Geologic Trip, Rogue Valley

Table Rocks

Upper Table Rock and Lower Table Rock are two horseshoe-shaped buttes located on the west side of the Rogue Valley immediately north of the Rogue River. These two buttes are excellent examples of inverted topography, where the bottom of an older river valley now forms the top of the mountain. The Table Rocks are capped by a 200-foot thick lava flow that came into this area from the High Cascades about seven million years ago and covered much of the floor of the ancestral Rogue Valley at that time. After the lava flow, the Rogue River established a new channel and continued to erode and deepen the Rogue Valley so that the floor of the valley is now 600 feet lower than it was at the time of the lava flow.

 

Not only the floor of the Rogue Valley was eroded, but the land surfaces in the Rogue Valley and Bear Creek Valley would have eroded at about the same rate, wherever the tributary creeks and streams to the Rogue River maintained their gradient. This may seem like a lot of erosion, but it is really not that bad. If you do the math, 600 feet of erosion over seven million years means that the rate of erosion would have been about one inch per 1000 years. The Klamath Mountains have been uplifted at about the same rate over the same period. Another way to appreciate the amount of erosion in the Rogue Valley  and Bear Creek Valley is by looking at the Table Rocks from the top of Barneburg Hill. The channel of the ancestral Rogue River is at an elevation of 1800 feet at the Table Rocks, over 100 feet higher than the top of Barneburg Hill. The hill certainly must have been a few hundred feet higher than the ancestral Rogue River seven million years ago. 

 

The Table Rock lava flow has well-developed columnar joints and looks much like a typical basalt flow. However, based on chemical composition it is classified as an alkaline-rich andesite. The lava is glassy and nearly black, with distinctive tabular specks of white plagioclase and small crystals of green olivine and dark augite. The Table Rock flow ranges from a maximum thickness of 730’ at the south bank of Lost Creek Lake to about 200’ at the Table Rocks.

 

The top of the lava flow at Table Rocks is very hard and has not eroded much over the last seven million years. When you walk on the top of the Table Rocks, you are walking on a land surface that has not changed much since late Miocene time. However, the soft sedimentary rocks of the Payne Cliffs Formation below the lava flow are easily eroded and provide a weak foundation for the flow. Thus, most of the erosion of the Table Rocks lava flow is from the sides of the lava flow, where chunks of the flow break off along the columnar joints and make their way down the slope of the hill. These broken pieces of the flow can be seen in many places along the lower slopes of the Table Rocks.

 

Places to See the Rocks

The Table Rock lava flow can be seen along the trails to Lower Table Rock and Upper Table Rock, and also in road cuts along Highway 62 at Lost Creek Lake:

Lower Table Rock Trail head— Drive north from Medford on Table Rock Road to Wheeler Road. Turn left on Wheeler Road. Trail head is 1/2 mile ahead on the left. 2.7 miles to top.

 

Upper Table Rock Trail head— Drive north from Medford on Table Rock Road. Turn right on Modoc Road and continue for 1 mile,  trail head on left. 1.4 miles to top.

 

Road Cuts at Lost Creek Lake—Highway 62 2.0 miles northeast of the turnoff to McGregor Park has road cuts with excellent exposures of the Table Rock lava flow.

 

 

External Websites

BLM:  Welcome to Table Rocks

Oregon Geology (Hladky): Origin of Lava...Table Rock  

Wikipedia:  Upper and Lower Table Rock

 

 

 

 

Return to: Rogue Valley

The Table Rock andesite lava flow is well exposed at the rock quarry on the south side of Lost Creek Lake. In this  area the lava reaches a maximum thickness of 730 feet where it fills a narrow paleochasm of the ancestral Rogue River. The lower part of this lava flow is exposed in Highway 62 road cuts along the south side of Lost Creek Lake.

The lava that caps the Table Rocks came from volcanic vents near Olsen Mountain in the High Cascades and flowed west about 40 miles, generally following the ancestral Rogue River and spreading out to cover much of the floor of the ancestral Rogue Valley. Remnants of the lava flow also occur in a few other places, as shown of the map.

The horseshoe shape of Upper Table Rock and Lower Table Rock likely follows meanders of the ancestral Rogue River.

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