*Local geology*
The rock outcroppings along the North Platte River Valley at Lake McConaughy provide a glimpse into what life in Nebraska was like back in days gone bye. It is along the south shore of Lake McConaughy that excellent exposures of the Ogallala group are seen. The sedimentary rocks were deposited during the Miocene epoch of the Tertiary period of geologic time.This group of rocks is not just here, but throughout much of Nebraska. It is however best seen here whereas most of the rest of the state is covered by younger sediments. A close up inspection of these rocks will show you abountance of locally abundant fossils of seeds, casts of pedotubules, and occasional fossils of vertebrate animals such as rhinoceroses, horses, elephants, and camels. Pedotubules are tubular openings in soil. They may have been made by plant roots or by worms, insects, or other animals.
If deposition occurred in this region during Cambrian, Ordovician, Silurian, Devonian, or Mississippian periods of geologic time, subsequent erosion removed all traces of it. During the Pennsylvanian and Permian periods, the midcontinent of the United States was covered by a succession of shallow seas. Evidence that the Lake McConaughy region is underlain by rock layers deposited in those seas consists of limestones and shales containing fossils characteristic of Pennsylvanian and Permian marine deposits. The shallow seas of the Pennsylvanian and Permian periods teemed with algae, other marine vegetation, protozoans, sponges, corals, bivalves, snails, brachiopods, echinoderms, trilobites, primitive fishes, and sharks. The upper Permian rocks in this area include many maroon-colored beds of shale and sandy shale. Rocks of Triassic, Jurassic, and Early Cretaceous ages also are missing. Like rocks in the Cambrian through Mississippian periods, they either were not deposited or were deposited and then entirely removed from the area by erosion. In Late Cretaceous time, however, shallow seas again invaded the central part of North America. The first Cretaceous deposits in the Lake McConaughy region consist of sandstones and shales laid down on beaches and in tidal flats. As the marine transgression advanced, a younger sequence of marine limestones and shales was laid down over the older near-shore deposits. Where exposed in other places, Cretaceous marine rocks contain fossils of protozoans, clams, ammonites, fishes, sharks, and swimming reptiles and Cretaceous nonmarine rocks contain fossils of dinosaurs, primitive mammals, and nearly modern plants. Both types of deposits also contain fossils of flying reptiles and birds. Almost certainly such fossils occur in the Cretaceous rocks underlying the Lake McConaughy region, even though they are deeply buried. By the end of the Oligocene epoch of the Tertiary period, a sheet of continental sediments covered all or nearly all of western Nebraska. Known as the Brule Formation, it is exposed at the south end of Kingsley Dam on the downstream side. Most of this formation originated as wind-deposited silt that later was partly consolidated into massive siltstone. As much as 60 percent of the silt consists of shards, or fragments, of tiny bubbles of volcanic glass. For such a tremendous quantity of shards to have accumulated, a great many volcanoes must have been spewing ash into the atmosphere. Since the Brule has few vertical or lateral differences in lithology, it is unlikely that streams played much of a role in its deposition. Although conditions then probably tended to be arid and dusty, the existence of lime-rich lenses containing a few fossil snails, numerous fossil ostracods, and possibly fossils of charophyte algae indicates that precipitation at times was sufficient to produce overland runoff and to maintain ponds. Fossils occurring in the Brule are similar to those from the Ogallala. Vertebrate animals such as oreodonts lived during this time. Rocks of the Ogallala Group, deposited during the Miocene epoch, are typical of stream and floodplain deposits in that they differ greatly both vertically and laterally. The presence of fossil seeds of grasses and borage herbs indicate that the floodplains and probably the interstream uplands supported meadows. Skeletons of some of the vertebrates that lived during this time are preserved as fossils in the Ogallala Group. The present topography near Lake McConaughy developed in several steps. First, the wind-deposited Brule sediments were eroded. Then, floodplain deposits of the Ogallala Group were deposited on the uneven erosional surface. Subsequently, the Ogallala deposits were eroded and later were covered with more floodplain deposits and lastly with loess, or wind-deposited silt. Erosion since the loess was deposited has carved numerous deep canyons into the Cheyenne Table, which is adjacent to the south side of the North Platte River valley. Exposures of the Ogallala Group are common in the canyons immediately south of Lake McConaughy, and exposures of both the Brule Formation and Ogallala Group are common in the canyons on both sides of the North Platte upstream from the lake. Test drilling by the Conservation and Survey Division of the University of Nebraska Lincoln has shown that loess, which is the surficial deposit on the upland south of Lake McConaughy, is also present north of the lake but there it is mantled by younger (Recent) sand dunes.
- Pabian, Diffendal, Gould (1981)
Most of the core of the dam is made of the Brule Clay from the Nebraska Loess Hills along the south side of the Platte River. The loess here is of Pleistocene age and is predominantly a fine sandy loam of pale brownish-buff color. The color here is not mostly brownish-buff in color, in fact, it has a pinkish color mixed with the brown. The pinkish color is Brule clay. The cliff is made up mostly of this mix of Loess and clay with some limestone and sandstone rocks mixed in. Looking along the edge of the bay here you will see mostly limestone and sandstone that make up the cliffs along the edge of the water. Loess comes from the German language meaning loose or crumbly. Loess is a sedimentary deposit composed largely of silt-size grains that are loosely cemented by calcium carbonate and clay. Loess can be deposited by three main ways; eolian (deposited by the wind) fluvial (deposited by a river) or lacustrine (formed in a lake). On the estern side of Nebraska the famous loess hills there where created by eolian.