Topography: Land form and Drainage
The Tothill Range is a portion of the northern Mt Lofty Ranges. Appila Tillite forms the spine of the range, but the tillite is often weathered and outcrops poorly. [Tillite is sedimentary rock that consists of consolidated masses of unweathered blocks (large, angular, detached rock bodies) and glacial till l (unsorted and unstratified rock material deposited by glacial ice) in a rock flour (matrix or paste of unweathered rock) (Britannica)].The tillite contains numerous sandstone and quartzite members which form most of the visible outcrops.
The main ridge runs north-south, branching into two ridges with a high central valley south of Webb Gap. The highest and lowest points are 676m and 500m respectively. Slopes vary from moderate to steep except for a small area of gently sloping land in the north-east corner. The Niblet Gap land is also moderately to steeply undulating and the highest point is 644m.
Most of the eastern boundary is a watershed and drainage is chiefly to the west by several steep gullies which run into Tothill creek, a tributary of the Light River. However, Webb Gap Section 2, is on the eastern side of the watershed and drains north eastwards into Brady creek which runs into Robertstown lagoon.Similarly the eastern half of Niblet Gap drains to Brady creek which the western half drains into Apoinga Lagoon.
Climate and Hydrology
The climate is typically Mediterranean with warm to hot dry summers and cool moist winters. There are no records of mean average rainfall. The nearest recording station, Marrabel, receives an average of 550mm; because of its elevation the Tothill Range land probably gets more.
There are no permanent streams , but several steep creeks flow briefly after rain. There is a reliable dam on Webb Gap, Section 2, and a small less reliable dam no section 353. Both attract wildlife, especially Western grey kangaroos, Euros, birds and frogs. There is a disused well near the house ruins.
The rocks of the Tothill ranges area were originally deposited as sediments some 800 million years ago. Sands, silts, clays and minor dolomite (Saddleworth Formation and Mintaro Shale) were deposited under relatively quiet marine conditions. A period of non-deposition followed, heralding the later onset of cold climate sedimentation, comprising further sands and silts, but dominated by chaotic mixtures of coarse rock fragments scattered through a fine muddy matrix.These sediments were deposited both directly from glacial action in marginal marine settings (Appila Tillite) and also in a deeper marine environment with significant sediment contributed from melting sea ice ( WIlyerpa Formation). Most of the sediments were sources from the extensive land mass (Gawler Craton) which existed to the west. These sediments were progressively buried by younger deposits and in time became lithified, then compressed into broad folds 500 million years ago.
Gradual erosion has stripped all the younger rocks to produce the current topography, with the more resistant beds, usually sandstones, now forming the prominent ridgelines (‘strike ridges’) of the Tothill Range.
Because of the steep slopes and the nature of the parent rock the soils are chiefly skeletal, shallow with minimum profile development, and dominated by the presence of weathering rock and rock fragments. Nevertheless there is a good cover of native vegetation, chiefly sclerophyllous trees and shrubs. Shallow loamy red-brown earths occur on the flatter land on both sides of the property.