The river and its immediate flood plain does have soil - don't know how deep it is, but often, when there's a stand of aspens or birch, there's often more topsoil around them. There are fields near the PP home that would have
some diggable dirt and it's clear that some trees have been planted here and there (so at least 3 feet of diggable soil?) I'm guessing that in some lower lying places, it's deeper - but a lot of it is out in the open, not really conducive to hiding a body.
I doubt PP has any place where a six foot hole could be easily dug - I'm thinking about the augur attachment (did he even have ready access to one?) and why the Bobcat has been ruled out as aiding in this crime. A couple of augur holes right next to each other...I guess it could work but how does he hide the fact that he's doing this? It would be a big ole operation - where does the dirt go? You can't just dig a huge hole and not have a pile of dirt (some of which needs to be available to refill the hole once the body is inside - otherwise the big rock is going to sink into the hole unless it's like 15 feet in diameter - or bigger.)
I still think he left her south of PP in a place that, as
@sk716 mentioned, might provide more cover for mountain lions. His hope lay in rapid decomp and disarticulation of her body (IMO). Animal scavenging/mountain lion would be consistent with his "lion took her from a bike," a thing which has actually happened within the last 50 years here in California; or "abductor tossed her from a car," (abductor would obviously have had to U turn or turn back toward Salida at some point, but why not?) or "she wandered off in a daze after her accident and collapsed."
@sk716 is correct about that area having more "cover," (tons more aspens, dense enough to make it hard to walk through them - but no problem for a hunter who is used to such terrain). Take a look at the pictures on the
Green Creek Hiking trail sometime (on maps.google.com).
Not that I think Barry actually used a trail - he knew better, but the trail does give a glimpse of the wildly varying terrain right there, very difficult if not impossible to grid search. Suzanne would not have walked to her final resting place - Barry would have though, so the scent dogs needed to be looking for Barry's scent on all that land, not Suzanne's...