Many granitic rocks of the Coast Range Arc are plentiful in the North Cascades of the Cascade Range, which is the southernmost boundary of the arc. Here, these granites intruded highly deformed ocean rocks and assorted fragments from pre-existing island arcs, largely remnants of the ancient Bridge River Ocean which lay between North America and the pre-existing Insular Islands. Massive amounts of molten granite injected over this period, burning the old oceanic sediments into a glittering medium-grade metamorphic rock called schist.
