Complex sentence: Four million years ago, the Colorado River started to enter into the area, and the formation of the Colorado River Delta separated the Salton Trough during the Pleistocene from the Gulf of California; during the Pliocene the connection still existed.
Split into simpler sentences: Four million years ago, the Colorado River started to enter into the area. The formation of the Colorado River Delta separated the Salton Trough during the Pleistocene from the Gulf of California; during the Pliocene the connection still existed.