of their past passage. They are also on the other side of the spreading centers, though that does suggest a pure speculation that these postulated zones of weakness could be related to transform faults from the spreading center. Precambrian basement Following the Great Divide Megashear into the mid-continent reveals something interesting: a widespread pattern of similarly trending (roughly NW-SE) fault zones, rifts, and aeromagnetic and gravitational anomalies. Although some of the faults are recent, the NW trending zones themselves have been attributed to continental-scale transcurrent shearing at about 1.5 Ga – that's billions of years ago – during the assembly of Laurentia (the North American continent). Curiously, there is another widespread pattern of parallel fault zones, etc., of various ages trending roughly NE-SW, including the Midcontinent Rift System, the Reelfoot Rift (in the New Madrid Seismic Zone), and others. These fault zones and rifts occur on tectonic boundaries that date to the Proterozoic – that is, 1.8 to 1.6 billions of years old. They are also roughly parallel to the Ouachita— Appalachian mountains, raised when Laurentia merged with the other continents to form the Pangaea supercontinent some 350 million years ago. It is now believed that these two predominant patterns reflect ancient weaknesses in the underlying Precambrian basement rock, which can be reactivated to control the orientation of features formed much later. Such linkage of older and younger features seems very relevant to the OWL's troubling age relationships. The possible involvement of the deep Precambrian basement does suggest that what we see as the OWL might be just the expression in shallower and transitory terranes and surface processes of a deeper and persistent ur-OWL, just as ripples in a stream may reflect a submerged rock, and suggests that surficial expression of the OWL may need to be distinguished from a deeper ur-OWL. But neither the applicability of this to the OWL nor any details have been worked out. Summary: What we know about the OWL First reported by Erwin Raisz in 1945. Seems to have more depressions and basins on the north side. Associated with many right-lateral strike-slip fault zones. Seems to be expressed in Quaternary (recent) glacial deposits. Does not offset Columbia River Basalts, so older than 17 million years. Not offset by the Straight Creek Fault, so probably younger than 41 million years. (Maybe.) Approximately separates oceanic-continental provinces. Not an oceanic-continental crustal boundary. (Maybe.) Not a hotspot track.