First Rhyolite Gorge (mile 6.5)

The narrow, orange-colored canyon just downstream of the mouth of Crooked Creek is informally named First Rhyolite Gorge. It is a very short gorge, a little over one mile long, which allows only 20-30 minutes to see it by boat at typical river speed.

The vertical walls, which rise 150 feet high, consist of a single layer of rhyolite lava—a type of volcanic rock commonly formed by eruptions along the Yellowstone Hotspot. The volcanic vent’s location is unknown but likely nearby. Rhyolite lava is so viscous and slow moving that it usually only flows a few miles before it solidifies into rock.

Age: The gorge walls may be about 11 to 12 million years old, the same age as several rhyolite lava flows to the north (downstream) that have been laboratory dated. At least 24 different rhyolite volcanic centers were active in this corner of Oregon around this time.

Uplift: Several geologists who studied Rome valley stratigraphy think this rhyolite lava flow may underlie the Rome sediment at depth, out of sight. We can see it here today because this portion of the lava flow was uplifted by ancient east-west faults as sediment was filling up the basin. The rhyolite lava flow was buried for millions of years until the Owyhee River encountered the erosion-resistant rock and cut a gorge through it. The base is not exposed so we are not able to determine its total thickness.

Flow patterns: Rhyolite lava flows like toothpaste squeezed from a tube, piling up on itself, forming flow patterns visible as curls or swirls or bands. All those flow patterns are visible in the cliff face as you pass through the gorge. Also notice the vertical rock fractures called “jointing” caused by contraction as the hot volcanic rock cooled.

Vesicles: If you stop and walk along the base of the cliff on river-right at mile 7, you can find several places where the cliff face looks like Swiss cheese, loaded with gas cavities of assorted sizes and shapes (vesicles). The vesicles are often filled with secondary minerals that formed long after the lava cooled and was buried by sediment.

First Camp: At First Camp the river corridor changes from the narrow rhyolite walled gorge to a V-shaped, basalt lava rimrock lined canyon. From First Camp we can look across the river and see where the cliffs of First Rhyolite Lava Gorge end and interbedded basalt lava flows and sediment begin.

Little Owyhee Butte rimrock (mile 8): The canyon rimrock is a basalt lava flow from a nearby volcanic vent named Little Owyhee Butte located about 2 miles to the northeast. Its most recent eruption is dated 1.9 million years ago. Upriver, the rimrock contains features called “lava deltas”—evidence the hot lava flow encountered water. The water was probably the lake that temporarily filled the Rome valley after the river was blocked downstream by a lava dam. (See Chapter 4, section “Lava Delta in Ancient Lake” for details.)

Fletcher Point dogleg bend: At Fletcher Point, the river makes an abrupt dogleg bend before resuming its north-flowing course. The dogleg bend likely avoids an erosion-resistant mass of rhyolite lava concealed beneath the sagebrush and loose soil of Fletcher Point. The cliff at the dogleg bend, along with the 100-foot cliffs downstream and the 150-foot cliffs upstream, are all composed of rhyolite lava. The lava flow shapes the river’s course here.

Abandoned watercourses: The side canyons located across the river from Fletcher Point are abandoned watercourses. They loop back to a lava divide above the Rome valley near Crooked Creek. These channels could be former courses of the Owyhee River or Crooked Creek, or they may represent a lake overflow from the Rome valley caused by a lava dam that blocked the Owyhee River downstream. The head of the watercourses near Crooked Creek is situated at an elevation of 3,700 feet, approximately 350 feet above the current river channel, suggesting its formation occurred in ancient times. Further research is required to ascertain the specifics regarding the origin and timeline of these canyons.

Sources: (Orr, 1985), (Evans, et al., 1990), (Evans, 1991), (Ellison, 1968), (ODGMI Map, 2021), (Wolf & Ellison, 1971), (Shoemaker, 2004), (Swenton, et al., 2022)