on this. CLEW and Columbia Plateau Further east is the "CLEW", the segment of the OWL from approximately the town of Cle Elum (marking the western limit of the Columbia River basalts) to the Wallula Gap (a narrow gap on the Columbia River just north of the Oregon border). This segment, and the associated Yakima fold belts, do include many northeast-trending faults crossing the OWL. However, these are largely dip-slip (vertical) faults, associated with compressional folding of the overlying basalt. As there is typically 3 km of sedimentary deposits separating the basalts (also about 3 km thick) from the basement rock, these faults are somewhat isolated from the deeper structure. The geological consensus is that any strike-slip activity on the OWL predates the 17 Ma old Columbia River Basalt Group. There is some evidence that some of the northwest-trending ridges may have some continuity with the basement structure, but the nature and details of the deeper structure is not known. A 260 km long seismic refraction profile showed a rise in the crustal basement beneath the OWL, but was unable to determine if that rise was aligned with the OWL, or just coincidentally crossed the OWL at the same location as the profile; gravity data suggested the latter. The seismic data showed a uniformity of rock type and thickness across the OWL that discounts the possibility of it being a boundary between continental and oceanic crust. The results were interpreted as suggesting continental rifting during the Eocene, perhaps a failed rift basin, possibly connected with the rotation of the Klamath Mountain block away from the Idaho Batholith (see Oregon rotation, below). There is a curious change of character of the OWL in the center of the CLEW where it crosses the roughly north-trending Hog Ranch—Naneum Anticline. West of there the OWL seems to follow a ridge in the basement structure, to the east it follows a gravity gradient, much like the Klamath–Blue Mountain LIneament (see below) does. The significance of all this is not known. Hite Fault System Past the Wallula Gap the OWL is identified with the Wallula Fault Zone, which heads towards the Blue Mountains. The Wallula Fault Zone is active, but whether that can be attributed to the OWL is unknown: it may be that, like the Yakima Fold Belt, it is a result of regional stresses, and is expressed only in the superficial basalt, quite independently