Do I really need to dig a foundation if I'm building on SOLID ROCK and SAND MIX (NO soil whatsoever)

Call me lazy - but the spot in our backyard where I want to build, used to be the neighborhood sand/rock dump for mixing foudations for the neighboring homes. It is literally 100% coarse sand and rock mix (rocks about 8 inches in diameter). Am I wasting my time digging a foundation? Seems silly to dig rocks OUT just to replace it with... rocks. Drainage won't be a problem, because it's near the top of a small slope. Does the concept of frost heave only apply to soil and things that retain moisture? What I would like to do is get some 2-3 ft. diameter boulders delivered from the neighboring rock pit, arrange them in a circle, and cob on top of that. (The cob comes from the same neighboring pit. Roof will be a huge overhang of repurposed old metal shed roofing.) The location is Spokane, WA (eastern WA) - frost depth is 2 ft.

I wouldn't trust it. Sand drains easily, but not SO easily that you can't get a little frost heave going on. It's still winter now; why not test it? Build a 5'x5' test wall with concrete blocks and clay mortar, and see if it's still plumb and free of cracks come spring. If not, you've got a crystal-clear answer. If yes, then you can keep doing homework, and who knows, maybe you've got the coolest location in the country, no foundation needed.

I'm not a building engineer so you can take my words with a grain of salt. But that doesn't sound like a very stable base for building. Frost heave is part of the equation, but there's also seasonal temperature variations, and the resulting expansion/contraction which can cause shifting of the ground.

With a Frost Depth of 2 feet, if the sand your mentioning doesn't go at least 2 feet 6 inches, you will have frost heave and your building will crack. That is the Large and small of it, no matter that you bring in huge boulders to use as the foundation, they will be moved by the freezing ground water.

Yes it sucks, but if you want any building to survive you have to do the right things to make sure the worst doesn't happen.