Geologic Trip, Rogue Valley Area

Rogue Valley

The Rogue Valley is an unusual valley in that it is completely surrounded by mountains. The rugged Klamath Mountains form the western boundary of the valley and the Cascade Range lies to the east. The valley is pinched off north of Sams Valley, where these two ranges merge, and south at Siskiyou Pass where the mountains again merge. The Rogue River escapes the valley by cutting through the Klamath Mountains from Gold Hill to Grants Pass and then across the Klamath Mountains until it reaches the Pacific Ocean at Gold Beach. Why is the valley in the middle of all these mountains and how did the valley form?  

 

Rogue Valley formed because the valley is underlain by soft sedimentary rocks and these rocks are more easily eroded than the hard metamorphic and igneous rocks in the Klamath Mountains and the volcanic rocks of the Cascade Range. Where there are no sedimentary rocks, there is no valley. Without these sedimentary rocks there would be no valley. These sedimentary rocks are referred to as the Hornbrook Formation and the Payne Cliffs Formation. 

 

The Hornbrook Formation occurs mainly along Bear Creek and the west side of Bear Creek Valley. Most of these rocks are mudstone and siltstone, but locally there are beds of thick sandstone and conglomerate. The mudstone and siltstone is very soft and easily eroded, and Bear Creek follows these soft rocks for most of its course. The thick sandstones and conglomerates are hard and resist erosion. These rocks form many of the small hills along the west side of Bear Creek Valley. Barneburg Hill is formed from some of these thick sandstone beds. The Hornbrook Formation was deposited in a seaway that extended along the western margin of the North America from Canada to California during mid- to late Cretaceous time. 

 

The Payne Cliffs Formation consists of sandstone and mudstone that was deposited on a broad alluvial plain that had formed in south central Oregon during Eocene time. These rocks occur mainly in Sams Valley, the northern part of Rogue Valley and along the eastern side of Bear Creek Valley. The Payne Cliffs Formation is overlain by volcanic rocks of the Roxy Formation, and these hard volcanic rocks form a protective cap for the underlying softer rocks of the Payne Cliffs Formation.

 

During Miocene time, about 20 million years ago, the Klamath Mountains began to be uplifted. As uplift continued, the overlying sedimentary and volcanic rocks were tilted eastward and the sedimentary rocks began to be eroded. Rogue Valley is formed from these tilted and eroded sedimentary rocks.

 

 

Tilted Rocks. The photo, right, from Dead Indian Memorial Road, shows tilted rocks of the Payne Cliffs Formation along the east side of Bear Creek Valley, overlain by volcanic rocks of the Roxy Formation at the top of the slope. The volcanic rocks of the Roxy Formation are hard and form a protective cap over the softer rocks of the Payne Cliffs Formation. The floor of the valley is mainly soft shale and sandstone of the Hornbrook Formation.  

 

Pompadour Bluff is on Dead Indian Memorial Road one mile east of the Ashland Airport. The bluff is formed from a thick conglomerate bed in the upper part of the Payne Cliffs Formation and was probably deposited in a river channel on the Eocene alluvial plain. The east dip of the conglomerate occurred during Miocene uplift of the Klamath Mountains.  

 

Emigrant Lake. Conglomerate beds of the Payne Cliffs Formation are well exposed in road cuts near the boat ramp at Emigrant Lake. Pebbles in these conglomerates were derived from erosion of the Klamath Terranes and were deposited in a stream on a broad alluvial plain that once covered much of south central Oregon. The beds are tilted east.

 

Barneburg Hill is formed from beds of sandstone, conglomerate and shale of the Hornbrook Formation that are tilted gently eastward. The crest of the hill is formed from an especially thick, hard sandstone bed that has resisted erosion and has given the hill its distinctive shape. Without this resistant sandstone bed there would be no Barneburg Hill.

 

I-5 S Road Cut. Fossiliferous sandstone of the Hornbrook Formation is well exposed in a road cut on I-5 South near Mile Marker 6 on Siskiyou Pass. Most of the fossils in this sandstone are clams and snails, but sharks teeth, sea urchins, oysters, starfish and coral have also been found. The sand was deposited as a beach along the shoreline of the ancestral Klamath Mountains when the late Cretaceous seaway extended through large parts of Washington, Oregon and California.

 

Roxy Ann Peak is not a volcano. The sharp top of the peak is formed from a thick diabase dike that has intruded the older sedimentary and volcanic rocks that make up the lower parts of the mountain. The diabase dike consists of very hard rocks that have protected the underlying rocks from rapid erosion.

 

Table Rocks. Seven million years ago a lava flow from the Cascade Range followed the Rogue River west from Lost Creek Lake to Gold Hill and covered much of the valley floor at that time. Since then, most all of the lava flow has been eroded from the valley and the valley floor has been lowered several hundred feet. The flat tops of the Table Rocks are remnants of the lava flow that have not yet been removed by erosion.

 

External Websites

ODGM:  Geologic Map, Bear Creek Valley

SEPM:   Stratigraphy, Payne Cliffs Fm.

 

 

 

Return to: Rogue Valley Area

Roadcut, Emigrant Lake

Pompadour Bluff

Tilted Rocks