Gondwana which themselves began to break up. Gondwana comprised all the southern continents of the modern world, as well as peninsular India, Turkey, Iran, Afghanistan, Tibet, and parts of South-east Asia. Flowering plants appeared before the breakup of Gondwana and evolved rapidly.
During the Jurassic (c.166-145my), into the Clarence-Moreton basis spilled the sediments which became the Walloon Coal Measures which are overlain by the Kangaroo Creek Sandstone.
The Walloon Coal Measures are fine claystones, sandstones, and coal seams and they weather rapidly. The abundant coal in some areas indicates the large quantity of organic matter and plants growing at the time. The Walloon Coal Measures can’t be seen in our valley but have been mined for coal at Nimbin. In Queensland, dinosaur footprints have been seen in rocks of the Walloon Coal Measures and some fossil fish have been observed in the creeks near Nimbin. It is in this layer that CSG is potentially found.
Kangaroo Creek Sandstone was deposited in a river setting resulting in larger grain size than Walloon Coal Measures. It consists mainly of white to cream coloured quartz sand which glistens more than usual because while buried, fluids in the rock caused extra silica (quartz) to crystallise on the existing sand grains creating new tiny crystal faces that reflect light in a vivid way. Because they are relatively well-cemented, they tend to weather less readily than Walloon Coal Measures and can be seen in the sometimes steeply sloping hills and ridges, particularly on the western side of the valley. The recrystalisation of quartz in the Kangaroo Creek Sandstone means that this unit is now essentially dry –there are very few spaces left for the water to travel through.
The soils formed from these sedimentary deposits in high rainfall areas and under forest cover are called Podsols. Topsoils are shallow, highly erodible grey to brown loam overlying clay subsoils. They are infertile with low mineral content, low pH, poor structure and low organic matter.
By this time, ferns, treeferns, conifers (pines) and cycads and, finally, flowering plants had appeared while reptiles, especially dinosaurs roamed. Mammal-like reptiles had become common.
Australia split from Antarctica about 50mya and began its northward journey, developing its unique flora and fauna.
The next major event to form the beautiful landscapes of our valley was the Tweed or Mount Warning Volcano.
The Tweed Volcano, of which Mount Warning is the centre, erupted some 24mya. The lavas flowed over a country already landscaped by erosion from the old metamorphic rocks, from the sedimentary layers, and from the remnants of the earlier Chillingham (c.220my old) and Focal Peak (43my) volcanos. While some of the lavas may have erupted from Mount Warning, others arose through vents on the flanks. Over several million years, many flows of lava occurred. Flows were of two types – dark basalt of oceanic origin and pale rhyolite of continental origin.
Molten basalts are fluid, and these flows covered a large area (into southern Queensland and south past Lismore). The old shield has since largely eroded away. You can see in many parts of our sandstone hills where basalt has tumbled down earlier sandstone valleys. Often small ephemeral creeks course down here in heavy rains.
Krasnozem (Red Ferrosol) soils are formed from basalt parent material under conditions of high rainfall. They are red-brown to orange in colour depending on organic matter content, generally a well-structured clay but with localised stoniness. The good structure facilitates high water infiltration, good drainage and aeration. They are commonly deep topsoils but once cleared have poor chemical properties, e.g. low pH, trace elements and other minerals and nutrients. Under the original rainforest vegetation, they are high in organic matter and of good nutrient status.
Mountain Top plateau illustrates these Krasnozem soils; here they are largely cleared and used for commercial agriculture. Patches of recovering rainforest indicate other areas where these soils can be seen.
Chocolate soils are formed from the basalt parent material usually under conditions of lower rainfall and for a shorter time period – they usually occur on steep slopes where the basalt plateau is being worn away or on basalt caps covering sandstone. Topsoils are shallow, brown to black in colour with a loamy clay or clay overlying a paler heavy clay. The subsoil is poorly drained. These soils have good mineral content, but with a high magnesium to calcium ratio making soil structure less stable.
Nimbin Rhyolite, because of its resistance to weathering, is mainly responsible for the rugged steep cliffs and valleys to the north. Rhyolite rocks, being relatively stable and comprising mainly silica, alumina and potassium, weather to shallow, pale-coloured soils of poor fertility.
Alluvial soils are usually well-drained loams, fertile because they are composed of eroded topsoil from the catchment. Fertility depends on the degree of volcanic input particularly from basalt.
In his Rocks and Landscapes of the Gold Coast Hinterland, Warwick Willmott describes geological processes and these are applicable to our valley.