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For millions of years, river flows and tidal action deposited sediment in the Delta, the low point of the Central Valley. Thick organic soil, commonly referred to as peat, was formed as tules and other plants were covered by this sediment throughout many years. These organic soils, up to 60 feet deep in some areas, were first farmed in the mid-1800s. Although highly productive for agriculture, peat is also very prone to subsidence. The map shows the various thicknesses of organic materials that can be found in the Delta.