Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 19-10
Presentation Time: 11:20 AM


HLADKY, Frank R., National Association of Geoscience Teachers Pacific Northwest Section, 3225 Washington Avenue, North Bend, OR 97459

One interpretation that has recently gained the weight of published tradition, as to the origin of the lava-capped buttes of Upper Table Rock and Lower Table Rock near Medford, Oregon is that two buttes represent examples of inverse topography. The story goes that lava flowed down the channel of the ancestral Rogue River 7 million years ago, and after it solidified, preserved the meandering shape of the river channel as the river re-established itself elsewhere. That river eventually established the present valley floor. The two prominent, horse-shoe-shaped buttes rise about 210 meters above the valley floor. They owe their prominence to a cap of distinctive andesite of Miocene age, locally more than 33 meters thick, mapped initially by Francis G. Wells in 1939. This lava unconformably lies atop softer fluvial sediments of the Payne Cliffs Formation of Eocene age.

The notion that the buttes are representations of inverse topography comes from a number of observations. The horse-shoe shape of each butte suggests to the mind the arcuate shapes of river meanders. The buttes’ proximity to the present-day Rogue River tempts one to think that the ancestral Rogue River could have anciently carved a meandering channel for the lava to follow. Being more resistant than underlying mudstones and sandstones the capping lava would have naturally protected the underlying sediments from erosion and forced the river to move elsewhere. With the passage of time, some of the ancient meanders would have been preserved while the river graded the rest of the valley.

The latest Google Earth satellite imagery restores an alternative interpretation, which is not new at all, but which was explicitly advanced by Smith and others in 1982: the lava flow or flows were valley-filling rather than channel-filling. This view is supported by the vast scale of the mapped distribution and thickness of andesite of Table Rock outcrops. New terrain data derived from Google Earth cross-sectional profiles show that the thickening and thinning of outcrops at the Table Rocks are at odds with the downstream direction of flow of a fluid lava following and confined to a pre-existent channel. Though a scientific notion may be advanced by popular publication, that view may not necessarily be supported by the full weight of the scientific evidence.