 The latter tholeiite igneous rocks have a strong magnetic signature, and this can be detected from the rock underneath Bass Strait.The band of rocks is 35 km wide.It extends north northeast to Phillip Island, Victoria, and also south 40 km to just off the west coast of Tasmania stopping at the Braddon River Fault

In the Rocky Cape Block west of Wynyard and north of Granville Harbour, the Precambrian rocks consist of the Rocky Cape Group from the Stenian period, with Cowrie Siltstone, Detention Subgroup, Irby Siltstone, and Jacob Quartzite.