Geologic History of the Columbia River Gorge

The geologic history of the Gorge is represented in deposits which date back about 36 million years before present time (36 mybp). The oldest of these deposits were emplaced by the Western Cascade volcanoes, a volcanic chain located west of the current Cascades about 35 million years ago. Younger deposits have since covered most of these volcanoes. The lavas extruded from the Western Cascades were primarily andesite with some basalt mixed in. The deposits we find today consist primarily of weathered lavas that have been interlayered with mud deposits. They are known as the Ohanapecosh Formation and have been altered to saprolite (extremely weathered (i.e. rotten) rock) and argillite. Generally these deposits are only found on the WA side of the gorge due to the orientation or dip of the rocks in the area (more about this later).

About 15-20 mybp, a series of mudflow and lahar deposits were deposited on top of the Western Cascade lava flows. These younger deposits represent a continuation of the Western Cascade volcanism and are known as the Eagle Creek formation. Because we don’t see any lava flows associated with these deposits we believe the source of these deposits was to the north and the deposits were carried here through mass wasting and erosional processes. The Eagle Creek formation can be found on both sides of the gorge but is primarily exposed on the WA side.

Overlapping with the deposition of the younger Western Cascade mudflows and lahars, was the deposition of the Columbia River Flood Basalts (CRBs). These basalts represent a time when the northwest was undergoing a period of crustal extension due to changes in the westward subduction zone and the development of a back-arc basin. One result of this crustal extension was the development of normal faults and fractures which produced large amounts of basaltic lava. These lavas erupted over approximately 11 million years and have been divided into four basic groups (see attached stratigraphy for the flows within each group). From oldest to youngest they are the Imnaha, Grand Ronde, Wanapum and Saddle Mountain Basalts. These basalts flowed westward from vents in eastern OR and WA, with some flows eventually reaching the Pacific Ocean. The most extensive of these four groups was the Grand Ronde which makes up about 85% of all the CRBs. Because basalt has a low viscosity (it flows very easily) it fills in low depressions on the surrounding landscape, creates a very flat surface topography, and often will follows stream valleys. In the gorge, many of these intercanyon (stream valley) flows can be seen and are recognizable as “V”-shape flows with river cobbles located at their bases. Some examples include Crown Point, Latourelle Falls, and Bridal Veil Falls. The primary flows of the CRBs found in the gorge include the Pomona flow (Saddle Mtn. Basalts), Priest Rapids flow, Frenchman Springs flow (Wanapum Basalts), and the Grand Ronde Basalts. Many of these basalts have unique jointing patterns that allow for distinction between them.

At the same time the CRBs were being emplaced, the Columbia River was flowing from north to south, eventually emptying in to the Pacific around Florence OR. As the CRBs began erupting they often filled and blocked the ancestral Columbia River channel. The result of this was to force the river northward into its present course. Deposits of the ancestral Columbia River are known as the Troutdale formation and can be recognized as such by quartz pebbles in the deposits. Quartz is not a mineral readily found in this area and pebbles of it were carried in from the east by the Columbia River. Several old channels of the Columbia River that were filled with CRBs are found in the gorge. These include the Bridal Veil channel and The Priest Rapids Channel (Crown Point). See the additional handout for the locations of the old Columbia River Channel.

Near the end of the depositional period for the CRBs, compression forces caused folding, faulting and tilting (to the south) of the rock layers in the gorge . One possible explanation for this event is the development of the Ancestral or Plio-Cascades. These Cascade ancestors developed about 7 mybp and deposits from them include andesite lava flows, diorite stocks (Wind Mountain), lahars, and tuff deposits known as the Rhododendron Formation. Folding and faulting of the existing rock types created features such as the Mosier Syncline and the Ortley Anticline. Additionally, the tilting of the rock units began a period of mass wasting events which continues today. These mass wasting events are due to a combination of the tilt of the rocks and area’s stratigraphy (heavy layers of basalt (CRBs) overlying slick slip-surfaces (the Eagle Creak and the Ohanapecosh Formation)). As the Columbia River became “captured” in its present course, undercutting of the lower rock units caused slides. Because the rocks dip to the south (towards OR), most landslide events occur on the WA side. This is also why the Ohanapecosh formation is only exposed on the WA side of the river. It does exist on the OR side but is covered up and can’t be seen. It is thought the Ohanapecosh is responsible for the continuous roadwork that is required on the OR side of the gorge.

About 1 million years ago the current Cascade chain or the High Cascades began developing. Few to no deposits of these volcanoes are found in the gorge. However, during the development of the Plio-cascades and the emplacement of the Rhododendron Formation, a series of basaltic eruptions began. These eruptions lasted from about 4-1 mybp and are of the same age as the Boring Lavas found in the Portland area. These basaltic lavas are thought to be associated with the Boring Lavas but are slightly different compositional. Many papers list them as Boring Lavas to avoid confusion. This eruptive episode produced many buttes and shield volcanoes including Underwood Mtn, Larch Mtn, and Mt. Defiance. Additionally, a couple of the lava flows blocked the Columbia River (see additional handout) causing the gorge to fill with water until the blockage could be breached. An example of this is where Wind River enters the Columbia. At this point, the gorge was filled up with water to elevations over 900 ft. This created a high bench across the mouth of Herman Creek east of Cascade Locks.

In the late Pleistocene (12,000-15,000 years ago) a series of glacial floods flowed southward from Montana through the gorge and into Portland. These floods created several channels in the Portland area and deposited many gravels and sands. Several of these flood bar deposits are seen along the gorge as are several erosional features. The most recognizable erosional feature is the scablands created by the scouring and stripping of soil from the top of the lava flow deposits. Most of the gravel deposits and scablands are found on the east side of the Gorge near the Dalles.

The final stage of the gorge's development is still being written. Most prominent today is the continuation of landslide deposits and the development of talus slopes.

Stops (and Non-Stops) Along the Way

Stop 1 Along Old Columbia Hwy. Stop and look at ancestral Columbia River Deposits.

Stop 2: Portland Woman’s Forum. Great view of the Gorge. See Larch Mountain, Crown Point (intercanyon flow (Priest Rapids)), Rooster Rock Slide Block, Mt. Zion, and Landslide deposits. Get Bearings and discuss history of area.

Stop 3: Crown Point/Vista House. Bathroom stop and history

Non stop: ½ mile from Vista House Right side of the road: Great Pillow lavas of the Priest Rapids flow. Note that these pillow are fairly round in shape indicating they were emplaced in a pond or lake where there was no current. Originally a stop but kind of dangerous to pull out and look.

Stop 4: Latourelle Falls. Lower path down to falls. See a great example of an intercanyon flow where the falls are and also see a dacite flow on the way down to the falls. The dacite is probably from a Western Cascade Vent (36 mybp) and because we see the flow this indicates the vent was near by. Notice the difference in the jointing patterns for the basalt flow making up the falls and the dacite lava flow. Why are they different?

Stop 5: Oneota Gorge Tree casts in lava flows.

Stop 6: Exit 41 (Fish Hatchery). Great example of Eagle Creek Formation. Notice how the deposit is unsorted. This is a volcanic mudflow deposit from the Western Cascades. Bottom of the stratigraphic column

.Lunch. Exit hwy at the town of Stevenson and travel through the town. Stop at park. Have lunch and talk about landslide topography and the Bridge of the Gods.

Stop 7: Mitchell Point Park (around exit 58)

Stop 8: Dalles Bridge Really cool outcrop. Tell me what type of deposits you see and then we’ll talk!

Stop 9: Rowena Loop… Missoula flood talk

Stop 10: Hood River Boat Ramp…Filled lava tube

Stop 11: Multnoma Falls (bathroom stop). Great falls but lots of people usually. There are 11 different Grand Ronde basalts exposed here with six of them outcropping along the river to the top of the upper falls. Can you find them all? Look for the pillow lavas..where are they?