Final answer:
The North American plate is moving southwest relative to the northward-moving Pacific plate, with the interaction along the San Andreas Fault causing significant shifts in California's geography.
Step-by-step explanation:
The North American plate is moving in a southwesterly direction relative to the Pacific plate, which is moving northward. These plates interact along the San Andreas Fault, a transform boundary. At this boundary, the Pacific plate carries the coastal cities of Los Angeles and San Diego to the north. Over time, the movement has led to significant shifts in the landmass, evidenced by the displacement of landmarks and the formation of the southern Californian landscape. The relative motion of the North American plate over hotspots can be measured by analyzing the series of hotspot eruptions related to their age; this motion is key to understanding the geological future of areas such as the Cascade Range.