They are underlain by thousands of feet of mostly sedimentary rocks that are the remnants of the numerous previous mountain systems that have formed to the west.
(They; are underlain; by thousands of feet of mostly sedimentary rocks) (mostly sedimentary rocks; are; the remnants of the numerous previous mountain systems) (the numerous previous mountain systems; have formed; to the west)
They mostly sedimentary rocks the numerous previous mountain systems
are underlain are have formed
by thousands of feet of mostly sedimentary rocks
the remnants of the numerous previous mountain systems
to the west