Ulva Island, Patterson Inlet, Stewart Island, has a sea cave (not a big one).
Ulva Island is a pristine forested Island in Patterson Inlet managed by the Department of Conservation. A local support trust looks after rodent trapping/monitoring and bird reintroducing. It is one of the few pest free islands open to the public. A brief infestation of rats occurred in 2010 so please check your self/boat when visiting.
You can go there by your own vessel or by the ferry from Golden Bay.
Walking tracks are easy some are wheel chair accessible. Lots to see on the walk from Post Office bay to the West end of Island.
At the West end of the track there is a beach that should give you access to the cave at all tides except for real high ones.
Be aware to keep your distance to marine animals on the beach (Seals, Sea-lions) if you are lucky to find them.
A bit on the rocks we are here to investigate.
At this place you will find black basalt and what looks like a lime stone rock or it could be just sandstone rock. One from lava the other sedimentary. An unusual mix.
Limestone rock is a sedimentary rock with colours ranging from white, cream to light brown. Usually having a variety of sized particles, seams of gravel, and darker redish colour. It is soft and can be gouged out easily . It can be composed of skeletal fragments of marine organisms such as coral, forams and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate.
Sand stone is a rock that is also soft and is formed by sedimentary sand being compressed removing a lot of moisture and becoming solid. Depending on the sand and minerals with it, its colour varies and can have seems of larger stones. The grains in sandstone also can be fine or courser. Colours vary from grey, white, reds, browns or a mixture of them all. If there are dark red streaks there could be iron particles.
Basalt rock is black in colour and is formed by volcanic lava that has flowed and cooled slowly on land. Lava that has cooled underground forms granite rock which is made by the lava crystallizing. This is why some rock in this part look grainy and have shinny flakes. A good example of this granite is Bluff Dorite (Bluff Granite). When lava is cooled rapidly as when it enters a lake or ocean it becomes very brittle almost powdery and quickly erodes into black sand. Large areas of Basalt as on the Chatham Islands demonstrate basalt columns split into geometric octagonal patterns. In fact there are basalt columns on Cargill Hill Dunedin where another Earthcache is as well as some on the cliffs south of Dunedin. Basalt rock looks like a rock that has undergone a melting, other rock may have been trapped in this process or you may even see layers of basalt from different lava flows.
Sea caves are formed by the pressure of the waves carving out soft material that is surrounded by slightly harder rock.
This cave faces Fovaux Straight and must have experienced some mighty storms.
Things to notice and calculate.
1. Describe the two rock types at the cave entrance. Try identifying them from their appearance. From your observations which is eroding faster?
2. Estimate the volume of material that has been worn out so far to create this cave. Calculation formula as follows.
volume for a cone. (pi x diameter squared x height) divided by 12
Lean more on the estimate as I am sure you wont have a measuring tape and calculator.
Enjoy your visit.
Post answers to CO. Log as soon as possible. CO will confirm quickly.