| Washington Formation | |
| Type: | Formation |
| Age: | Permian |
| Period: | Permian |
| Prilithology: | limestone, sandstone, coal |
| Otherlithology: | shale |
| Region: | and |
| Country: | United States |
| Unitof: | Dunkard Group[1] |
| Subunits: | Upper Washington Jollytown coal Hundred sandstone Upper Marietta Washington “A” coal Middle Washington Lower Washington Lower Marietta Washington (No. 12) coal Little Washington coal Mannington sandstone Waynesburg “A” coal Waynesburg Sandstone Elm Grove limestone Cassville |
| Underlies: | Greene Formation |
| Overlies: | Waynesburg Formation |
The Washington Formation is a coal, sandstone, and limestone geologic formation located in Ohio, Pennsylvania and West Virginia. It dates back to the Lower Permian period, with its base at or near the Permian/Carboniferous boundary.[2] [3] The Washington formation and the Dunkard Group as a whole was deposited at a time when the continents were in the process of forming the "Super Continent" Pangaea as well as a gradual drop in sea levels.[4] The result during this period was coals being thinner and impure with high ash content. The limestones found with in the formation are exclusively freshwater deposits.