What caused the flat "plateau like" regions in the Southern California regions?
Plateaus are flat land-forms at relatively high elevations. They can be caused by several different factors that are responsible for producing such land-forms. Volcanic activity would be one distinct possiblity. Shield volcanoes are among the largest in the world. They have quiet lava flows, primarily because of low silica content. The lava oozes out of the vent and spreads across the ground, very similar to the way thin pancake syrup spreads across a stack of pancakes. This produces a large, flat land-form; repeated eruptions would be a mechanism to construct a large, flat land-form such as a plateau. Glacial erosion would be another distinct possibility. Glaciers are renown for their ability to raze mountains into flatter terrain. The North American continent was in the middle of an ice age thousands of years ago, with glaciers protruding into the areas you are describing.