Grindstone Creek Formation, expanded (TXPAgr;0)

State	Texas
Name	Grindstone Creek Formation, expanded
Geologic age	Phanerozoic \| Paleozoic \| Carboniferous Pennsylvanian [Des Moines]
Original map label	IPgr
Comments	shale, in part sandy, contains local coal beds and sandstone lentils, gray, thickness of the exposed part of Grindstone Creek Formation, 225 ft. Buck Creek Ss. is top part of the map unit and is coarse-grained, massive, reddish-brown, forms prominent scarp, thickness about 30 ft; Brannon Bridge Limestones in lower part of map unit, fine-grained, some interbedded shale, dark chert lenses, bedding uneven, indistinct to medium gray units as much as 15 ft thick, form scarps and broad dip slopes; about 10 ft of shale separates the two limestone units.
Primary rock type	shale
Secondary rock type	sandstone
Other rock types	limestone
Lithologic constituents	Major Sedimentary > Clastic > Mudstone > Shale > Black-shale (Bed)gray shale, locally sandy and silty Minor Sedimentary > Clastic > Sandstone (Bed) Sedimentary > Carbonate > Limestone (Bed)Brannon Bridge Limestones with 10 ft of shale between
Map references	Bureau of Economic Geology, 1992, Geologic Map of Texas: University of Texas at Austin, Virgil E. Barnes, project supervisor, Hartmann, B.M. and Scranton, D.F., cartography, scale 1:500,000
Unit references	Plummer, F.B., and Hornberger, J., Jr., 1936, Univ. Texas Bull. 3534, p. 16. Bureau of Economic Geology, 1972, Dallas sheet, Geologic Atlas of Texas, Bureau of Economic Geology, University of Texas at Austin, scale 1:250,000.
Geographic coverage	Erath - Palo Pinto - Parker