Date of this Version
The occurrence of Quaternary anorthosite-rich sand and gravel deposits in south-central Morrill County, Nebraska, supports the idea that Pumpkin Creek formerly flowed farther east than it does today. This eastern extension of the creek was abandoned when another headward cutting tributary of the North Platte River cut through the divide between Pumpkin Creek and the North Platte just east of Jail and Courthouse rocks and captured Pumpkin Creek.
Unusual clast types found in Quaternary deposits along Pumpkin Creek in Banner County may be used to determine some characteristics of the streams that carried them. For example, armored mud balls and friable sand megaclasts occur in exposures of a complex sand and gravel alluvial fill which caps a strath terrace in southwestern Morrill County, Nebraska. The mud balls are limited to a tributary arroyo. The sand megaclasts occur in sediments deposited along a trunk stream. Size and sphericity of mudballs and coherency of the sand megaclasts may be used to determine distance of transport of the clasts and velocity of the streams which transported them.