I'm sure they are correct Roak. But any one can check on land ownership in Montana . I do not know if this is valid in other states but here you go to
http://svc.mt.gov/msl/mtcadastral You can find out ownership and contact info for the owners if you wish to contact them. It's a pretty cool site. Chances are that clay bank is in the roadway rite away. A
bucket or two will probably not get you tossed in jail but, the land still is owned and taxes paid by the land owner. Look them up , give them a call and ask, its just a polite thing to do...
Truth is there is no lack of clay pockets here in W, Montana. Just have to look around, check with the county road crew, any excavation going on should have some, and I promise they will want you to take some away... they usually offer to load it with their machine...