A type 1 sequence is bounded below by a Type 1 sequence boundary, which is an unconformity marked by subaerial exposure and significant subaerial erosion associated with stream rejuvenation (increase in stream gradient and rate of flow caused by a lowering of base level). Type 1 boundaries are also marked by an abrupt basinward shift in facies so that nonmarine or shallow marine facies above the boundary overlie deep marine rocks below the boundary. Basically, a type 1 sequence begins with relative sea level at its lowest point.