Document Type

Poster

Location

Ellensburg

Event Website

http://digitalcommons.cwu.edu/source/

Start Date

19-5-2016

Keywords

Sandstone, Swauk Watershed, Spatial Distribution

Abstract

Large groups of gigantic sandstone and conglomerate monoliths populate the Swauk Watershed of northern Kittitas County. These monoliths rest on side slopes in the watershed and distinctively project from their surroundings. The origins of these features are unknown. We studied these monoliths in the field by mapping their spatial distribution, describing their morphology and composition, and measuring their orientation and sizes in order to determine their origins. We used Google Earth and topographic maps to locate the monoliths and map their distribution. Interpretations were based from field work data and past research. Our field results show commonalities between the features related to overall structure, composition, and geomorphology. All monoliths studied were associated with dipping strata. Dip slopes are gently sloping while anti-dip slopes are much steeper. The monoliths also have distinct and traceable conglomerate layers that are highly resistant to erosion, as well as thick sandstone layers and some smaller pebble layers. These features also share similar geomorphology: they are surrounded by channels; fresh surfaces are lichen-free; honeycomb weathering and overhangs dominate the anti-dip slopes; and prominent vertically aligned jointing parallels the dipping beds. These results indicate that geologic composition and structure play a significant role in the initial shaping of these landforms. Differential weathering, fluvial erosion, and mass movement weakened the sandstone to cause low bedrock escarpments to retreat on the slopes, which carved out vertically aligned joints. The repetitive cycle of weathering, mass movement, and stream erosion has ultimately been the cause of the isolation of the sandstone monoliths over time.

Poster Number

7

Faculty Mentor(s)

Karl Lillquist

Department/Program

Geological Sciences

Additional Mentoring Department

Geography

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May 19th, 12:00 AM

The Spatial Distribution and Origins of Sandstone Monoliths in the Swauk Watershed, Kittitas County, WA

Ellensburg

Large groups of gigantic sandstone and conglomerate monoliths populate the Swauk Watershed of northern Kittitas County. These monoliths rest on side slopes in the watershed and distinctively project from their surroundings. The origins of these features are unknown. We studied these monoliths in the field by mapping their spatial distribution, describing their morphology and composition, and measuring their orientation and sizes in order to determine their origins. We used Google Earth and topographic maps to locate the monoliths and map their distribution. Interpretations were based from field work data and past research. Our field results show commonalities between the features related to overall structure, composition, and geomorphology. All monoliths studied were associated with dipping strata. Dip slopes are gently sloping while anti-dip slopes are much steeper. The monoliths also have distinct and traceable conglomerate layers that are highly resistant to erosion, as well as thick sandstone layers and some smaller pebble layers. These features also share similar geomorphology: they are surrounded by channels; fresh surfaces are lichen-free; honeycomb weathering and overhangs dominate the anti-dip slopes; and prominent vertically aligned jointing parallels the dipping beds. These results indicate that geologic composition and structure play a significant role in the initial shaping of these landforms. Differential weathering, fluvial erosion, and mass movement weakened the sandstone to cause low bedrock escarpments to retreat on the slopes, which carved out vertically aligned joints. The repetitive cycle of weathering, mass movement, and stream erosion has ultimately been the cause of the isolation of the sandstone monoliths over time.

https://digitalcommons.cwu.edu/source/2016/cos/3