One of the most unique features of the Rogue Valley are Upper and Lower Table Rocks. Located about 10 miles northeast of Medford, Oregon, USA, the Table Rocks are the remnants of a huge lava flow that covered the valley around seven million years ago. Their formation however, begins long before the outpouring of lava that provides the two mesas with their distinctive crowns. The Table Rocks are founded on an ancient seafloor and built of depositional sediment laid down by erosion.
An Ancient Seaway is Formed
The Table Rocks sit upon the Payne Cliffs Formation, which overlays the much older Hornbrook Formation. The Hornbrook Formation was created between 100 to 65 million years ago (M.Y.A.) during the late Cretaceous period after the Klamath Mountains broke away from the continent to create a shallow seaway. The channel was about 60 to 80 miles wide and approximately 230 miles long.
During this 35 million year period, many layers of conglomerate, sand, calcite and silica rich silt were deposited upon the seafloor and hardened into sandstone, mudstone, limestone and chert. Trapped within these layers were the remains of ammonites, gastropods and bivalves. By the time of the middle Eocene epoch, approximately 50 M.Y.A., a substantial amount of sediment had filled a portion of the channel to create a low plain, connecting the Klamaths to the continent.
A Period of Erosional Deposition and Uplift
Over the next 15 million years a system of rivers and streams, flowing from the Klamaths and the Idaho Batholith, began to deposit non-marine sediments to form the Payne Cliffs Formation. Reaching a depth of over 1000 feet in some areas, the Payne Cliffs Formation stretches north from the Siskiyou Summit to the Evans Creek area just north of Rogue River, Oregon, USA. The formation is comprised of sandstone and conglomerate layers that make up the flanks of the Table Rocks. The older layers consist of varying quantities of quartzite and quartz, granodiorite, slate-phyllite and chert. The newer layers contain more volcanic material in the form of tuff and andesite.
The Hornbrook and Payne Cliffs formations underwent a period of uplift beginning about 35 M.Y.A., when seafloor subduction shifted to its present zone along the Juan De Fuca Plate. As a result of the new subduction zone, the Klamaths began to push into the continent compressing and lifting the two formations and creating a depression that would later become the Rogue Valley.
An Increase in Volcanic Activity
In addition to causing uplift, the shift in subduction began the formation of the Western Cascades. Volcanic activity from this area began in the late Eocene and early Oligocene epochs and contributed volcanic material to the upper layers of the Payne Cliffs Formation.
Volcanic activity in the Cascades continued until the late Oligocene, when it ceased for a period of about 10 million years. During this time, the central and eastern portions of Oregon were being flooded with basaltic lava. Eruptions resumed in the Cascades approximately 12 M.Y.A. creating what is now the High Cascade Range.
The Eruption of Olson Mountain
In the late Miocene epoch, the Table Rocks were beginning the last stages of their development. Having been founded on the Hornbrook Formation and built up by the Payne Cliffs Formation, the geological stage was set to add the final layer. This came in the form of huge lava flow, which erupted from Olson Mountain approximately 7 M.Y.A.
Olson Mountain was a large shield volcano located a few miles southwest of Lost Creek Lake. Its eruption produced a lava flow that was over 44 miles long and covered an area from Prospect, Oregon, USA., to Sams Valley. Ranging in thickness from 730 feet at its source to about 100 to 200 feet at the Table Rocks, the lava spread out over the entire Rogue River valley and its tributaries.
Upper and Lower Table Rock
During the past seven million years over 90 percent of the lava and much of the underlying sediment has been eroded away by the Rogue River, leaving only the two horseshoe shaped mesas. The lava itself is trachyandesite containing bits of plagioclase feldspar. Composed of olivine, augite, andesine, labrodorite, magnetite, hematite, apatite and orthopyroxine, the lava caps display irregular column jointing and uneven displacement.
The Table Rocks are managed jointly by the Nature Conservancy and the Bureau of Land Management and are open to the public for hiking. There are over 300 species of plants and wildflowers and 70 species of animals including the federally protected Fairy Shrimp that reside in vernal pools during the spring.
Leaf fossils and petrified wood have been found on the surrounding lands at the base of the rocks. However, most of that land is now privately owned and inaccessible to the public. Searching the gravel bars at Touvelle Park in the late summer, may yield an occasional fossil, but you will have better results up at the Siskiyou Summit where sections of the Hornbrook formation are exposed off Interstate 5.
Sources:
- Table Rocks, Geology, Bureau of Land Management, US Department of the Interior, 1999.
- Hladky, Frank R., Age, Chemistry and Origin of Capping Lava at Upper Table Rock and Lower Table Rock, Jackson County, Oregon. (pgs 81 - 91) Oregon Geology, Vol 60, Number 4, Oregon Department of Geology and Mineral Industries. Jul/Aug 1998.
- Seevers, Joan., Borgias, Darren., Oregon Plants, Oregon Places: Upper and Lower Table Rocks, Jackson County, Kalmiopsis, 1993.
- Graham, David L., EMS., Broadrick, Alyss, SOU., Description of the Sevens Basin Watershed, Chapter 1, Introduction Sevens Basin Watershed Assessment 2004.
- Bestland, Eric A., Volcanic Stratigraphy of the Oligocene Colestin Formation in the Siskiyou Pass Area of Southern Oregon., (pgs 79-86) Oregon Geology, Vol 49, Number 7, Oregon Department of Geology and Mineral Industries. Jul 1987.
- Santino, Michael., SW Oregon, Cretaceous Hornbrook Formation, Search and Rescue Report. North America Research Group., Jun 2007.
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