The "multicolor igneous rocks" you saw (Did they look sorta like this?) were probably what the locals call "fieldstone" and what geologists call "glacial erratics." Glaciers brought them. The bedrock it came from is somewhere up in Canada. They are found all over Minnesota and Wisconsin (except in the "driftless zone") but especially in eskers.

Jackson Pollock? That's a bit harsh. Mondrian, maybe.