26. Bentonite and gypsum, dark-yellowish-orange, seam consists mostly of gypsum which locally thickens to 0.06 foot . . . . . . . . . . . . . . . . . . 0.02 25. Shaly chalk, weathered, light-gray to very pale orange, tough, very obscurely laminated, speckled, with three seams of prismatic selenite lying parallel to bedding and some platy selenite along joints and bedding; FOSSILS - Inoceramus sp., Ostrea congesta . . . . . . . . . . . 1.4 24. Bentonite and gypsum, grayish-orange, seam consists mostly of prismatic selenite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.04 23. Shaly chalk, weathered, light-yellowish-gray to pale-grayish-orange, somewhat mottled, obscurely laminated, speckled, tough, weathering softer and flaky, with 0.2-foot-thick lensing bed of chalk 1 foot above base; platy selenite abundant on slope; FOSSILS - Inoceramus sp., Ostrea congesta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.15 22. Gypsum, prismatic, forms conspicuous marker . . . . . . . . . . . . . . . . . . 0.03 21. Shaly chalk, light-olive-gray with light-gray mottling, top 0.3 foot weathered grayish orange to light yellowish gray, like 23; 0.01-foot- thick layer of powdery gypsum lies 1.9 feet above base; FOSSILS - Ostrea congesta, Inoceramus platinus, Clionscaphites choteauensis, smooth bacu- litid, fish remains . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 20. Gypsum, two layers of prismatic selenite separated by a limonite seam; unit forms conspicuous re-entrant . . . . . . . . . . . . . . . . . . . . 0.04 19. Shaly chalk and chalk, light-olive-gray weathering very light gray, mottled, very tough, obscurely laminated, speckled; FOSSILS - Inoceramus simpsoni, Inoceramus platinus, Clionscaphites choteauensis, Ostrea congesta, smooth baculitid, fish remains . . . . . . . . . . . . . . 0.55 18. Bentonite, dark-yellowish-orange, with very thin seams of prismatic selenite and some powdery gypsum above and below . . . . . . . . . . . . 0.08 17. Shaly chalk, light-olive-gray weathering light gray, mottled with very pale brown, very tough, especially hard in upper 0.4 foot, all obscurely laminated, speckled, very fossiliferous; FOSSILS - Ostrea congesta, Inoceramus platinus, smooth baculitid . . . . . . . . . . . . . . . 3.2 16. Bentonite and gypsum; bentonite is 0.03 foot thick, moderate yellowish brown; seams of prismatic selenite above and below bentonite; unit forms conspicuous re-entrant . . . . . . . . . . . . . . . . . . . . . 0.09 15. Shaly chalk, light-olive-gray to moderate-yellowish-brown, weathering pale olive gray with light-gray banding near top and darker banding in lower part, tough, obscurely laminated, speckled, softer and flaky in dark-banded part; FOSSILS - Inoceramus platinus, Ostrea congesta . . 4.65 14. Bentonite and gypsum, bentonite is light brown, limonitic, unit composed mostly of granular and powdery gypsum . . . . . . . . . . . . . . . . . 0.04 13. Shaly chalk, light-olive-gray to moderate-yellowish-brown, weathering pale olive gray mottled with grayish orange; near base unit is tough and hard, blocky, grading upward to softer and flakier weathering rock, all speckled and obscurely laminated, with much powdery gypsum and some platy gypsum along joints and bedding planes; FOSSILS - Inoceramus platinus, Ostrea congesta . . . . . . . . . . . . . . . . . . . . 1.45 12. Gypsum, mostly white and powdery with a streak of limonite, probably a weathered bentonite seam . . . . . . . . . . . . . . . . . . . . . . 0.02 11. Shaly chalk, light-olive-gray weathering pale olive gray and locally mottled with light gray, with two darker bands in lower half; like 23; darker bands weather very soft and flaky; much powdery gypsum through- out; FOSSILS - Inoceramus platinus, Ostrea congesta . . . . . . 9.6 10. Bentonite, dark-yellowish-orange, less than 0.01 foot thick, sandwiched between seams of limonite and nearly white powdery gypsum, with scattered limonite nodules up to 0.1 foot thick and 0.7 foot across . . . . . 0.04