Date of Award

12-1-1993

Document Type

Thesis

Degree Name

Master of Arts (MA)

Department

Geography and Geology

First Advisor

Dr. John F. Shroder

Abstract

The Meadville slope failure, 24 km north of Ainsworth, on the north facing cut bank of the Niobrara River, is the largest such feature in Nebraska. Tertiary sands, gravels and siltstones, overly the unstable Cretaceous Pierre Shale which lies beneath the slope failure. The slope failure in this area has caused damage to roads, power lines and a bridge and has the potential to damage several buildings. Numerous slump and glide blocks, ground cracks, flow lobes and shear zones, have effected tree species including Pinus ponderosa (Ponderosa pine), Juiperus virginiana (red cedar), Quercus macrocarpa (bur oak), Fraxinus pennsylvanica (green ash), Tilia americana (American linden) and Populus deltoides (cottonwood). Trees situated on an unstable slope can help explain the internal morphology of a landslide. Dendrogeomorphological analysis of the temporal and spatial changes within the trees can lead to the identification of event-responses. This study involved widespread sampling of 168 Pinus ponderosa which had been affected by various slope movement activities. Pinus ponderosa were used because they have proven to be good time keepers in other studies. Core samples and cross-cut samples were used to determine how the trees reacted both internally and externally to slope movement activity. Strongly replicated events, i.e. something that affected the tree physically, either internally or externally, were recorded on a skeleton plot using event-response phenomena plotting. Index values were then computed which are used as an indication of movement. From these index values an event-response curve was produced which showed peak periods of movement. Recurrent slope movement within the past 100 years is attributed to the underlying stratigraphy, slope water and the continuous removal of the slope failure’s toe by the Niobrara River. Based on tree characteristics, long term movement was shown in 1906, 1909, 1916, 1923, 1924, 1952, 1973, 1978, 1983, 1985, 1986, and 1987. Years 1923 and 1983 produced the highest index values of 52.3% and 37.8% respectively. Above average precipitation was recorded in the two years (24 cm above average in 1923 and 17 cm in 1983). The data suggest a relationship between precipitation and slope movement. The trees were divided into 15 groups to gain a better understanding of the slope failure. One group of trees proved to be the most active of all groups, indicating the area of the shear zone.

Comments

A Thesis Presented to the Department of Geography/Geology and the Faculty of the Graduate College University of Nebraska In Partial Fulfillment of the Requirements for the Degree Master of Arts University of Nebraska at Omaha. Copyright 1993 Susan Kim Smulling.

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