Whenever land rises, the streams flowing across the plain cut down and make deep, large valleys. This happened along the San Diego County coastline. Then, as climate warmed, ice sheets melted and sea level rose, flooding the valley into an estuary, and causing the deposition of soft sediments, which would be eroded easily when sea level dropped again. These deposits make up the Bay Point Formation of Pleistocene age, less than 1 million years old. Look for very plentiful, but broken, oyster and clam fossils looking very modern near the East end of the lagoon trail. No collecting please.
Because we are so close to a plate boundary, sections of the land continue to move sideways or up or down along faults. A small up-down fault cuts through the sediments in the road-cut on La Costa Avenue near the freeway. It is difficult to see with the brush that has grown on the face of the cut. Geologists think it crosses the lagoon toward the northeast. The fault is older than the terrace sediments above it, which are not displaced by it, but appear to have been deposited after faulting.