The black cracking clay of the Casterton Rolling Hills
the Merino Group rocks are soft, rich in lime and soda, and high in clay-forming minerals, lack a coarse fabric, are well supplied with potassium and moderately supplied with phosphorus
“The features of this landscape are the generally treeless dark heavy soils, a sharp escarpment at the junction with the tablelands, deep valleys with concave lower slopes and flat valley floors... The Casterton land-system extends over most of the area between and around Merino, Coleraine and Casterton and separates the two sub-systems of the Glenelg land-system so that to the north-east, the Glenelg land-system is mainly of earlier Palaeozoic rocks, whilst to the south-west, it is mainly of later rocks, early Cainozoic capping upper Mesozoic. Both Kenley and Boutakoff refer to the soft nature of the Merino Group sediments. From all this, it is clear that the Merino Group rocks are soft, rich in lime and soda, and high in clay-forming minerals, lack a coarse fabric, are well supplied with potassium and moderately supplied with phosphorus.. an area of moderate rainfall, where the parent material is Mesozoic sediments, which are calcareous, soda-rich, without an open fabric and containing abundant clay-forming minerals or clay particles, the soils have the following features:
- uniformly heavy texture down the profile ; swelling clay throughout and therefore deeply cracking when dry but tightly closed when wet ; strong medium-sized structural units at the surface due to calcium
- larger, well-formed units in the subsoil, because of sodium on the exchange complex, the faces of the units being slippery when first wetted.
http://vro.depi.vic.gov.au/dpi/vro/glenregn.nsf/pages/sw_land_system_casterton more info: http://hamilton-field-naturalists-club-victoria.org.au/images/Dec2013/Hist%20flora%20fauna%20WVic%20Apr2011%20updated.pdf http://www.researchgate.net/…/0912f50ebd60ac4750000000.pdf includes a map (Fig. 2) The Glenelg River Catchment showing the four geomorphic regions of Jennings and Mabbutt (1986)