Geology of the Snowy Mountains

Up to 1948 the geology of the Snowy Mountains was known only in the very broadest outlines. Systematic study of the geology then commenced with geological mapping of the main projects of the scheme carried out for the Joint Technical Committee by geologists of the Commonwealth Bureau of Mineral Resources, Geology and Geophysics and the New South Wales Geological Survey (Ref. 1). Following the establishment of the Snowy Mountains Hydro-electric Authority, the investigations for the engineering projects were taken over by the Authority's Engineering Geology group. The N.S.W. Geological Survey continued with systematic regional reconnaissance mapping on a scale of 1 inch : 1 mile, in conjunction with the Bureau of Mineral Resources which mapped areas adjacent to the Australian Capital Territory (Ref. 2), and now most of the Area has been covered at least by reconnaissance mapping.

The delineation of the regional geology thus achieved serves as a very useful and necessary basis for broad engineering planning, and as a background for the detailed geological investigation of project sites.

Physiography:

The Snowy Mountains area lies within the Australian South Eastern Highlands physiographic province, and includes the highest and most deeply dissected parts of these Highlands. It is made up essentially of several distinct partly dissected tablelands or plateau remnants, lying at different elevations and often abruptly separated by distinct scarps, which trend most commonly in directions varying between north-south and north-east to south-west.

These tablelands are the remains of a former extensive continuous peneplain which was broken up by faulting, flexuring and updoming of the earth's crust. The final major period of disturbance, causing the greatest uplift, occurred during the Kosciusco epoch at the end of the Tertiary period about one million years ago, when the mountains were raised to their present elevations.

The highest country, most of it above 4,500 feet, with a maximum elevation of 7,313 feet at Mt. Kosciusko, forms a central belt 15 to 20 miles wide, trending in a north-north-easterly direction. On the west, where it is drained by headwater tributaries of the Murray River, this high tableland country is sharply bounded by great fault escarpments which lead very rapidly down to the low- lands. To the east the descent from the high plateau to the lower tablelands country is more gradual, occurring in part along a broad warped zone, and in part along a series of step-like fault escarpments.

The high tableland country is considerably dissected, principally in the north by the Tumut River which, in its upper middle reaches, is entrenched in a steep-walled valley 2,500 feet below the plateau surface, and in the south-east by the deep valleys of the Snowy and Crackenback rivers, and on the west by the Geehi River, Bogong Creek and the Tooma River. Nevertheless, there are extensive areas of the tableland consisting of mature topography—undulating country with broad, open, often swampy valleys drained by streams which form the sources of the main rivers. These streams often flow for considerable distances through such country before becoming entrenched in the steep-walled youthful valleys.