In searching for a nearby Traditional Cache (GCB3ABK Frog Holloway), I stumbled across this interesting feature.
NOTE: The actual earthcache location is at S 37° 55.341 E 144° 59.180, but I didn't want to clobber the Frog Holloway traditional icon on the map with this earthcache icon, so these posted coordinates are 30 metres NE along the beach from the feature on the point.
At first glance, the rocks here might look like they’ve been laid out by people, almost like giant paving stones. But this feature is completely natural!
All the similar features I've seen are the result of cooking basalt, but what struck me about this feature is that it is Sandstone!!!
I have discovered that the rock you see is sandstone that formed during the Cenozoic Era (possibly 60 million years ago, about the time mammals inherited the earth after the cataclysmic asteroid event that evicted the former tenants, the dinosaurs. This event, known as the Cretaceous-Paleogene (K-Pg) extinction event, is attributed to the impact of the Chicxulub impactor. This ushered in the Cenozoic Era, but that's another story.
Geologists describe this sandstone as undifferentiated, poorly consolidated, and highly weathered. This means:
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Undifferentiated - it hasn’t been split into separate layers or types, just one mixed unit.
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Poorly consolidated - the sand grains are only loosely held together, so the rock is softer and can crumble more easily than hard sandstone.
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Highly weathered - wind, rain, and waves have been breaking it down for a very long time, rounding edges and widening cracks.
If you look at the shelf here, it appears convex, as though geological forces have pushed it up from beneath, causing cracks as the rock yields to the pressure. This reminded me of opening a sliced mango, though that analogy doesn't hold up in other respects. Because it is fairly uniform in structure, the rock cracks along straight orthogonal lines
Over millions of years, these natural stresses caused the sandstone to crack into blocky shapes called joints. Weathering then wore away the weaker areas, leaving behind the harder blocks. This process created the striking rock pavement you see today.
This illustrates one type of weathering.
As we know, these "tesselated pavements" can form in other ways, too. For example, magma flows from volcanos and if it is the right consistency, it forms a uniform slab of solidifying basalt. Then, a different process forms the cracks and blocks, which are usually hexagonal rather than rectangular. The Giant's Causeway in Ireland is a magnificent example of this, but we have local examples at Organ Pipes National Park, Merri Creek, and Darebin Parklands, close to Melbourne, which are well represented by earthcaches.
Logging Tasks
You may log your Find immediately to keep your find details in order, but please send me your answers by GC Messenger or email. If I do not receive them within 14 days, your Find log may be deleted.
Q1. Look at the rock pavement. On average, are the blocks mostly square/rectangular, or are they more irregular? Does this support the process described above as being the cause?
Q2. Describe how the sandstone blocks are breaking down. Do you see rounded edges, crumbling into small rocks, or loose sand grains? What has caused this wear and tear?
Task 3. Please take a photo of yourself, your team name, or a personal object with either the Brighton Bathing Boxes or Green Point in the background to post with your log.
References :
Rockd App
Geocience Australia
Wikipedia