I was at Lake Powell this last week and was able to collect quite a few slabs that I saw as usable. Some only for Shohin-sized trees, but I was curious what you look for when collecting slabs that make it a good slab. Does it require a rough surface? Thin overall? Color that stands out? Bird’s eye view shape? Smooth or jagged edges?
Here are photos of the slabs I collected. Any ideas or comments about the usability or viability of them would be much appreciated.
@Nate_Andersen: I don’t know much yet about slabs for bonsai, but as a geologist I do love my rocks and these look pretty cool lol. Do you live in a climate that freezes?
It does not freeze here much. It occasionally gets a bit below 32, but that is for an hour or two and at the most drops down to 18.
From what I can see, this one is my only concern and that’s because sandstone weathers more quickly, especially if water gets in the rock and especially those holes and freezes. If you find yourself in situations where the temperature rises and drops below freezing frequently (i.e. 34 degrees one day, 28 the next, then 33, etc…), then ice wedging might be something to consider.
On what I consider to be an interesting side note, it looks like those holes might be “potholes”; when a smaller rock gets trapped between a larger rock and a river current and just spins in circles while the force of the river drives it deeper. There are potholes in boulders in the Niagara River valley that you can walk through; it’s pretty cool.
I realized later @Carl that this half of it was sandstone, so sadly I’m not sure I’d use it for anything due to that. The others I made sure to drop it a few times to make sure it wasn’t sandstone. My plan for this was the flip this side and put it on the bottom and use the rocky side of it to plant, but maybe next time 
As a geologist, is there anyway to create slabs like these? Can I do something to a rock to get it to piece off in slabs or is that just a naturally occurring thing I need to keep searching for.
Haha sandstone can be compact and more cemented too, so dropping rocks won’t always tell you if it’s sandstone. As far as creating slabs by getting a rock to piece off naturally, you’ll have to decide what shape you’re looking for. Some rocks have what’s called cleavage, where the rock tends to break in the same pattern due to the molecular structure of the rock. For instance, look at basalt flats, slate, mica, and many types of limestone. Other rocks don’t have cleavage, like a lot of igneous rocks. The Lake Powell area was once a sea and so you’ll find a lot of sandstone, limestone, and shale.
You could probably take a small hammer and hit away at small areas to reveal the shape you want. That might be the only thing I can think of without doing any kind of cutting which would get away from that natural look.
