Rock formation of East Risdon.
Park at the end of Saunderson's Road and follow the bush track a short distance to GZ.
At GZ you are standing on an elevated promontory projecting into the Derwent estuary.
The rocks present are fine sandstone and siltstone of the Malbina Formation and area of Permian age. The sandstone forms hard, compact massive beds up to 2mts in thickness which alternate with siltstone beds generally less than 1mt in thickness. The siltstone weathers more readily and is readily distinguished in outcrop, but it is not markedly softer than the sandstone. These rocks were formed at sea level prior to the break-up of the Gondwana Super Continent. During the final stages of the Gondwana break-up (which sees the formation of Australia) significant crustal movement occurred leading to the uplifting of the sediments in a Graben/Horst formation which left the Derwent valley very much lower than currently seen.
Graben Horst activity (The Derwent flows in the "Graben").
For a while these sediments were exposed in cliffs well above river and sea level. As the new formation settled down there was a significant rise in sea level which led to the flooding of Bass Strait and similarly the flooding of many river estuaries in Tasmania. A notable estuary flooded in this manner was the Derwent, in its lower sections, which explains why the sediments here are now (again) at river/sea level. Just like any river the current Derwent River erodes and deposits dependent upon river flow. As this area now forms part of a tidal eatuary wave action, more like that found in the ocean, can be seen in rough weather conditions.
To log this cache, answer the questions below and send them to the CO.
Standing at GZ.
Question 1.
Describe the rock at your feet (examples are look and feel, colour and shape) and include the direction and amount of tilt of that rock. Explain how these rocks were formed.
Question 2.
(a) Look directly to your south and compare the rock exposed on the next headland, to that you have described in Q.1. Where thses rocks formed in the same was as those at your feet and give your reasons.
(b) Look further to the south (to the headland with the power line tower) and again compare the rock exposed to that in Q.1.
Question 3.
Under your feet is some beautifully etched erosion with obvious bands.
(a) Describe the feature and explain how it may have been caused.
(b) If the present trend continues, hypothesise what will happen to the pebbles and the track that can be seen on top of the rock formation.
Optional: Take a photo of yourself/a team member or with your caching name with an obviously, “non-environmental”, location in the background.
Please make sure you read the logging details carefully.
If you wish to preserve the sequence of your caches you may log immediately but failure to submit the relevant answers within 14 days, will be mean that your log will be deleted.