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Geology Oregon/Washington BLM




How did the Table Rocks form?

The following represents the most recent theory of the formation of the Table Rocks:

Rogue Valley 40-66 million years ago

Approximately 48 to 35 million years ago, the Payne Cliffs Formation was being deposited by rivers in the area of the Table Rocks. The Payne Cliffs Formation is made up of river deposited sandstone and conglomerates. From 20 to 10 million years ago the uplift of the nearby Klamath Mountains and the formation of the Rogue Valley took place.

Volcano Erupted and Spread Lava Over the Entire Valley

About 7 million years ago, a shield volcano erupted a lava flow that was approximately forty-four miles long and spread out over the entire valley, from the Prospect area to Sams Valley. This mass of lava caused the valley floor to rise in elevation to the height of the top of Table Rocks.

Rogue River eroded and carved Away Most of the Lava Rock

Over the following seven million years, the ancient Rogue River meandered through the valley, eroding or carving away most of the andesite lava rock.

Lava-filled Valley Washed Away to the Ocean via Erosion

Approximately 90% of the lava-filled valley was washed away to the ocean due to the erosional force of the Rogue River.

Modern Day Table Rock Mesas

Currently, all that remains are a few solitary large rock masses or monoliths and two horseshoe shaped andesite capped mesas known as the Table Rocks.

Does Erosion Sstill Affect the Rocks Today?

Erosion, or a breaking down of rocks and natural material, is an ongoing process. Earth material is constantly being eroded, transported, and deposited into other areas, creating our ever-changing geology.

While hiking, look for large rock masses along the side of the trail. These are called monoliths. They were created by erosional forces that broke down the original lava mass.

As you climb, look for the talus slope to your right near the top of both Table Rocks. Talus slopes are formed by an accumulation of debris at the base of a cliff. These areas look like fields of small rocks. Similar to monoliths, these rocks were originally part of the top, but due to erosion they are slowly moving downward.

What Causes Erosion?

  1. Water is one of the main forces causing erosion. The Rogue River helped form the Table Rocks as we know them today. This process took millions of years.
  2. Freezing and thawing of water in and around the rocks is another erosional force. Water gets into cracks of rocks, freezes, and expands like a water bottle in the freezer. Eventually, cracked rocks can break off or change shape from these weathering forces.
  3. Plants, such as mosses or poison oak, can cause erosion as their roots grow into rocks and soil. Root systems loosen soil, which decreases stability and increases the chance of erosion. Most lichens also cause erosion by releasing acid that breaks down rocks.
  4. Humans and animals can cause erosion by compacting and transporting soils. For example, a gopher tunneling underground searching for food can move an estimated two tons of soil per year. Simply walking on the trails compacts the soil.
  5. Wind can also cause erosion by dispersing soil, seeds, and other material from place to place.
  6. Earthquakes have also been known to move large masses of earth. This displacement increases vulnerability to erosional forces over time.

All of these erosional forces have played a role in making the Table Rocks what they are today! It is amazing to think that 90% of the Table Rocks have already eroded away. We are very lucky to experience the Table Rocks as they are today.

Do you think the Table Rocks will be around forever?