From a tectonic standpoint, where the Sierras ends and the Cascades begin are not quite so easily defined. Because the relative movement of various faults do not change at Almanor (although the Mohawk Valley fault zone-think of this as the equivalent of Owens Valley- bends dramatically northward right at Almanor to form the spectacular west-facing escarpment that proceeds northward from the peninsula) the "Sierra Nevada microplate" as geologists call it can be argued to extend up INTO the Cascades. However, I think the cleanest geologic definition is to place the "province" boundary at the southernmost active volcanic center, which would be Lassen and environs. We could say from there on north, we're in the Cascades no, matter what. Keddie Ridge, by this definition, as well as the ridge that includes Crystal Lake north of Quincy, fall outside the "geologic" definition of the Sierra because they are east of the Mohawk Valley fault zone. A lot of folks include the block between the Honey Lake fault zone and Mohawk Valley fault zone in the Sierras, but this is the geologic equivalent of calling the White-Inyo Mtns (between the Fish Lake Valley fault zone and the Owens Valley fault) "Sierra".