No works come within the small area affected by this third glacial stage, The effects of the older second and third glaciations have been much modified by normal river erosion and denudation in the period following the glaciations, particularly at the lower levels where the streams are now entrenched well below their former glaciated floors. The glaciations generally, therefore, do not have a direct effect on many works. One project, however, the proposed Kosciusko Dam on Spencer’s Creek, a tributary of the Upper Snowy River, is greatly affected.

Rock Weathering:

Much of the undulating plateau country, the broad mature valleys and even most of the higher slopes of the steep-walled youthful valleys entrenched below the plateau surface, such as those of the Tumut and Upper Snowy Rivers, are covered with a mantle of residual soil often of great thickness formed by the weathering in place of the bed-rock. Only along the beds of actively degrading streams and the lowest hundred feet or so of the major valleys is it common to find fresh rock exposed at the surface.

Determination of the extent of the weathering and the nature and engineering properties of the products of weathering are among the most frequent and important geological problems encountered. They arise in connection with the investigation of the sites of practically every dam, tunnel and power station. Weathering often severely limits the number of possible quarry sites for concrete materials, rock fill and rip-rap. Residual soils often provide the most suitable and abundant sources of earth-fill for dam embankments and road building materials. Weathering greatly affects the location and design of roads in steep country.

Apart from these direct effects on the engineering works, the widespread blanket of soil makes geological mapping difficult by concealing the underlying rock and, in particular, it often tends to obscure weak-zones in the bed-rock, since both sound rock and weak rock are reduced by weathering to a similar soil.

Weathering is by no means uniformly developed throughout the area. The most important factors influencing its development are the rock type, the local geological structure and the erosional history of each particular locality.

The granitic rocks are the most abundant and widespread rocks in the area, and it was therefore necessary at an early stage in the investigation to closely define the terms used to describe the various degrees of weathering of granitic rocks in order that descriptions of weathered rock exposed in outcrops, excavations, or in drill cores made by different geologists would be consistent and would indicate the engineering properties of the materials. Weathering of granite usually follows a regular pattern of development, and the degree of weathering can be consistently judged. Weathering in this case is mainly chemical weathering, recognized by the softening and decomposition of the felspars to clays, decay of the biotite mica, frequent brown discoloration throughout the fabric of the rock due to limonite, and a tendency for the rock to break into individual mineral grains, all leading to a reduction in its strength compared with its fresh state.