Dropstones 🌏 EarthCache

For your personal safety and viewing pleasure, this cache may be best visited at low tide. Tide times can be found here

The Questions:

On the rock platform there are stones embedded in the sandstone that look nothing like the surrounding rock. These are dropstones, dropped by glaciers.

Questions:

1.) Describe the colour and texture of the dropstones at GZ. How does this rock compare to the surrounding rock? Are they different, why?

2.) Which stone here would be older, the sandstone or the dropstone, give a reason to support your answer.

Then post a photo of you at the location with your log, (please do not show the subject of the questions in your photo). Of course, if you do not want to appear in the photo, a personal item in the photo is enough proof of your presence. You may log the cache as soon as you submit your answers to us via messenger.

Logs without accompanying answers sent or without a photo uploaded may be deleted without notice. If you are part of a group feel free to submit your answers via one profile making sure you include the names of each team you are submitting for.

You can find other dropstones on the Wasp Head rock platform:

One here: S35 40.043 E150 18.374

And here: S35 40.121 E150 18.419

The Lesson:

DROPSTONES

On the rock platform here, embedded in the sandstone are several large, well rounded rocks that are too large to have been washed along with the much fine grains of sand that make up the matrix of the rock. These dropstones have glacial origins, having been dropped from glaciers and ice rafted from polar seas into the newly forming Tasman sea during the Permian period. Many of the boulders are felsic volcanic rocks (a type of igneous rock characterized by a high silica content and the presence of feldspar and quartz minerals) which were carried hundreds of kilometres from their place of origin.

*Examples of how glacial dropstones end up a long way from their formation sites

Dropstones are isolated fragments of rock found within finer-grained water-deposited  sedimentary rocks or pyroclastic beds.  They range in size from small pebbles to boulders. The critical distinguishing feature is that there is evidence that they were not transported by normal water currents, but rather dropped in a vertical manner. 

When deposited into fine layered mud there is often evidence of the vertical descent, this includes an impact depression underneath the dropstone and an indication that the mud has been squeezed up around the edges of the falling rock. Subsequent deposits of mud drape over the dropstone and its impact crater. Glacial dropstones, involving rocks falling out of icebergs are one of the most common types of dropstone preserved in the geological record, particularly when deposited in low-energy deep sea or lake environments. Dropstones differ from erratics found in glacial till in that they are deposited in an aquatic environments.

There are five natural mechanisms that produce dropstones, they are (a) Volcanic, (b) biological rafts, (c) Turbidity currents, (d) meteorites and (e) glacial…glacial is the mechanism we are concerned with at GZ.

Glacial: As glaciers move across a surface, they detach rocks from it and either push this along in front or incorporate the debris into their mass, mostly underneath. At the coast, fragments of glacier detach and float away as icebergs and may be laden with sediment. These sediments are often “ice rafted” (transported) great distances into the ocean. As the iceberg melts debris will gradually be released and deposited as dropstones in open marine sediments. Dropstones can be anything up to boulder size and their size is in marked contrast to surrounding fine-grained oceanic sediments they become incorporated into. Glacially deposited rock differing from the size and type of rock native to the area in which it rests is called a glacial erratic.

Dropstones are important indicators of the presence of ice shelves and hence provide evidence of past global climate conditions.

Resources:

https://ozgeotours.yolasite.com/resources/Wasp%20Head%20Myrtle%20Beach%20excursion%20notes.pdf

https://geologylearn.blogspot.com/2015/07/marine-glacial-environments.html