Geologic Trip, Rogue Valley

Roxy Ann Peak

Diabase Dike

Roxy Fm

Payne Cliffs Fm

Roxy Anne Peak (3,576’) is a distinctive landmark on the east side of Medford that can be seen for many miles in the Rogue Valley area. Although the peak may look like a volcano, it is not a volcano. The top of the peak is formed from a thick diabase dike that has intruded the older sedimentary and volcanic rocks of the Roxy and Payne Cliffs Formations that make up the middle and lower slopes of the mountain. If Roxy Ann Peak were a volcano, the entire peak would have been formed from volcanic rocks.

 

The photograph, right, looks down the southeast ridge of Roxy Ann Peak toward the rock quarry in the left center of the photograph. The large fractured rocks in the foreground are from an outcrop of the diabase intrusive at the top of Roxy Ann Peak. 

 

The diabase is massive, hard, and breaks into large blocks that have resisted erosion. This hard cap of diabase protects the underlying rocks from erosion and gives the peak its steep distinctive top.

 

Diabase and basalt have the same composition, but diabase forms from magma that does not reach the surface of the earth whereas basalt forms from magma that reaches the surface, generally as lava flows. The diabase at Roxy Ann Peak has a K-Ar age-date of 30.8 ± 2 Ma.

 

Roxy Ann Peak lies within Prescott Park, which is owned by the City of Medford. The park has a number of trails that offer good access to the peak. The parking area for Prescott Park is at the gate on Roxy Ann Road, 1.4 miles north of Hillcrest Rd. (42° 20’ 44’’ N, 122° 47’ 10’’ W).

 

The Payne Cliffs Formation consists of non-marine sandstone, shale and conglomerate of Oligocene age. The Roxy Formation consists of volcaniclastic sedimentary rocks, tuffs, and basalt and andesite lava flows. These rocks represent the earliest volcanic activity of the Western Cascades in this area.

 

External Websites

City of Medford:  Prescott Park Brochure

ODGM:  Geologic Map, Bear Creek Valley

Wikipedia:  Roxy Ann Peak

 

 

 

 

Return to: Rogue Valley