The 33.1-28Ma (million year) old Colestin Basin is filled with volcaniclastic rocks that are tilted ~20-25° eastward. The basin is bounded to the north by the Siskiyou Summit Fault (SSF) and to the south by the Stateline Fault (SLF). The ancient Colestin Basin, that deepened to the NE, has been faulted into its present position along oblique-slip faults. Faulting on the SSF, occurring before deposition of a 28Ma tuff, produced normal right-lateral movement with a net slip of 4.5 km; Colestin rocks older than 28Ma thin dramatically to the south and north of the faults. Slickensides with an average rake of 60°SE within the N.60-75°E-trending SLF yields a net slip of 0.77 km. Both northern and southern faults show evidence of reactivation: early primarily lateral motion with later dominant dip-slip motion. Those faults were reactivated during uplift of the Klamath Mountains as well as being influenced by later NW-trending faults.