Basalt on the plateau about 2 miles east of the river occupies the floor of an ancient valley which appears to be the ancestor of the Tumut, indicating that the present valley has been developed since early Tertiary time.

The river flows northward, and the valley, when viewed broadly, is straight, or only gently curved for about 25 miles. In the valley floor however, the river channel has many sharp bends and loops along its course, entrenched in rock. In the vicinity of the power stations, the river bed is 70-120 feet wide and is generally filled with boulders through which the river flows as a series of rapids.

The distribution of the main groups of rocks in the region is shown on Figure 2. T.1 and T.2 power stations are located in a complex mass of granitic paragneiss and granulite intruded by sheets of granites. The group of metamorphic rocks is referred to as Boomerang Creek granitic gneiss, and the granites as Happy Valley granite. Along its eastern margin adjacent to the contact with highly folded sedimentary and volcanic rocks, the granite is intensely deformed by shearing with the development of gneissic granite and mylonites. The marginal zone is heavily faulted, and there is evidence of renewed movement at widely separated intervals of time along these faults. The movements which caused the initial deformation of the granite appear to have been occurring at the time of the emplacement of the marginal gneissic granite and are possibly late Silurian in age. Renewed movement along one of these old faults is shown at the intersection of the granite and basalt of Eocene-Oligocene age about 2 miles northeast of T.1 power station. The area in the vicinity of the power station is cut by a series of overthrust faults with seams of crushed rock and clay and close jointing to the fault zones.

In the first stage several alternative sites and arrangements of T.1 power station were investigated.

The first proposal was for a surface power station on a narrow alluvial terrace, 1 ½ miles downstream from the present site. This is one of the few places along this part of the Tumut valley where there is a sufficient area of level ground close to the river for a surface power station.

The steep slopes down which a surface penstock line would extend were entirely soil-covered. During the investigations rather extensive sliding of the surface layer of soil and intensely weathered gneiss ocurred along the access track leading down the slopes. Subsurface exploration by diamond drilling at the headrace surge-tank site and down the slope showed that the underlying granitic gneiss was intensely weathered for 30-80 feet from the surface and became irregularly but progressively less weathered down to 170 feet from the surface. Drilling and surface mapping revealed a wide fault zone passing through part of the site, and it became evident that the widening of the valley bottom in this area was the result of the presence of weakened rock along the fault zone.

Because of the evidence of the instability of the surface soil and weathered rock it was considered that surface penstocks were impractical. The alternative of using underground vertical or sloping pressure shafts was rejected, in part because of the evidence of unfavourable rock conditions for underground work caused by deep weathering and faulting.

Other sites were examined progressively upstream, by surface mapping, airphoto interpretation and diamond drilling. Of these the present site appeared to be free from major faulting and to be the most favorable.