These are the trappings of vernal pools, otherwise known as hogwallows. Water is prevented from percolating underground thereby leaving evaporation the only means of escape. How mima mounds were formed is largely unknown and a subject of great debate. As the pocket gophers excavate their burrows, they push the soil toward these high points making them higher, consequently making the low points lower. Over time this, or a combination of the above processes, may account for the mounded topography around which vernal pools form.
Vernal pools are little known and no distribution map is complete.
Most of the known vernal pools in California are found in alluvial valleys like the Great Central Valley, coastal terraces of Southern California, and volcanic mudflows in Tehama and Riverside Counties. Beneath the surface swim fairy shrimp, a freshwater counterpart to the familiar brine shrimp that inhabit salt ponds around the bay. On the bottom of the pool scoot tadpole shrimp resembling miniature horseshoe crabs. These organisms depend on the ephemeral nature of vernal pools in order to complete their life histories. They have adapted to a habitat that is inundated with water during winter and spring, and completely desiccated the rest of the year.