Thanks, I'll give the vet a call tomorrow.
I just figured this was a behavioral problem, since he always seems fine before the hikes and always eats grass and dirt if I take my eye offof him. His poops look like a sand pit the next day.
Seems to be a problem any time we take him outside of town, even lower elevation hikes. In town and on local trails we don't have any issues I can remember.
I was hoping maybe he just needed to adjust to the local bacteria or something but it's been going on long enough that you might be right, could be an underlying issue.
ETA: I guess I should add that he'll munch on grass from time to time in town, but the dirt eating happens when we take him outside of town.