Salton Sea concretions

Along the southwest side of Salton Sea, northeast of San Diego, in Imperial County, southern California, sandstone concretions weather out in abundance from outcrops of the non-marine Borrego Formation of Pliocene age (about 2 million years old). These concretions have a variety of shapes, and some are very unusual. A selection of the shapes is shown here.

The largest concretion (the one that looks like a rabbit's head) is about 18 cm long (about 7 inches).

Concretions like these form by waters percolating through sediment. Over time, dissolved minerals in these waters cement
the sediment together and create these bizarre shapes. Some of the concretions found in this formation can several feet in size.