These rocks are mostly gently folded. Scattered areas of almost horizontal basalt lava flows of early Tertiary age, up to 120 feet in thickness, occur in the vicinity of Cabramurra, capping some of the highest ground. The basalt often overlies and protects from erosion beds of unconsolidated sand, clay, ligmite and gravel up to 300 feet in thickness. These sediments are the remnants of former extensive river valley bottom and lake deposits, over which the basalt lavas flowed. Other similar but more extensive basalts and sediments occur, particularly in the Cooma-Berridale district.

Deposits of alluvium along the present-day streams are quite limited in extent. Within the main mountain block where the main streams are chiefly in narrow steep-sided valleys alluvium is either absent or consists of poorly sorted boulder gravels. Fairly extensive flats containing gravel deposits occur along the more open valleys where the rivers leave the main mountain block.

Geological Structure:

The Snowy Mountains area is traversed by many faults—that is, ruptures in the rock masses accompanied by differential displacement of opposite sides of the fractures. The rock is usually crushed in zones of varying width along the fractures, and is often more closely jointed than normal adjacent to them.

The faults can be classified into two groups:

(a) Old faults developed during the crustal disturbances which caused the repeated folding of the strata. Many of such faults are “healed” and the crushed rock cemented by later deposition of silica and other minerals. Some are not healed.

(b) Faults associated with the warping and uneven elevation of the area during the movements of uplift in the Tertiary period. During this period some of the old faults were reactivated, and new ones formed.

Few faults are exposed at the surface. Many have been inferred from study of air photos and surface mapping and proved by drilling or trenching; and many have been found in exploratory and construction tunnels, road cuttings and other excavations.

Pleistocene Glaciation

The highest part of the Kosciusko tableland is the only region of the mainland of Australia known to have been extensively affected by glaciation during the Ice Ages of the Pleistocene period (about one million years ago up to the present) when much of northern Europe, Canada, Northern U.S.A,, south-westem Tasmania and the South Island of New Zealand were profoundly affected. Most of the mainland of Australia escaped because of its position in latitude and absence of high altitudes. The work of W. R. Browne (Ref. 3) has shown that the area affected by the glaciation covers about 400 square miles of the highest country along the Main Divide in the vicinity of Mt. Kosciusko, extending down to 4,800 feet above sea-level (Fig. 1).

Three stages of glaciation have been recognized. The first stage was an ice cap glaciation with ice sheets on the high ground causing a general smoothing and rounding of the granite surfaces, and depositing extensive ground moraine. The second stage, possibly 100,000 to 200,000 years ago, was characterized by valley glaciers which occupied valleys already formed by river erosion, and modified them by truncating spurs, widening the valley floors, developing rock basins along the valley floors and depositing glacial drift and moraines. In the third and final stage, possibly 10,000 to 20,000 years ago, the glaciation was confined to the highest altitudes mostly above 6,000 feet, when small glaciers occupied the valley heads and excavated cirques and rock basins and deposited lateral and terminal moraines, and formed Blue Lake and the several other small cirque lakes in this area.