Coral From Alluvial Slope

[InTheDark]

My collecting occurs on an alluvial slope down hill from some assorted Paleozoic mountains north of Las Vegas, Nevada. Dating is difficult but the last couple items I posted appeared to be order Rugosa and roughly Mississippian. The productive region is about 2 KM from the base of the mountains and that's roughly 1/4th of the slope in the photo.

Here's the find of the week:

I slab saw these finds (sacrilege to some, I'm sure) with the intention of paving another table top.

My question today is more geology than ID. This specimen spent some unknown years on the desert floor heating and cooling, getting abraded by the wind and the occasional rain but it doesn't look to have moved down the slope in quite a while. The exposed side is badly cracked (bad for slabbing) and the protected side is much more intact. I'm wondering how long it takes for this sort of damage/weathering to occur. Is it just the time between thousand year floods or ...? I just don't know.

But some desert rat out there does, and I'll thank her in advance for any insight shared.

[squalicorax]

This weathering happens over time. Likely these rocks have been sitting there for thousands of years. Especially if there is not lake or river near by. Flooding is not a likely culprit due to its proximity along the hillside and they usually only cause minor damage to the rock. Here you have likely a series of free thaw expansion happening many times over the year. Also minor rain adds up over a thousand years.

I'd best most of these rocks have been sitting there for quiet awhile. Stuff like this can happen pretty fast geologically speaking. I would try to find where these rocks are coming from and being able to take fresh blocks off the outcrop unweathered.

Edited September 21, 2012 by squalicorax

[cvi huang]

corals with black background is cool, thank You sharing

[PetrolPete]

The las-vegas area is also interesting in why the mountains look the way they do. Since it is part of the basin and range province, you are in an old rift system where large blocks have sheared off in an asymmetric graben, which is why when you look off one direction (east or west I forget which has which slope) you will see a steep slope, but if you look in the other direction there is a much more gentle slope. Which is why you can go to the steep slope and put one foot on that wall, and another on the ground and be straddling millions of years. So, with all the tectonics in the area, there is probably also some damage/fractures induced just from the large blocks sliding down the rift. But I'm sure a great deal of damage is still due to what squalicorax was saying.