FRONTISPIECE. Aerial photograph of Denver. 1. Index map 3 2-3. Maps showing: 2. Physical divisions of the United States and maximum extent of the continental ice sheets 6 3. The Great Plains province and its sections 8 4. Photograph of Mescalero escarpment and southern High Plains 9 5. Geologic time chart 12 6-8. Maps showing: 6. Paleogeography of U.S. in Late Cretaceous 14 7. Structural setting of the Great Plains 14 8. Progressive southward expansion of areas of deposition 17 9. Photograph of Big Horn strip mine at Acme, Wyo. 18 10. Black Hills diagram 21 11-16. Photographs showing: 11. Weathering of granite at Sylvan Lake in the Black Hills 22 12. Capulin Mountain National Monument, N. Mex. 24 13. Mesa de Maya, Colo. 25 14. Spanish Peaks, Colo. 26 15. “The Gangplank,” Wyo. 27 16. Scotts Bluff National Monument, Nebr. 28 17. Aerial photograph of the Nebraska Sand Hills 30 18. Map of the Nebraska Sand Hills 31 19-30. Photographs showing: 19. Loess plain in Nebraska 32 20. Ground moraine on the Coteau du Missouri in North Dakota 34 21. Slump blocks in North Unit of Theodore Roosevelt National Memorial Park, N. Dak. 36 22. Highwood Mountains, Mont. 37 23. Devils Tower National Monument, Wyo. 38 24. Glaciated valley in Crazy Mountains, Mont. 39 25. Powder River Basin in vicinity of Tongue River 40 26. Badlands National Monument, S. Dak. 41 27. Badlands of Little Missouri River in South Unit of Theodore Roosevelt National Memorial Park, N. Dak. 42 28. Hogback ridges along the Front Range, Colo. 43 29. Pawnee Buttes, Colo. 44 30. Edwards Plateau, Tex. 47 TABLE 1. Generalized chart of rocks of the Great Plains 15 The GEOLOGIC STORY of The GREAT PLAINS By Donald E. Trimble INTRODUCTION