Billions and billions of shells or cheniers...
What we're looking for are these white bands here. This is an aerial photo of a section of the mud flats. Here's the ocean out here. These white bands are called cheniers and they're shell ridges. What happened in the past is that occasionally the river would be diverted into the Salton Basin, something that happened inadvertently at least once. You get diversion of the Colorado into the Salton Basin. During this brief period of time when the river is no longer flowing into the gulf, the tides of the gulf which are the third highest in the world and are really spectacular tides, would re-work all of the sediment that was being deposited by the river and being of course accumulated by the biological organisms in the northern gulf.
So, the tides would re-work the sediment and essentially sort the sediment according to particle size and particle density. So what you get are these concentrations of large and heavy shells separated by these very broad expanses of fine, very, very fine particles and mud. These ridges are called cheniers. It's a French word; actually, it's a Cajun word. The only other cheniers that we know of in the world for sure are found in the Mississippi Delta of Louisiana. As we move away from the shore and inland, the cheniers, on average, contain older and older shells. They represent earlier and earlier diversions of the river.