The most notable geological feature crossing the OWL is the Cascade Range, raised up in the Pliocene (two to five million years ago) as a result of the Cascadia subduction zone. These mountains are distinctly different on either side of the OWL, the material of the South Cascades being Cenozoic (<66 Ma) volcanic and sedimentary rock, and the North Cascades being much older Paleozoic (hundreds of millions of years) metamorphic and plutonic rocks. It is unknown whether this difference is in any way linked with the OWL, or is simply a coincidental regional difference.