Paper No. 3-4
Presentation Time: 8:30 AM-5:30 PM
COMPARISON OF BEACH GRAVEL SHAPES, WESTERN WASHINGTON STATE
PONZINI, Caroline J., Geology, Univ of Puget Sound, 1500 North Warner, WSC Box 3519, Tacoma, WA 98416, cponzini@ups.edu, BARRON, Caleb, Geology, Univ of Puget Sound, 1500 North Warner, WSC Box 2496, Tacoma, WA 98416, and HAHN, Matt R., Geology, Univ of Puget Sound, 1500 North Warner, WSC Box 2149, Tacoma, WA 98416

COMPARISON OF BEACH GRAVEL SHAPES, WESTERN WASHINGTON STATE

Various studies have been conducted which investigate the properties of river and beach gravels. These studies have concluded that characteristic properties can be related with the environment in which gravel is formed. The purpose of our study was to compare shapes of beach and river gravels and determine if they are consistent with the findings of past studies (Dobkins and Folk, 1970; Els, 1988; Stratten, 1974). Gravels from Owen Beach at Pt. Defiance Park in Tacoma, Washington State, were collected. These gravels originate from the coastal bluffs, which expose Pleistocene glacial and interglacial sediment deposits in the near shore area. Other gravels, comprised of mostly granite, were collected from the Nisqually River, a runoff channel of Mt. Rainier in Elbe, Washington State. Samples from both sites, comprising more than 400 pebbles from five different sample locations were measured for their long (a), intermediate (b), and short (c) pebble axis. From these, the coefficient of flatness, sphericity, and oblate-prolate index were calculated. Axis ratios b/c and c/a were plotted on a Zingg diagram to determine pebble shapes.

The data collected from the Nisqually samples shows a strong consistency with Dobkins and Folk’s approximate lower index limits for pebble shape in a fluvial environment. The Owen Beach gravel also shows similar dimensions, which do not follow the common shapes of beach gravel. Previous studies conclude that Beach gravels contain more blade and rod shaped pebbles, but the Owen Beach samples do not follow this pattern. The coastal bluffs that feed the shore in South Puget Sound are composed of glacial and fluvial sediments. Therefore, we have concluded that since these gravels were glacial-fluvial in origin, they retained their original properties and show a greater similarity with river gravels than beach gravels. The Owen Beach samples have not undergone enough shoreline processes to warrant a change to common beach gravel dimensions.

Cordilleran Section - 99th Annual (April 1–3, 2003)
General Information for this Meeting
Session No. 3--Booth# 20
Sedimentology and Sedimentary Rocks (Posters)
Hotel NH Krystal: La Capilla
8:30 AM-5:30 PM, Tuesday, April 1, 2003
 

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