I'm going to set a bunch of markers over the weekend. I've decided to tie them out to a 4 inch Dura Nail with stainless washer directly underneath the markers. To do that I need to dig a hole about three feet deep. So I'm going to try an auger attachment for the Bobcat. I've go it rented sitting in the back of the truck. 12 inch diameter by 4 foot long auger. It may destroy my affection for a shovel and crowbar for the rest of my life.
Don't throw your shovel away -- you'll need it to move the dirt away from the auger so it doesn't fall back into the hole.
And don't forget to call in your sites to your local one-call utility marking service before you dig. There's nothing quite like the thrill of seeing a yellow plastic gas line come spinning up out of a hole being augered. (Fortunately, it was a line that was never charged. The gas company didn't enter it into their database, so it didn't get marked out.)
I've done a couple of projects that required a bunch of 4-foot-deep 12-inch holes scattered around various counties. I hired an owner-operator who has a little track-mounted Cat with an auger. It worked pretty slick, and was well worth the money.