Dan McShane has posted photos of some really huge basalt blocks stranded on ice-age lake terraces in eastern Washington. Read Dan’s story here: http://washingtonlandscape.blogspot.com/2011/08/glacial-erratics-near-grand-coulee-dam.html
The basalt blocks sit on or in lake deposits. They were carried by very large icebergs that had calved off the nearby terminus of the glacier covering the northern part of the region in the Pleistocene, and which plugged Columbia river drainage in the area of Grand Coulee Dam.The underlying bedrock is Columbia River basalt or, in places at least, much older gneiss. The basalt blocks did not travel very far, either within the glacier or in the ‘bergs, since CRB doesn’t extend very far north of the southern limit of ice flow and consequently not much available to the glacier to erode.