Date of this Version
Detailed field mapping of outcrops in southern Garden and Morrill counties, Nebraska, has revealed a drop of more than 60 m (200 ft) in 4.8 km (3 mi) in the elevation of the contact between the Brule Formation and the Ogallala Group as exposed on the east side of Rush Creek. Beds of silt, sand, siltstone, and volcanic ash locally dip to the northwest at angles up to 8.5'. The contact between the two formations cannot be seen on the west side of Rush Creek but beds in the Ogallala Group there dip to the north and northeast. Rocks north of the North Platte River in the vicinity of Lisco dip to the south, completing the outcrop belt of the Rush Creek-Lisco Structural Basin.
The unusually long valley of Rush Creek runs along the axis of a north plunging synclinal fold within the structural basin, which probably controls the position of spring in the Ogallala Group feeding the only large perennial stream on the south side of the North Platte River in Garden County. The occurrence of Ogallala outcrops at river level along the south bank of the North Platte River just southeast of Lisco, Nebraska, is also related to this structural basin.