Well, surprise, surprise. NOT. I live in the Coachella Valley (Palm Springs area) of Southern California, also known as the Sonoran Desert or Colorado Desert. At one time, it was underwater. One can dig and find sea shells.
Quite a few years ago, I purchased a new home on a mesa above San Diego Stadium. So, it was about 50 to 75 feet above sea level. Every place I dug in the yard when planting my garden, I found sea shells.