Field Guide to the Geology of the Harlan County Lake Area, Harlan County, Nebraska — with a History of Events Leading to Construction of Harlan County Dam

The year 2002 will mark the fiftieth anniversary of completion of the Harlan County Dam and Multi-Purpose Reservoir. This seems a good time to write about the history of and reasons for building the dam, the effects of the dam and reservoir on the Republican River valley, and the geologic features seen along the shores of the lake and areas nearby. As many junior authors helped produced this educational circular, they are listed in the table of contents. All other sections were written by the senior author. Includes July 2016 update: Addendum of photographs showing fault traces on drought-exposed lake bottom. Cretaceous Niobrara Chalk Formation and the younger Pierre Shale are also shown. Table of Contents Introduction 1 Location 1 Republican River Floods 1 The Republican River Flood of 1935 - D R Mohlman 1 Cautionary Notes (sidebar) 1 Events After the Flood Leading to Creation of Harlan County Lake 9 Stormer Ford Dog Town (sidebar) 10 Water 10 Surface Water Hydrology - S Summerside 10 Shoreline Springs and Seeps - F: E Harvey and K J Warren 12 Introduction 12 Using Water Chemistry to Identify Spring and Seep Origin 1 2 Geology 15 General Conditions Leading to Good Exposures 15 Stratigraphy of Unconsolidated Sediments and Rock Strata 15 Cretaceous System - Upper Cretaceous Series 15 Niobrara Formation - Smoky Hill Chalk Member 15 Pierre Shale 15 Tertiary System - Miocene Series 16 Ogallala Group -Ash Hollow Formation 16 Quaternary System - Pleistocene Series 16 Crete (?) Formation (Sands and Gravels) 1 6 Loveland Loess 16 Gliman Canyon Formation 16 Peoria and Bignell Loesses 16 Quaternary System - Holocene (Recent) Series 1 6 Unnamed Fluvial, Eolian, and Colluvial Deposits 16 Paleontology of the Various Formations 16 Plants 16 Invertebrates - R K Pabian 17 Cretaceous Marine Fossils 17 Niobrara Formation 17 Foraminifers 17 Post-Devonian Stromatoporoids 1 9 Mollusks 19 Bivalves 19 Ammonites 19 Pierre Shale Formation 19 Bivalves 19 Ammonites 19 Quaternary - Pleistocene (Ice Age) Invertebrate Fossils 19 Vertebrate Fossils - R G Corner 24 Late Cretaceous Fossils 24 Niobrara Formation 24 Niobrara Fishes 24 Niobrara Mosasaurs 29 Pierre Shale Formation 30 Protosphyvaena gladius: Giant Mystery Fish 30 Pierre Mosasaurs 32 Late Miocene Ogallala Group 32 Bone Cove I Fauna 32 Bone Cove II Fauna 32 Quaternary - Pleistocene (Ice Age) 33 Crete (?) Sands and Gravels 33 Gilman Canyon Formation 33 Peoria Loess 35 Late Pleistocene Fauna 35 Late Pleistocene Environments 37 Geologic Processes Affecting the Strata 37 Modem and Ancient Landslides 37 Modern Landslides D A Eversoll 37 Ancient Landslides - D A Eversoll and R F Diffendal, Jr 39 Geologic Structures 41 Introduction and Previous Work 41 Angular Unconformities 45 Joints 45 Faults and Folds 45 Ancient Landscapes (Paleotopography) 50 Quaternary River Terraces 50 Concluding Remarks 50 References Cited 51 Glossary Keywords: flooding 1930s, vertebrate fossils, earthquake faults, landslides, groundwater qualit