If you're here in Ohio, and you're just parking cars and pickups on it, 6" of #304 limestone (#4's down to bug dust) will do the job. Just be sure to strip all of the topsoil off, down to clay. Depending on how thick the topsoil is, you'll want the top of your compacted material a little higher than surrounding grass, so as to shed water. If you have some slope, grade the subgrade to match, to get water off to one side/end. If large trucks will be running/parked on it, then some 2's for a base is suggested, otherwise, straight #304. The problem with #1's, or #2 as an initial base, and subgrade not built in, and drainage installed to get rid of water, water will collect between gaps of the larger stone, and can get pumpy. Water is basically trapped there, until it soaks away down through the clay. Not a big problem if just cars and pickups, but a larger truck can pump mud up through to the surface. That's what's used here in Ohio for highway base, although it's 12" thick, compacted. Over time, it will get a hard surface, and shed most water. In the winter in freezing temps, it will absorb water, and may get a little mushy on top, until it thaws completely through, and lets the water down. That's just Ohio, and the materials we have to work with. That's why you'll see secondary roads get potholes during the Spring thaw. Soft flexible base, under brittle pavement, carrying weight heavy traffic. In years where the temps are below 20º, and calcium chloride is used in road salt, makes it worse. It will go down through the cracks in the pavement, into the base, and actually draws moisture. You'll likely see potholes in those area's well into June.
Depending on where you are here in Ohio, there's normally 2 types of limestone. One has a more higher clay/calcium content, and the other is a more sandy type. The sandy type will have a redish cast to it. I prefer the grayer type with more calcium. just seems to pack tighter, and shed water better.. Along with that, there are two types of limestone, freshwater, and saltwater. The saltwater type is harder, and will hold up better in the longrun. The freshwater is softer will break down, and be a little muddy over the long run. There is another type, which has a blue cast to it, that is really hard, and gets down close to a layer of marble. Good stuff but rare. The last I knew of in Central Ohio was the Maxville quarry near Logan, but that was mined out, probably 20 years ago.
It would help to know what part of Ohio you're in. I could suggest some quarry's to get it from, at least from Columbus, to the Ohio River. North of that, not real sure. If there are any highway projects going on in your area, and they are using a limestone base out of a certain quarry, that's where you want it from. It has passed State spec's for that purpose. I put in a cross over drive between my driveways just over a year ago, to keep from running through the yard, or going out on the road, to get from point A, to point B. I cut the topsoil out which was approx. 8" deep. I wedged it deeper to approx. 12" deep at the existing drives edges, because that's where most of the surface friction would be, when turning, anything from pickups, pickups with trailers, tractors and manure spreader, to a tandem fuel oil truck. Nothing fancy to work with, just my old backhoe, 1 ton dump, and my little Farmall A with front blade. Once it was bladed out,like in the last picture, I rolled it with the backhoe. the 304 was damp-ish, and packed pretty decent. I rolled it again a few days after the Nov. monsoon's quit, for a final compaction. It held up well over the winter, through the Spring thaw, and was pretty much a hard surface this summer.
What I got here, was from Columbus Limestone. Delivered price was $19 a ton, but got the "good buddy" discount because a friend owns a 6 axle dump. Usual material trucking fee,ismaterial per ton, and $3 a loaded mile. It may be more now with the rise in fuel prices. And if you have Deer around, if whatever material you decide on sits in a pile for a couple of days, almost guarantee you'll see hoof prints in the pile.
