On the other hand, the light-coloured mica-poor granites, aplites, fine-grained porphyries and acid lavas such as the Devonian rhyolites and dacites, are usually very little weathered chemically and have only thin soil cover. The sedimentary rocks including phyllites and schists are variably weathered usually only to moderate depths. Quartzites are commonly highly resistant to chemical weathering.

Apart from the composition of the rock, weathering has been found to depend upon the local geological structure. Weathering is usually more intense and penetrates more deeply below the surface in zones where the rock is crushed by faulting or more closely jointed than normal.

The erosional history and topography of the particular site are also important, since weathered rock may be quickly removed by erosion. Weathering is normally a very slow process and a thick weathered zone takes a very long time to develop. For example, boulders of Kosciusko granite embedded in the David Moraine at the Kosciusko Dam site on Spencer's Creek, possibly 100,000 to 200,000 years old, are completely weathered to a depth of about 6 inches; boulders and pebbles less than 12 inches in diameter are usually weathered throughout, while boulders larger than I2 inches have fresh cores. Similar boulders and pebbles of similar rock in the youngest moraines, possibly 10,000 to 20,000 years old, are usually only slightly or moderately weathered. In the beds of most of the youthful valleys of the main streams in the mountainous parts of the area, it is usual to find fresh rock exposed in the stream bed, and often it is but little weathered up to 50 to 100 feet above stream bed.

These streams are still actively down- cutting, and have removed the weathered materials. Above this level weathering commences and becomes progressively more intense and deeper. Thus at many of the dam sites investigated, the rock in the river channel, even if partly obscured by boulders and gravel, has been found to be fresh at river level, but in the abutments it is considerably weathered in a manner, and to an extent, depending on the rock type and structure.

Methods of Geological Investigation.

The geological investigations for every project are based on geological mapping supplemented by such exploration as may be required to clarify and prove the geological conditions, to obtain samples for laboratory tests, or to enable tests to be made of the material in place. Exploration is carried out by means of diamond and cable-tool drilling, trenches, test pits and tunnels, auger borings, and seismic refraction surveys. Results of such exploration also provide a means whereby prospective tenderers for construction contracts may judge for themselves the conditions likely to be encountered during construction.

Geological Mapping:

The broad geological mapping usually associated with the reconnaissance stage of investigation is carried out on a scale of 4 inches = 1 mile, based on maps contoured from vertical air photos. Study of these photos under the stereoscope yields considerable geological information. For example, many of the major fault zones in the area are indicated on the photographs as an alignment of topographic features such as depressions, saddles, parts of streams and swamps. These same faults are frequently very difficult or impossible to find on the ground by surface mapping, because of the thick soil cover.