Detailed geological examination of the area in the vicinity of the proposed tunnel has shown that, provided suitable precautions are taken, there are no geological factors which will prevent the successful construction of a tunnel through the ridge north of the Myponga Gorge. Along the tunnel route the rocks consist of tough blue-grey, green and purplish slates with interbedded quartzites dipping uniformly north-northeast. The slates are intersected by closely spaced cleavage planes and two sets of joints. Two parallel faults were discovered, the western-most of which intersects the present proposed route. The faults have caused some shearing and increased cleavage, but have not resulted in the formation of clay gouge. Two alternative tunnel routes are suggested, route A having the greatest geological advantages. Lining of the tunnel for most of its length will be necessary along any of the routes. Some drilling is recommended.