Coal Resource Classification System of the U.S. Geological Survey
By Gordon H. Wood, Jr., Thomas M. Kehn, M. Devereux Carter, and William C. Culbertson
GEOLOGICAL SURVEY CIRCULAR 891
Table 7. - Areas of resource categories from figure 19 listed in proper reliability and thickness of coal and overburden categories
[Average thickness of coal in each are can be determined by utilizing coal isopachs]
| Geologic | Coal thickness | ||
| 14 to 28 inches | 28 to 42 inches | 42 to 84 inches | |
| 0 to 500 feet overburden: | |||
| Measured | A, F, G, H | B, E | C, D |
| Indicated | I, MM | I, J, P | K |
| Inferred | CC, Y, S | AA, Z | W |
| 500 to 1,000 feet overburden: | |||
| Indicated | L, R, M | N, Q | O |
| Inferred | U, BB | V, X | T |
| 1,000 to 2,000 feet overburden: | |||
| Inferred | DD, FF | EE | None |
| Hypothetical | GG, JJ | None | Do. |
| 2,000 to 3,000 feet overburden | |||
| Inferred | LL, II | HH | OO |
| Hypothetical | NN | None. | None |
| 3,000 to 6,000 feet overburden | |||
| Hypothetical | KK | None. | None |
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