Fracking occurs in layers of rocky shale formations, typically deeper than 7000 ft below the surface. There are a few major areas where these shale formations are accessible and fruitful enough to be economically feasible. Major shale formations include the Permian and Eagle Ford formations in Texas, the Bakken Shale Formation in Montana and North Dakota, the Marcellus Shale Formation in Appalachia, and the Nibrara formation in South Dakota, Colorado, Nebraska, and Wyoming. The only significant shale formations in California lie in Kern, Kings, Tulare, and San Luis Obispo counties. These are relatively small formations.