THE ORE BIN - Volume 39 - No. 4 - Field guide to the geology of Corvallis and vicinity, Oregon

Sep 29, 2026

Field guide to the geology of Corvallis and vicinity, Oregon, prepared from an environmental geology class originally taught in spring 1971. The guide is organized as a self-guiding, two-part bicycle trip emphasizing practical local geology, bedrock structure, and physiographic setting. It explains regional geomorphic provinces, the Corvallis fault and major landforms, and summarizes four key bedrock units and their Eocene to Oligocene history, including volcanic, sandstone, and intrusion-related processes, plus later unconsolidated valley deposits.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages24

Reading Time1 h 0 min

AccessFree to download

Introduction and Geologic Background

Two-part self-guiding field trip overview

Geomorphic provinces and regional setting

Corvallis fault and bedrock units

Siletz River Volcanics

Tyee Formation

Spencer Formation

PRIL1977VoLUME 39,No,4 THE ORE BIN The Ore Bin Published Monthly By STATE OF OREGON DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIESHead Office:1069 State Office Bldg.,Portland,Oregon 97201Telephone:[503]229-5580 FIELD OFFICES 2033 First Street 521 N.E。"E"StreetBaker 97814 Grants Pass 97526 Subscription Rates1 year,$3.00;3 years,$8.00Available back issues,$.25 at counter,$.35 mailed Second class postage paid at Portland,Oregon GOVERNING BOARD R.W.deWeese,Portland,ChairmanLeeanne MacColl,PortlandRobert W.Doty,Talent STATE GEOLOGIST Ralph S.Mason GEOLOGISTS IN CHARGE OF FIELD OFFICESHoward C.Brooks,BakerLen Ramp,Grants Pass Permission is granted to reprint information contained herein. Credit given the State of Oregon Department of Geology and Mineral Industriesfor compiling this information will be appreciated. The 0RE BINVolume 39,No.4April1977 FIELD GUIDE TO THE GEOLOGY OF CORVALLIS AND VICINITY,OREGON R.D.Lawrence,N.D.Livingston,S.D.Vickers,and L.B.ConyersGeology Department,Oregon State University,Corvallis In troduction and Geo1ogic B a ck groun d This field guide had its origin in a class in environmental geol-ogy taught in the spring of 1971.The original road log has beenmodified so that those with an introduction to elementary geologywill find it self-guiding.It emphasizes the practical aspectsof local geology.It is divided into two parts,each of whichprovides a pleasant bicycle trip for an afternoon.The firsthalf of the trip discusses the area west and north from Corvallisand deals largely with the bedrock geology of the area and thestructure responsible for major landforms.The second half coversthe area east and south of town and deals largely with surfacesediments and the behavior of the Willamette River. Western Oregon is divided into four geo- norphic provinces:the Cascade Range,the Wil-lamette Valley,the Coast Range,and the Kla-math Mountains.Corvallis,located at the west-ern edge of the Willamette Valley adjacent tothe Coast Range(Figure 1),lies on the westbank of the Willamette River,mostly north ofthe mouth of Marys River.The Coast Range reachesits maximum elevation about 15 miles west of townat Marys Peak.From Marys Peak,a long ridgecalled Vineyard Mountain(MacDonald State For-est)trends northeast past town.The Corvallisfault,located along the southeastern edge ofthis ridge,separates erosion-resistant volcanicrocks on the northwest from soft sedimentaryrocks which are mostly covered by valley alluviumon the southeast(Figure 2).The Corvallis faultforms the eastern boundary of the Coast Range nearCorvallis. The four bedrock units under the CorvallisFigure 1.Geomor-area are,from oldest to youngest:the Siletzphic provincesRiver Volcanics,the Tyee Formation*,the Spencerof western Ore-Formation,and diabase dikes and sills(Vokes andgon.others,1954).The first three units were *Mapped as Flournoy Formation by Baldwin(1974). deposited during Eocene time(40-60 million years ago),and thedikes and sills were intruded into them during the mid-01igocene(25-30 million years ago).No younger materials were preserveduntil the accumulation of unconsolidated terrace,flood,andfloodplain deposits of Pleistocene to Recent age(1ess than100,000 years_old).The bedrock units are more widely exposedin the Coast Range,whereas the unconsolidated materials aretypical of the Willamette Valley. The Siletz River Volcanics are basaltic rocks many thousandsof feet thick that erupted onto the early to early-middle Eocenesea floor from submarine vents(MacLeod and Snavely,1973).Theyare the oldest rocks in the area and include basaltic pillow lavas,breccias,tuffs,and associated basaltic sedimentary rocks.Thelower part of the unit is similar to oceanic crustal basalts andis thought to_be oceanic crust which accreted onto the continen-tal margin. In the upper part of the unit,rocks of more variedcomposition accumulated locally in sufficient thickness to formisland similar to modern seamounts(Snavely a

FAQs

What is the guide’s structure and purpose for readers?

It originates from a class on environmental geology and is modified into a self-guiding, two-part bicycle trip. The emphasis is on the practical aspects of local geology.

How does the Corvallis fault relate to major landforms in the area?

The guide describes the fault as separating erosion-resistant volcanic rocks to the northwest from softer sedimentary rocks covered by valley alluvium to the southeast, forming the fault’s role as an eastern boundary near Corvallis.

Which bedrock units are highlighted as underlying the Corvallis area?

The guide lists four main bedrock units from oldest to youngest: the Siletz River Volcanics, the Tyee Formation, the Spencer Formation, and diabase dikes and sills intruded into these units.

Sep 29, 2026

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