What is there 500 acres of on Upper Table Rock?
Starting approximately 40 million years ago in the middle Eocene, a braided river system called the "Ancestral Rogue River" flowed through the region where the Rogue Valley is now carved. For about 2.1 million years, the river deposited what is now known as the Payne Cliffs Formation by laying down a thin conglomerate, followed by arkosic sandstone and siltstone. Between 10 and 20 million years ago, the uplift that created the nearby Klamath Mountains caused an incision that formed the Rogue River valley. Vertical erosion, or downcutting of the Rogue River continues to keep pace with the recent uplift, with about 690 feet (210 m) of erosion occurring in the past seven million years.Approximately seven million years ago in the upper Miocene, a 44-mile (71 km) long trachyandesitic lava flow that likely came from Olson Mountain near present-day Lost Creek Lake flowed down the Ancestral Rogue River and its tributaries and spread throughout the valley. This lava formed a hard cap over the Payne Cliffs Formation. At Lost Creek Lake, the lava attained its maximum thickness of 730 feet (220 m) and thinned to about 100 to 200 feet (30 to 61 m) to the north of Medford.
 
Since the Olson eruption, the Rogue River has eroded 90 percent of the solidified lava. Though the andesite prevented much erosion to the caps of the Table Rocks, the andesite-capped cliffs eroded from the side as the softer sedimentary units of the Payne Cliffs Formation gave way. This erosion created expansive talus fields which surround the plateaus on all sides, creating slopes capable of supporting abundant plant and animal life. Upper and Lower Table Rock both stand 800 feet (240 m) above the valley floor, and just over 2,000 feet (610 m) above sea level. There are approximately 300 acres (120 ha) of level ground on Lower Table Rock, and 500 acres (200 ha) on Upper Table Rock.The Table Rocks offer an example of inverted relief, in which previous topographic lows are filled with a resistant rock and become new topographic highs after the erosion of the surrounding region. Each plateau is shaped roughly like a horseshoe because the lava followed the meanders of the Ancestral Rogue River.
Two caves and two former gold mines are located at the base of the andesite cap on Upper Table Rock. The caves were created by natural fractures in the cap, and the gold mines were excavated by prospectors searching for gold in the 19th century. Three are large enough to walk into, with an average width of 8 feet (2 m), while one is a small pit, dropping 30 feet (9.1 m) vertically into a pond of water.
level ground