In southern Arizona, it's just plain dirt, Ive never seen gravel laid down for road bed unless it's a private road. Our dirt roads get slick and mucky when it rains, dry and dusty when hot and dry........and always washboard, unless the county grader has passed thru, which ain't often.
That's pretty much the case in NM as well.
They do get some gravel out of old stream beds that may happen to cross the dirt roads though. But then you get the large rocks with the small. I've seen some exposed by the grader you damn sure didn't want to hit.
