The mine extracts lignite coal from the Jackson formation, seams A, B, C and D ranging in thickness from 1.0 to 4.5 feet with a combined average total lignite thickness of approximately 9 feet. The seams are separated by relatively thin partings which vary in thickness from 0.5 to 1.5 feet.

The deposit consists of four or more thin (generally < 1 m or 3.3 ft thick) lignite benches that are separated by claystone and mudstone partings (fig. 2). The partings are composed of altered volcanic air-fall ash that has been reworked by tidal or channel processes associated with a back-barrier depositional environment (Snedden and Kersey, 1981; Gowan, 1985; Ayers, 1986; Senkayi and others, 1987; and Warwick and others, 1996).

In the mine area, the Jackson strata dip to the southeast about 17 m/km (90 ft/mi) (Ayers, 1986). Gowan (1985) and Ayers (1986) suggest that the mud-dominated sediments the comprise the floor, partings, and roof of the San Miguel lignite deposit
originated in lagoonal mires landward (west) of the sandstone-dominated barrier island complex. The lignite-bearing interval is laterally persistent along strike for more than 40 kilometers (25 mi) (Ayers, 1986, 1989; Tewalt and others 1983).

The San Miguel lignite interval consists of four (or more) thin (generally <1 m or 3.3 ft thick) lignite benches that are separated by claystone and mudstone partings. The upper bed is designated as the A bed and is underlain by the B, C, and D beds (figs. 2, 3a) (Gowan, 1985). Tewalt and others (1983) referred to this interval as South Jackson seam no. 10. The rocks above the lignite beds are dominated by fine siltstone and claystone with occasional thin layers of gastropod and bivalve shells.

Kaiser (1982), Gowan (1985), and Ayers (1986, 1989) suggested that the rock partings within the San Miguel lignite deposit are composed of volcanic air-fall ash that has been reworked by possibly tidally influenced channel processes. Senkayi and others (1987) examined the partings in detail and found that most contained altered kaolinite and mixed detrital and volcanic quartz grains, thereby supporting the interpretation of a volcanic origin and subsequent detrital mixing of the sediment that forms these layers. These findings are consistent with the conclusions of Warwick and others (1996).

The overburden removal is achieved using two draglines, a 76-cubic-yard machine and a 30-cubic yard dragline. The 76-cubic-yard dragline moves nearly 21 million yards per year of overburden, while the 30-cubic-yard dragline moves about 7.7 million yards per year. Assisting the overburden operation is a 19-cubic-yard mass excavator with a fleet of 100 ton end dump trucks. The lignite is mined using a specialized mining tool called an Easy Miner. These machines resemble milling machines used to remove and recover asphalt and concrete highway surfaces and provides tight depth control to selectively mine the lignite or remove the parting, as well.