The rock material that composes the high peaks is sandstone which was laid down from rivers during the Devonian period 380 million years ago. This sediment slowly accumulated to a depth of 7 km. This was later raised and tilted for its present form. Forty million years ago the Southern Ocean reached the base of the northern and western base of the mountain range, the deposition from the range forming the sea floor which is now Little Desert National Park.
Sandstone
The Grampians Group is a sequence of approximately 3700 m of quartzo-feldspathic to quartzose sandstone and micaceous mudstone in which many new units have been defined. The sediments were probably deposited in structurally controlled basins developed in the Delamerian bedrock, but the age of this deposition is poorly constrained. The Grampians Group may be a fluvial to marginal marine portion of the sediment fan that deposited the Cambrian to Silurian deep marine turbidite sequences of western and central Victoria.
The Grampians Group has been tectonically thickened by a series of duplex thrust faults that intercalate and repeat large parts of the stratigraphy. This thickened sequence was then segmented, and transported over the Cambrian bedrock to its present position in a thin-skinned manner along an extensional décollement—giving the Grampians Group an allochthonous relationship to the underlying Cambrian bedrock. The timing of these deformation events is poorly constrained, but they are older than Early Devonian, when the deformed sequence was intruded to subvolcanic levels by several granitic plutons. Contemporaneous eruptions produced the extensive, felsic Rocklands Volcanics in the west. The granites and volcanics are described in detail and have been subdivided into a number of units.
The walking track climbs gently from the car park, through rock outcrops and a stringybark forest to the Balconies lookout. This Earth Cache is only possible during daylight hours.
Q1 Estimate the height and width of the 'Jaw'
Q2 Describe the colours and the texture of the rock in your own words.
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