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The ground beneath our feet is not uniform; in fact it consists of many different sediment layers that can vary in consistency from fine particles (clay) to coarser sands and gravels. Generally, coarser materials like sands and gravels allow water to move through them more freely. Geologic layers consisting of these coarser materials are called aquifers. It is important to note that an aquifer is not usually a cavern under the ground that is filled with water, rather it is geologic formation in which the spaces between the granular particles are filled with water.

The shapes of the aquifers in our county have been subjected to slow deformation over very long periods of time (millions of years). Although the sediments in our aquifers were originally laid down horizontally, over geologic time they were bent upwards and downwards. So, the same aquifer that is several hundred feet deep at one location in the county can be exposed at the ground surface at another location. This is the case in Santa Cruz County where the geologic formations that serve as deep aquifers in the Pajaro Basin are exposed at the ground surface in the mid-county area of Aptos.