They were 600 to  long and rode atop a microplate known as the Intermontane Plate. Over early Jurassic time the Intermontane Islands and the Pacific Northwest drew closer together as the continent moved west and the Intermontane Plate subducted.
These entities were 600 to long and rested on top of a microplate called the Intermontane Plate. During the early Jurassic period, the Intermontane Islands and the Pacific Northwest gradually converged as the continent moved west, causing the Intermontane Plate to subduct beneath them.