Phillip Island Excursion Site 2: Cat Bay—Summerland

As you drive south on Ventor Road to The Nobbies, just as you encounter the empty, former town of Summerland, there is a turn off on the right to a parking lot. You can access the beach at Cat Bay by wooden steps.

Here is another example of black basalt outcrops and boulders strewn on the beach having been eroded from the basalt cliff. You can examine the sand on the beach (check the grains, size, roundness, and composition).

Check the basalt outcrops and boulders. Note the appearance, jointing, colour, presence of minerals in the rock, presence of holes, and mineral deposits in the holes and in the joints. Check the horizontal layers in which the basalt has been deposited. Look for examples of columnar jointing.

Note the contact between the sand and the basaltic headland.

As you walk north along the beach note the huge sand deposit which joins the rocky basaltic outcrop (The Nobbies) with the rest of Phillip Island. This sand deposit and the rock island system are together known as a tombolo, and this one is called the Summerland Tombolo Complex. If you have time check whether there are any signs of structures in the sand (layers, drainage, mineral deposits, etc.).

Significance

These deposits illustrate the extent of the volcanic eruptions and give an indication of the thickness of each lava flow. The holes (vesicles) in the basalt point to the release of gases during the eruptions. The clay rich ‘weathering profile’ may be another indication of concurrent sedimentation with the volcanic eruptions. The sand tombolo deposit, connecting The Nobbies with the rest of Phillip Island, points to different sea levels in the past and abundant sand transport. The sand is likely a post-Flood accumulation.

Guide available as a booklet
This entry is taken from the geological excursion guide for Phillip Island. The complete guide is available as a booklet on creation.com Aussie store.