VERNAL POOL GEOLOGY

Mather Geology: The Landforms and Soils of Mather Regional Park  The vernal pools in Mather Regional Park are special because of the rare flowering plants and invertebrates that live in them. Yet vernal pools and the life that depends on them could not exist if it were not for the long history of landscape formation and soil development that created the conditions in which vernal pools can exist. Earthquakes, floods, and volcanoes have all played an essential role in the formation of vernal pools. How did such powerful earth forces create such a peaceful landscape? Many questions come to mind.

How did the Central or Sacramento Valley originate?  During the time of the dinosaurs a hundred million years ago, most of what is now California did not exist. The rocks that later became California were forming on the floor of the Pacific Ocean. What was to become the Central Valley was a two-mile deep trench in the floor of the ocean. California was gradually constructed over millions of years by the collision of segments or plates of the earth's oceanic and continental crusts and the motions of earthquakes. Throughout this time, sediment eroded from ancient mountains and shorelines to completely fill the deep trench, thereby forming the floor of the Valley.

How did the present land surface form?  The land surface of Mather Regional Park resulted in floods that deposited sediment on the floor of the Sacramento Valley. The floods occurred starting about two to three million years ago, shortly after the rise of the Sierra Nevada mountains that we can see today (on a clear day!) to the east. The floods occurred during a past ice age, when the climate was wetter and cooler than today, and at the end of the ice age when the glacial ice in the mountains melted. Wooly mammoths, saber-toothed tigers, and giant sloths lived on the floor of the Valley at this time. Human beings were still evolving in other parts of the world. Geologists or geomorphologists call the land surface the Laguna Formation; it is the oldest alluvial (flood deposited) land surface in the Central Valley.

How did the mounds and depressions form? Geomorphologists and soil scientists are not certain how the shallow mounds and depressions formed on the land surface, and many theories have been proposed. The most likely explanation is that later floods that occurred during more recent ice ages washed over and scoured depressions in the land surface of the Laguna Formation. These floods then deposited new sediment on lower terraces to the south (the Turlock Lake Formation ) about 600,000-700,000 years ago, and to the west (the Riverbank Formation, named for the town of Riverbank in Stanislaus County), about 200,000 to 400,000 years ago. Even more recent floods formed mounds and depressions in the surface of the Turlock Lake and Riverbank Formations. Human beings still did not live in the Sacramento Valley at that time, as humans did not arrive in California until about 12,000 years ago.

What kind of soils form vernal pools in Mather Regional Park? Soil scientists give names to different types of soils that often refer to a geographic feature near the area where the type or series of soil was first discovered. The two most common series of soils on the Laguna Formation in Mather Regional Park are called the Red Bluff and Redding series. The two most common types of soil on the Riverbank Formation to the east of Mather Regional Park are called the San Joaquin and Hedge series.

Below the first few inches of the soil, the San Joaquin, Redding, and Red Bluff soils are red in color. The red color comes from iron oxides, a material more commonly known as rust. There is iron in the soil that rusts as the soil weathers. The Redding and Red Bluff soils are redder than the San Joaquin soil because they are older and more rust has formed over a longer period of time.

The Redding, San Joaquin, and Hedge soils all typically contain vernal pools. The Red Bluff soil does not, as it occurs on the mounds and has a convex surface. All four soils contain subsurface hardpans, layers of rock-like material that formed in the soils about two to four feet below the soil surface. The hardpan is the weakest in the Hedge soil because it is the youngest soil. All four soils have layers of clay accumulation above the hardpan. For the Red Bluff, Redding, and San Joaquin soils, the clay has become thick and dense, forming a claypan.

The San Joaquin soil has special significance as it has been officially designated the State soil of California by the State Legislature. California has joined many other states in the country in designating a state soil. The San Joaquin soil was chosen as the State soil because it is very extensive in the Central Valley, it is an important agricultural soil as well as a vernal pool soil, and it has special features that are characteristic of soil formation in the climate of California.

Why does the soil hold water for long periods of time forming vernal pools?  Most soils hold water for only a short period of time after it rains, because the water seeps through the soil under the pull of gravity. The soils that have formed on the Laguna and Riverbank Formation hold rainwater during the winter and spring months because both the hardpans and claypans are mostly impervious to the downward percolation of rainwater. Where depressions occur on these soils, rainwater tends to pond, forming vernal pools.

Where did the cemented hardpans come from?  All soils on the surface of the earth evolve or change over long periods of time. Soils change largely because of the effect of climate, water, and vegetation on the mineral soil material. Soil scientists can learn much about the history and formation of land surfaces by learning about the different kinds of soil and the changes that have occurred in them. The cemented hardpan has formed as a result of soil weathering processes. The hardpan is cemented with a substance called silica. Silica is the most abundant mineral on the earth. It is the primary component in glass and sand. It is also the material used to make silicon chips for computer circuitry. Silica is abundant in the soils of the Laguna and Riverbank Formations.

Although silica is usually a hard solid, it can slowly dissolve over time. Silica most readily dissolves from volcanic ash. Volcanic ash could have been deposited on the soils of the Mather area by the ashfall of a large volcanic eruption. One possible source is the enormous eruption of the Long Valley volcano about 700,000 years ago. The Long Valley volcano is on the east side of the Sierra Nevada mountains, east of Yosemite National Park. Its eruption was one of the most powerful ever known to have occurred. The town of Mammoth Lakes now sits in the remains of the very large crater formed by the explosion. It is quite possible that large amounts of volcanic ash and pumice from this explosion landed on the Mather area. Another possible source of volcanic ash is the Sutter Buttes volcano to the north.

The abundant silica in the volcanic ash and pumice could have dissolved into the soilwater solution and filtered down through the soil profile. In the subsoil, the silica precipitated out of solution, filled in the pores between the soil particles, and hardened into a "cement", impeding the further downward flow of water and causing small ponds or vernal pools to form. The vernal pools could therefore be about half a million years old, although their age is not known for certain.

Where did the claypans come from?  After the hardpans formed, changes in the climate and vegetation resulted in different weathering and soil forming processes. Clays generally form in a more humid climate; hardpans form in a semi-arid climate, the same kind of climate that exists in the Sacramento Valley today. Certain types of clays form in certain types of soil weathering processes. The clay can also move downward in the soil profile by the percolation of rainwater where the clay was deposited on top of the hardpan. The clay is a type of clay that swells when it is wet, making it even more difficult for any more water to pass through. The claypan also contributes to the soil impermeability making it possible for vernal pools to form.

How fertile are the vernal pool soils?  Relative to many other soils in the Central Valley, vernal pool soils are not very fertile Under natural conditions very few trees can grow on them, because the shallow root zone above the hardpans and clay pans is too shallow for trees to get enough nutrients and water to grow. That is why grasses and small flowering plants are the native vegetation. The most important source of nutrients for the vernal pool vegetation and invertebrates is the small amount of organic matter that is slowly recycled in the vernal pool ecosystems, and the nutrients that flow into the pools from the mounds. The right balance of nutrients is necessary, too many nutrients could be as harmful to the plants and animals that grow in vernal pools as too few nutrients.

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