The Sister’s Hill Site And Its Bearing On The Wyoming Postglacial Alluvial Chronology

Abstract

Alluvial sediments exposed at the Sis ter's Hill Site near Buffalo, Johnson County, Wyoming shed new light on the stratigraphy of the Ucross and Kaycee formations and reveal that another strati graphic unit occur s between the se forma tions in some places. At the Sister's Hill Site, Hell Gap artifacts occur in the lower part of the new unit and in associa tion with a Carbon-14 date of 9650 B.P. On the basis of an extensive geological survey covering more than half of Wyo ming and parts of the neighboring states of Montana, South Dakota, and Nebraska, Leopold and Miller (1954) demonstrated that a consistent sequence of alluvial events is represented in the stream val leys of this area and that this sequence preserves a record of climatic events from late Wisconsin time to the present. The generalized alluvial sequence demonstrated by Leopold and Miller is shown in Figure 1. The alluvial record reveals that at some unknown time in the past the majority of streams in the area incised their channels into bedrock and subsequently filled with gravels to form an overlapping gravel fill called the Ar vada formation. After a period of stabil ity and subaerial weathering this sequence of events was repeated to deposit gravels and silts of the Ucross formation in a similar overlapping relationship. Again a period of subaerial weathering ensued followed by downcutting but this time the subsequent filling consisted of silt and sand that again overlapped the valley slopes. This fine-grained fill is called the Kaycee formation and because no sub sequent fill has attained the level of its surface it forms a persistent terrace, the Kaycee terrace, which is easily rec ognized along most drainage s of the area. After streams had attained grade at the level of the Kaycee terrace they deepened their channels into the Kaycee alluvium and established a new grade at a lower level. Further downcutting followed with out an intervening period of alluviation. The stream-cut surface thus formed is the Moorcroft terrace. The channel cut through this surface was filled by inter bedded silt, sand, and gravel that repre sents the last period of alluviation. In most valleys the modern streams are incised a foot or so into this youngest fill and thus form the Lightning terrace. In some valleys the Lightning surface is the modern flood plain. The weathering profile developed in the Arvada gravels is characterized by strong iron oxidation and involutions, but