Subsurface geometries of ancient faults mapped by high-resolution geophysical surveys use reflected seismic waves. Such surveys in southern Canada map faults which host significant gold or nickel deposits. Reflectors are considered to be brittle if upper-crustal, lava flows if folded or broken, and mid-crustal if mostly parallel. Steep fault zones possibly first formed in ocean settings when the lava erupted. Folding and horizontal shortening probably occurred during later mountain building.