The only airspace that VFR traffic is never allowed in is Class A, above 18,000 ft. VFR is permitted in Class B, C, D, E, and G.
Over sparsely populated areas, class E only surrounds airport transition areas and Victor airways. Any airspace not other wise classified is class G airspace, which is completely uncontrolled. That's what you're refering to over the UP, where the class G that is normally under all Class E extends up to 14,500 ft.
The difference between class E and G is visiblity requirements - below 10,000 you can legally fly VFR with 1 mile of visiblity in class G (legal, but still a really bad idea), Class E requires 3 miles of visiblity.
Note to JetDoc: He's asking about uncontrolled airspace, not controlled airspace.