Soil formation and the soils of Sydney
This is our second article in an occasional series on the origin of soils. In our first article, we saw how a soil develops into a series of layers, called horizons. Here we look at the crucial effect of geology on soil formation, with the resultant effects on modern land use in Sydney. Future articles will continue to explore the role of geology and other factors in soil formation.
Why one soil and not another?
The unique combination of geology, climate, organisms, topography and time at a site determines the soil that forms. The Swiss-American soil scientist Hans Jenny (1941) expressed this as:
Soil = function of (p, c, o, r, t)
where p = parent material (geology)
c = climate (temperature, rainfall, evaporation)
o = organisms
r = relief, or topographic position: crest (dry), midslope (damp), foot slope (wet)
t = time (the older the soil, the more developed or differentiated into horizons it is).
This formula, while only conceptual, is extremely useful in identifying the various forces that act on particular place to form a particular soil. The key factor is geology.
Let’s look at the five main geologies that give rise to the soils of Sydney, in order from oldest to youngest, and their consequent effects on how we use the land today.
1. Sandstone (up to 250 million years ago)
You don’t have to go far into Sydney Harbour to see how much Sydney is characterised by its sandstone. The beds of sandstone, around 2 km thick, are the result of an enormous, fast river (far bigger than the Amazon) eroding coarse sandy sediments from the then centre of Gondwana (now Antarctica) and depositing them in a wide delta. The result is a vast sandstone massif stretching from the Hunter Valley in the north to Jervis Bay in the south and Mount Victoria in the west. The sandstone weathers into flat-topped hills flanked by cliffs and rock falls and typical V-shaped valleys, as seen in the Blue Mountains.
Sandstone is made mainly of very nutrient-poor, coarse silica sands. It is found in all Harbour areas, north to Ku-ring-gai Chase, and south to Port Hacking and Wollongong. Soils derived from it are very infertile, so the early white settlers left it largely uncleared, giving us a vast National Park legacy in Sydney.
2. Shale (up to 230 million years ago)
Typically capping the sandstone, these finer silty and clay deposits were laid down as the ancient river became senile, when Australia separated from Antarctica. The river slowed, and instead of coarse sand, it could only carry silt and clay. In the final stages the deposits were laid down under sea water. The shale layers as a group are called the Wianamatta shales. They often contain plant fossils. The Minchinbury shales, a smaller formation, contain sands and lime, as they were laid down under the sea.
(The Narrabeen shales, which are visible in the cliffs of the Northern Beaches, actually underlie the sandstone and hence are older. They contain plant fossils up to 250 million years old.)
Being more fertile, the clayey soils derived from the shales were cleared early on for grazing and supported the first wheat crops at Parramatta. They cover all of Western Sydney, the North Shore ridge and as far east as Hyde Park. (In fact, when we were surveying in Hyde Park, we found sandstone soils on the Elizabeth St side and shale soils on the College St side.)
Uplift and upheaval. Beginning around 86 million years ago, New Zealand split off from Australia. This led initially to the uplift of the Blue Mountains. Later tilting of the Cumberland Plain exposed the underlying shale at Narrabeen and led to the erosion of the sandstone and the eventual creation of Sydney Harbour. To the north (Gosford and Central Coast) and the south (Port Hacking), no such tilting occurs, and sandstone dominates to about 300 km inland, forming a vast, largely undisturbed wilderness.
3. Basalt (about 18–14 million years ago)
Much later, volcanic activity formed Mount Wilson, Mount Irvine, Mount Tomah, Prospect and many other smaller basalt peaks and hills scattered around Sydney. The basalt caps and intrusions weathered to highly fertile soils, still evident at the Mount Tomah Botanic Garden. The volcanic rocks on the Cumberland Plain are now almost completely mined out.
Quaternary deposits (about 1.6 million years ago to now)
The most recent geological period is called the Quaternary. Within that period, the most recent 100 000 years of alternating glacial and interglacial climates has caused alternating dry and wet periods in the Sydney Basin (although no glaciers). This period of wetting and drying resulted in significant erosion and the deposition of two great Quaternary Alluvium deposits:
4. The Botany Sand Sheet. This formed from the wind-blown deposition of marine sand (both silica and shell-derived) swept constantly north along the NSW coast. It starts as a medium-sized sand on the coast and becomes finest furthest inland; for example, the sands at Victoria Barracks and Prince Alfred Park are finer than those at Kurnell. The sandy soils derived from the Botany Sand Sheet were largely developed for industry, the Airport, recreation and parks, with some use for horticulture.
5. The Nepean and Hawkesbury floodplain. From the beautiful horticultural soils of the Camden district all the way down to the rich Windsor flood plains, these fertile areas were discovered and cleared very early after agricultural settlement. The regular (now historic) flooding and flat landscape created moderately fertile soils supporting some of the best flood plain forest. In the lower reaches (e.g. Castlereagh and Agnes Banks) the sands are coarser, and form important sand mining sites (e.g. Penrith Lakes).
Further reading
Jenny H. 1941. Factors of Soil Formation. McGraw-Hill, New York.
Wikipedia: Hans Jenny