A geological mapping and drilling programme has shown that the making of a tunnel through Wylie Ridge is geologically feasible. The route will pass through folded quartzite and siltstone, and the rock mass possesses various discontinuities which will govern the ease of excavation, degree of overbreak, and the need for roof support. The rocks will be saturated over about half of the tunnel length, but large quantities of groundwater are not expected. Further work should be directed towards a study of the effect that the rock mass discontinuities will have on the tunnelling operation, and in determining the quantity of groundwater likely to be encountered.