The ultimate driver of the stresses that cause earthquakes are the motions of the tectonic plates: material from the Earth's mantle rises at spreading centers, and moves out as plates of oceanic crust which eventually are subducted under the more buoyant plates of continental crust. Western Washington lies over the Cascadia subduction zone, where the Juan de Fuca Plate is subducting towards the east (see diagram, right). This is being obliquely overridden by the North American plate coming out of the northeast, which has formed a bend in the subducting plate and in the forearc basin above it. This bend has distorted the subducting slab into an arch that has lifted the Olympic Mountains and prevented them from subducting. For the past 50 million years or so (since the early Eocene epoch) these have been thrust by subduction up against the North Cascades (fixed block in the diagram), which sit on the North American Plate. This forms a pocket or trough - what one local geologist calls the big hole between the mountains - between the Cascades on the east and the Olympic Mountains and Willapa Hills on the west. This pocket is catching a stream of terranes (crustal blocks about 20 to 30 km thick) which the Pacific plate is pushing up the western edge of North America, and in the process imparting a bit of clockwise rotation to southwestern Washington and most of Oregon; the result has been characterized as a train wreck. These terranes were covered by the basalts of the Crescent Formation (part of Siletzia). Folding and faulting has exposed these basalts in some places (black areas in diagram); the intervening basins have been filled by various sedimentary formations, some of which have been subsequently uplifted. Glacially deposited and shaped fill covers most of the lower elevations of Puget Sound. This is the Puget Lowland. The principal effects of this complex interplay of forces on the near-surface crust underlying the Puget Lowland are: