While the vernal pools at Mather Field range from 50,000 to 200,000 years old, the soils under them began forming more than two million years ago. Dissolved minerals slowly moved down through the soil, eventually forming a hardpan layer. This hardpan lies a few inches to a few feet below the soil surface.
In a vernal pool grassland, the movement of rainwater through the soil is stopped by a cement-like layer called hardpan. The water stays perched above the hardpan, where we see it in the shallow depressions called vernal pools. The pools host hundreds of species of tiny aquatic organisms.
The American Pillwort is related to the fern. It grows from spores instead of seeds and does not make flowers. It is most easily observed on the drying mud around and in vernal pools early in the spring. Learn more