The Missoula Floods carried large icebergs from the glaciers that dammed Lake Missoula or from the Okanagan lobe glacier that dammed Lake Columbia along with them as they made their way to the Pacific. As the icebergs melted or became “stranded” against ridges that formed Lake Lewis, they dropped the rocks that the glaciers picked up as they scoured their paths southward. Granite is present in Montana as well as northern Washington. But it is not present in central Washington. The origin of this single granite erratic on Rattlesnake Mountain is therefore not definite. It could have come from either Montana or northern Washington.
This chunk of granite is approximately 6′ in length. It is located at about 800 feet elevation (my estimate).