Uplift describes how the ground is lifted up during earthquakes. Many of southern California's mountains have formed because of repeated earthquakes. How tall the mountain depends both on how fast the faults are pushing it up and how long that fault has been active. How steep a mountain is tells you how fast its fault is moving (and from that, how often it will have earthquakes). Steep mountains will be eroded by rain storms into a smoother shape so a very steep mountain must be moving up faster than the rain is bringing it down.