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##### U.S. Geological SurveyScientific Investigations Map 2924

## Geologic Map of the Lacamas Creek Quadrangle, Clark County, Washington 

### *By* R.C. Evarts

### 2006

| The Lacamas Creek 7.5' quadrangle is in southwestern Washington, approximately 25 km northeast of Portland, Oregon, along the eastern margin of the Portland Basin, which is part of the Puget-Willamette Lowland that separates the Cascade Range from the Oregon Coast Range. Since late Eocene time, the Cascade Range has been the locus of an episodically active volcanic arc associated with underthrusting of oceanic lithosphere beneath the North American continent along the Cascadia Subduction Zone. Lava flows that erupted early in the history of the arc underlie the eastern half of the Lacamas Creek quadrangle, forming a dissected terrain, with elevations as high as 2050 ft (625 m), that slopes irregularly but steeply to the southwest. These basalt and basaltic andesite flows erupted in early Oligocene time from one or more vents located outside the map area. The flows dip gently (less than 5°) west to southwest. In the western part of the map area, volcanic bedrock is unconformably overlain by middle Miocene to early Pleistocene(?) sediments that accumulated as the Portland Basin subsided. These sediments consist mostly of detritus carried into the Portland Basin by the ancestral Columbia River. Northwest-striking faults offset the Paleogene basin floor as well as the lower part of the basin fill. In middle Pleistocene time, basalt and basaltic andesite erupted from three small volcanoes in the southern half of the map area. These vents are in the northern part of the Boring volcanic field, which comprises several dozen late Pliocene and younger monogenetic volcanoes scattered throughout the greater Portland region. In latest Pleistocene time, the Missoula floods of glacial-outburst origin inundated the Portland Basin. The floods deposited poorly sorted gravels in the southwestern part of the Lacamas Creek quadrangle that grade northward into finer grained sediments.

This map is a contribution to a program designed to improve geologic knowledge of the Portland Basin region of the Pacific Northwest urban corridor, the densely populated Cascadia forearc region of western Washington and Oregon. More detailed information on the bedrock and surficial geology of the basin and its surrounding area is necessary to refine assessments of seismic risk, ground-failure hazards and resource availability in this rapidly growing region.

 

 
**File         Name******File         Type and Description****File         Size**FILES for VIEWING and PLOTTING******LacamasCrk_text.pdfPDF file of the 22-page Geology pamphlet containing description of map912 KB LacamasCrk_map.pdfPDF file of 40" x 36" map sheet that can be used for viewing map in a browser, as well as for plotting (geologic map, description of map units, and correlation of map units)2.3 MBLacamasCrk_map.jpgJPG file of 10" x 8" 300 dpi version of the map sheet that can be used for viewing map in a browser or printing on letter-sized paper1.8 MB**README**readme.pdfPDF readme file that explains the contents of the two data files88 KB **DATA**metadata.htmlMetadata80 KBSIM2924_DB.ZIPZipped file that contains geodatabase data objects5.2-MB file that opens into a 20.6-MB folderSIM2924_SHP.ZIPZipped file that contains the shapefile package3.2-MB file that opens into an 8.6-MB folder
For questions about the content of this report, contact Russ Evarts 
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| Department of the Interior | U.S. Geological Survey |URL of this page: https://pubs.usgs.gov/sim/2006/2924/Maintained by: Michael DigglesDate created: June 19, 2006Date last modified: June 20, 2006 (mfd)******** |
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