Also, in the northern part of the mountains, are several areas of basalt lava of Tertiary age, younger again (but still about 40 million years old) and distinctly different from the Ravine-Talbingo lavas.

These basalts were erupted in relatively small quantities sufficient only to flow down into and fill the floors of several river valleys which covered, in the process, up to 100 metres of river-deposited gravel, sand, and clay as well as layers of lake or swamp vegetation, now preserved as thin beds of brown coal or lignite. The basalt resisted subsequent weathering and erosion better than the valley walls with the curious result that, in the end, the topography was inverted — the original valley floors, capped by durable basalt, now form flat-topped ridges and plateaux, with the present rivers and streams far below.

Gold occurs in some of the ancient gravels and the gold workings during the past century have produced good exposures of the sequence of strata, at New Chum Hill (Kiandra) and Nine Mile Diggings as well as other places. At New Chum, there is a full sequence from basalt, showing columnar jointing, at the top through layers of sand, clay and lignite (often with easily recognizable remains of leaves and wood) down into gravel resting on slate bed-rock.

Contractors to the Snowy Mountains Hydro-Electric Authority obtained sand from these ancient river deposits at New Chum and other workings nearby, and the southern end of Eight Mile basalt plateau, for building Tumut 1 and Tumut 2 power stations, and Tantangara Dam.

The highest parts of the Mountains, especially in the general vicinity of Mt Kosciusko, are of unique interest because they are the only part of the mainland of Australia which shows indisputable evidence of having been glaciated during the Great Ice Age, which ended some ten thousand years ago. Glaciers, “rivers of ice,” carry large masses of boulders and soil with them which are dumped when the ice melts in characteristic deposits called moraine. Glaciers, also, are able to cut basins in the rock below stream grade. There is a group of five lakes in the vicinity of the summit, which have been formed behind a bank of moraine, or in a rock basin, or by a combination of both.

Lake Cootapatamba, for instance, is dammed behind a curving embankment of terminal moraine; Lake Albina occupies a hollow in a large area of hummocky moraine; and Blue Lake occupies a deep rock basin with moraine on its lip.

In the Northern Hemisphere, where glaciation was severe and widespread over the land masses, four major glacial periods have been recognized, spread over about one million years. Quite probably there were several glacial periods on the Kosciusko Plateau also but, because glaciation was so limited in area and relatively mild, traces of the earlier glaciations are difficult to identify positively.

However, one definite relic of an older glaciation is the David Moraine at Spencers Creek on the Summit road. This is a mound of moraine up to 100 metres in thickness. The swampy flats upstream are formed of silt deposited in a lake, now drained, which was dammed behind the moraine.