About 50 million years ago, at the start of the Eocene Epoch of the Cenozoic Era of Earth’s history, western San Diego County was still attached to coastal Mexico, and was partly inundated by the ocean. (Dinosaurs had become extinct about 65 million years ago, probably killed off with many other species of animal and plant life by the impact of a huge meteorite near the Gulf of Mexico.). The coastline had bays and lagoons, much like the coast of today. The sediments brought by the rivers draining the land to the east were being deposited into the still water as a greenish-gray mud. Oysters, clams, worms and other marine invertebrates were making their home in the shallow water. This mud eventually was compressed into rock and exists today as the Del Mar Formation. It can be seen easily at the base of the sandstone cliff facing the ocean from La Jolla to Encinitas. The marine life is visible as very plentiful fossils. The tidepools at Swami’s Beach are in this formation.