DESCRIPTION:
The Boring Lava Field, Portland, Oregon

• Bob's Mountain: 2,110 feet elevation -- Allen, 1975
• Bob's Mountain (South): 1,690 feet elevation -- Allen, 1975
• Bob's Mountain (North): 1,770 feet elevation -- Allen, 1975

• Partial summit craters remain only at Bobs Hill, 33 kilometers (20.5 miles) northeast of Portland, and at a low cone enclosing a lake (Battleground Lake) north of Battleground, Washington, 33 kilometers (20.5 miles) north of Portland. Most other volcanoes still have a low cone shape and are mantled with loess above 122 meters (400 feet) elevation. Below this they were scoured by the cataclysmic Bretz floods from Glacial Lake Missoula around 13,000 to 15,000 years ago. -- Allen, 1990, IN: Wood and Kienle

• The densest concentration of volcanic vents lies west of the town of Boring, where 20 centers occur within an area of about 36 square miles. Because of this grouping near Boring, Ray Treasher (1942) first gave the name "Boring lava" to the lava, cinders, and ash which emanated from volcanic centers in the Portland area within a time span of from perhaps 10 million to less than 1 million years ago (Trimble, 1963). Some like Bob's Mountain in Washington, may be very young indeed. -- Allen, 1975

• Brunner Hill: 2 vents, 680 feet elevation -- Allen, 1975

• Cindery pyroclastic rocks of the Boring lava are present at several localities. The structure of the cinder cone on Mount Tabor is visible in an excavation that exposed the throat and the dipping cinder beds on the flanks. The cinder cone is the southwest patch of Boring lava shown on Mount Tabor. The small area of Boring lava northeast of the cone is flow rock that probably was extruded near the base of the cone in a manner similar to flows emanating from "bocas" at Parícutin in Mexico (Krauskopf, 1948,p.716). The peculiar topographic feature in SE1/4 sec.18,T.1S.,R.3E., about 2 miles southwest of Gresham, is a breached cinder cone. Weathered cinders are exposed along a small creek draining the crater through the breached wall on the west side. Cindery deposits are exposed also along Skyline Boulevard on the west side of the Tualatin Mountains, between Cornell Road and Thompson Road in sec.25,T.2N.,R.3E., on the small hill east of Brunner Hill in sec.24,T.2N.,R.3E., and on Prune Hill, near the road intersection in the SE1/4NE1/4 sec8.T.1N.,R.3E. -- Trimble, 1963

• Chamberlain Hill: 890 feet elevation -- Allen, 1975

• Cook's Butte: 2 vents, 718 feet elevation -- Allen, 1975

• Devil's Rest: 2 vents, 2,450 feet elevation -- Allen, 1975

• Elk Point: 2 vents, 975 feet elevation -- Allen, 1975

[Map,141K,InlineGIF]
Green Mountain, 1:25,000
-- Basemap courtesy TopoZone, 2003

[Image,85K,JPG]
Green Mountain as seen from the roof of CVO. View is from the south.
-- USGS Photo by Gene Iwatsubo, June 13, 2003

• Green Mountain: 804 feet elevation -- Allen, 1975

• The conical form of a volcano is preserved in many of the Boring eruptive vents, such as Green Mountain and Mount Sylvania. The lava plains slope gently, commonly less than 200 feet per mile and locally less than 50 feet per mile. The surface of the plain is nearly flat to gently rolling except where entrenched by streams or surmounted by cones. -- Trimble, 1963

• Highland Butte: 4 vents, 1,594 feet elevation -- Allen, 1975

• Northwest of the town of Boring, 20 eruptive centers are concentrated within around 100 square kilometers (39 square miles). Vents in the east part of this cluster average less than 2.6 kilometers (1.6 miles) in diameter and 333 meters (1,090 feet) in height above their bases. Lava from Highland Butte and Larch Mountain shield volcanoes form gently sloping plains covering many tens of square kilometers. Well logs indicate that in most places except near vents, Boring lava is between 30 and 60 meters (100-200 feet) thick. -- Allen, 1990, IN: Wood and Kienle

• Within a 13-mile radius of Kelly Butte there are over 32 volcanic vents; within a 20-mile radius centered at Troutdale there are 90 volcanic centers. Most of these were originally small cinder cones like Pilot Butte and Lava Butte near Bend, Oregon, but some of them, such as Mount Sylvania in southwest Portland, Highland Butte 10 miles southeast of Oregon City, and Larch Mountain south of the Columbia River Gorge, were low, broad lava domes of the type called "shield volcanoes". -- Allen, 1975

• The thickness of the Boring lava along the west side of Mount Sylvania, just east of the Willamette meridian, is about 150 feet. The thickness of the Boring lava less than a mile southwest of Highland Butte, in the SE1/4 sec.8,T.4S.,R.3E., however, is less than 150 feet, at least locally. -- Trimble, 1963

• Hunsinger Peak: 657 feet elevation -- Allen, 1975

[Map,144K,InlineGIF]
Kelly Butte, 1:25,000
-- Basemap courtesy TopoZone, 2003

• Kelly Butte: 2 vents, top of hill in Troutdale Formation, 400 feet elevation -- Allen, 1975

• Kelly Butte: 590.0 feet elevation -- Portland Office of Transportation Website, 2002

• The setting of the basalt field is puzzling and not understood. The vents lie well west of the crest of the Cascades, and those such as Kelly Butte, Mount Tabor, and the vents in the Portland Hills are in and even west of the Portland basin. -- Swanson, 1989

• Although the pyroclastic rocks at the Carver vent and the scoriaceous rock at Prune Hill are thought to be closely associated with their respective vents, and Troutdale pebbles are common in Boring lava, the only place that a feeder column for the Boring lava can be seen cutting Troutdale sedimentary rocks is in an abandoned quarry on the west end of Kelly Butte. Here a column of basalt about 20 feet wide cuts about 50 feet of conglomerate of the Troutdale formation, whose bedding has been arched adjacent to the feeder. The lava fed by this vent flowed down a preexisting slope and thickened beyond the point of emission. Local eruption on a sloping Troutdale surface also is represented by the Boring lava on Powell Butte, in NW1/4 sec.13,T.1S.,R.2E., on the hill south of Foster Road, in the south half of sec.13,T.1S.,R.2E., and on Mount Talbert, in sec.3,T.2S.,R.2E. On Mount Talbert a small vent apparently erupted near the eastern crest and a single flow poured southeastward down a previously carved slope. The eruption at Rocky Butte, in east Portland, must have buried a small hill of Troutdale formation, because conglomerate is exposed at an altitude of about 500 feet, just above the road parapet below the tunnel on the west side of the butte. These occurrences may represent eruptions on an irregular post-Troutdale topography, which was subsequently eroded. -- Trimble, 1963

• Larch Mountain: 4,056 feet elevation -- Allen, 1975

• Northwest of the town of Boring, 20 eruptive centers are concentrated within around 100 square kilometers (39 square miles). Vents in the east part of this cluster average less than 2.6 kilometers (1.6 miles) in diameter and 333 meters (1,090 feet) in height above their bases. Lava from Highland Butte and Larch Mountain shield volcanoes form gently sloping plains covering many tens of square kilometers. Well logs indicate that in most places except near vents, Boring lava is between 30 and 60 meters (100-200 feet) thick. -- Allen, 1990, IN: Wood and Kienle

• Within a 13-mile radius of Kelly Butte there are over 32 volcanic vents; within a 20-mile radius centered at Troutdale there are 90 volcanic centers. Most of these were originally small cinder cones like Pilot Butte and Lava Butte near Bend, Oregon, but some of them, such as Mount Sylvania in southwest Portland, Highland Butte 10 miles southeast of Oregon City, and Larch Mountain south of the Columbia River Gorge, were low, broad lava domes of the type called "shield volcanoes". -- Allen, 1975

• Visit A Volcano - Larch Mountain

• Lenhart Butte: 2,117 feet elevation -- Allen, 1975

• Lookout Point: 2,645 feet elevation -- Allen, 1975

• Mount Norway: 2 vents, 1,111 feet elevation -- Allen, 1975

• Mount Pleasant: 1,010 feet elevation -- Allen, 1975

[Map,144K,InlineGIF]
Powell Butte and Vicinity, 1:100,000
-- Powell Butte, Mount Scott, Mount Talbert, Willamette National Cemetery
-- Basemap courtesy TopoZone, 2003

• Mount Scott: 2 vents, 1,095 feet elevation -- Allen, 1975

• Mount Scott: 1,050.0 feet elevation -- Portland Office of Transportation Website, 2002

• Mount Scott is named for the outspoken editor of the Oregonian in the late 1800s and early 1900s, Henry W. Scott, brother of Abigail Scott Duniway for whom Duniway Park was named. -- Portland Parks and Recreation Website, 2002

• North of Mount Talbert, Metro purchased 20 acres on Mount Scott. A former Campfire Girls' day camp known as Camp Wekio, the site is heavily wooded with second growth timber. Wahoo Creek, a tributary of salmon-bearing Johnson Creek, runs through 1,300 feet of the property. When Metro acquired the property, it was in the process of being rezoned for 35 residential home sites. Mount Scott, one of a series of extinct volcanic buttes, has been under heavy development pressure in recent years, and this acquisition will help provide a permanent greenspace in this rapidly growing area. The City of Portland will assume responsibility for managing the property. -- Portland, Oregon Metro Website, 2002

[Map,172K,InlineGIF]
Mount Sylvania, 1:25,000
-- Basemap courtesy TopoZone, 2003

• Mount Sylvania: 2 vents, 975 feet elevation -- Allen, 1975

• Within a 13-mile radius of Kelly Butte there are over 32 volcanic vents; within a 20-mile radius centered at Troutdale there are 90 volcanic centers. Most of these were originally small cinder cones like Pilot Butte and Lava Butte near Bend, Oregon, but some of them, such as Mount Sylvania in southwest Portland, Highland Butte 10 miles southeast of Oregon City, and Larch Mountain south of the Columbia River Gorge, were low, broad lava domes of the type called "shield volcanoes".

• The conical form of a volcano is preserved in many of the Boring eruptive vents, such as Green Mountain and Mount Sylvania. The lava plains slope gently, commonly less than 200 feet per mile and locally less than 50 feet per mile. The surface of the plain is nearly flat to gently rolling except where entrenched by streams or surmounted by cones. -- Trimble, 1963

• The thickness of the Boring lava along the west side of Mount Sylvania, just east of the Willamette meridian, is about 150 feet. The thickness of the Boring lava less than a mile southwest of Highland Butte, in the SE1/4 sec.8,T.4S.,R.3E., however, is less than 150 feet, at least locally. -- Trimble, 1963

• The Boring lava is composed mainly of basaltic flow rocks, but locally contains tuff breccia, ash, tuff, cinders, and scoriaceous phases. The flows commonly are light-gray to nearly black, with lighter tones predominating, and are characterized by columnar jointing and flow structure which in places results in platiness of the rock. Typical jointing is exposed along Boones Ferry Road, where a tongue of lava from Mount Sylvania flowed down a small valley, cut in the underlying Troutdale sediments. The columns are perpendicular to the walls of the small valley. -- Trimble, 1963

[Map,156K,InlineGIF]
Mount Tabor, 1:25,000
-- Basemap courtesy TopoZone, 2003

[Image,163K,JPG]
USGS Aerial Photo - Mount Tabor, Portland, Oregon, 2000

• Mount Tabor: top of hill in Troutdale Formation, 535 feet elevation -- Allen, 1975

• Mount Tabor Park: 641.30 feet elevation -- Portland Office of Transportation Website, 2002

• Portland's Mount Tabor was named after another Mount Tabor, which sits six miles east of Nazareth in Israel. Our Mount Tabor makes Portland one of only two U. S. cities to have an extinct volcano within its boundaries; the other city is Bend, Oregon! The volcanic features of Mount Tabor became known in 1912, years after Mount Tabor became a public park. The volcanic cinders discovered in the park were later utilized in surfacing Mount Tabor Park's roads. Mount Tabor now contains a permanent exhibit of the volcanic cone from which the cinders were obtained. At the top of the park is a statue of a former editor of the Oregonian, Harvey W. Scott. -- Portland Parks and Recreation Website, 2002

• The best and most accessible exposure is the cross-section of the cinder cone in Mount Tabor Park ... Numerous quartzite-pebble xenoliths from the underlying Mio-Pliocene Troutdale gravels which make up the bulk of Mount Tabor have been found in the cinders here. -- Allen, 1990, IN: Wood and Kienle

• Cindery pyroclastic rocks of the Boring lava are present at several localities. The structure of the cinder cone on Mount Tabor is visible in an excavation that exposed the throat and the dipping cinder beds on the flanks. The cinder cone is the southwest patch of Boring lava shown on Mount Tabor. The small area of Boring lava northeast of the cone is flow rock that probably was extruded near the base of the cone in a manner similar to flows emanating from "bocas" at Parícutin in Mexico (Krauskopf, 1948,p.716). The peculiar topographic feature in SE1/4 sec.18,T.1S.,R.3E., about 2 miles southwest of Gresham, is a breached cinder cone. Weathered cinders are exposed along a small creek draining the crater through the breached wall on the west side. Cindery deposits are exposed also along Skyline Boulevard on the west side of the Tualatin Mountains, between Cornell Road and Thompson Road in sec.25,T.2N.,R.3E., on the small hill east of Brunner Hill in sec.24,T.2N.,R.3E., and on Prune Hill, near the road intersection in the SE1/4NE1/4 sec8.T.1N.,R.3E. -- Trimble, 1963

• Visit A Volcano - Mount Tabor

[Map,144K,InlineGIF]
Powell Butte and Vicinity, 1:100,000
-- Powell Butte, Mount Scott, Mount Talbert, Willamette National Cemetery
-- Basemap courtesy TopoZone, 2003

• Mount Talbert: top of hill in Troutdale Formation, 745 feet elevation -- Allen, 1975

• Mount Talbert rises as a forested green sentinel overlooking the web of development that surrounds it and the busy I-205 and Sunnyside Road interchange just to the west. In September 2000, Metro (Portland Metro) purchased 40 acres of woodland on the top of Mount Talbert, bringing Metro's holdings on the butte to 183 acres. This 40-acre parcel is particularly critical because it includes the top of the former lava dome, as well as the west and north facing slopes visible to tens of thousands of people who travel daily on I-205 or visit the Clackamas Town Center. -- Portland, Oregon Metro Website, 2002

• Although the pyroclastic rocks at the Carver vent and the scoriaceous rock at Prune Hill are thought to be closely associated with their respective vents, and Troutdale pebbles are common in Boring lava, the only place that a feeder column for the Boring lava can be seen cutting Troutdale sedimentary rocks is in an abandoned quarry on the west end of Kelly Butte. Here a column of basalt about 20 feet wide cuts about 50 feet of conglomerate of the Troutdale formation, whose bedding has been arched adjacent to the feeder. The lava fed by this vent flowed down a preexisting slope and thickened beyond the point of emission. Local eruption on a sloping Troutdale surface also is represented by the Boring lava on Powell Butte, in NW1/4 sec.13,T.1S.,R.2E., on the hill south of Foster Road, in the south half of sec.13,T.1S.,R.2E., and on Mount Talbert, in sec.3,T.2S.,R.2E. On Mount Talbert a small vent apparently erupted near the eastern crest and a single flow poured southeastward down a previously carved slope. The eruption at Rocky Butte, in east Portland, must have buried a small hill of Troutdale formation, because conglomerate is exposed at an altitude of about 500 feet, just above the road parapet below the tunnel on the west side of the butte. These occurrences may represent eruptions on an irregular post-Troutdale topography, which was subsequently eroded. -- Trimble, 1963

• Mount Zion: 1,465 feet elevation -- Allen, 1975

• Mount Zion is a Boring Lavas volcano that postdates the Troutdale Formation. A small basaltic-andesite intracanyon flow filling a channel cut into the top of the Troutdale Formation can be seen emanating from Mount Zion. Similar vents in the Portland, Oregon, area are inferred to be less than 730,000 years old. -- Beeson and Tolan, 1987, IN: GSA Centennial Field Guide, Vol.1

• Nesmith Point: 3,880 feet elevation -- Allen, 1975

• Nichol's Hill: 1,113 feet elevation -- Allen, 1975

• Pepper Mountain: 2 vents, 2,137 feet elevation -- Allen, 1975

• Pohl's Hill: 1,395 feet elevation -- Allen, 1975

[Map,144K,InlineGIF]
Powell Butte and Vicinity, 1:100,000
-- Powell Butte, Mount Scott, Mount Talbert, Willamette National Cemetery
-- Basemap courtesy TopoZone, 2003

[Map,151K,InlineGIF]
Powell Butte, 1:25,000
-- Basemap courtesy TopoZone, 2003

[Map and Aerial Photo, 156K,InlineGIF]
Powell Butte, 1:100,000
-- Images courtesy USGS and TerraServer, 2003

[Image,140K,JPG]
USGS Aerial Photo - Powell Butte, Portland, Oregon, 2001

• Powell Butte: top of hill in Troutdale Formation, 560 feet elevation -- Allen, 1975

• Powell Butte: 620.0 feet elevation -- Portland Office of Transportation Website, 2002

• Although the pyroclastic rocks at the Carver vent and the scoriaceous rock at Prune Hill are thought to be closely associated with their respective vents, and Troutdale pebbles are common in Boring lava, the only place that a feeder column for the Boring lava can be seen cutting Troutdale sedimentary rocks is in an abandoned quarry on the west end of Kelly Butte. Here a column of basalt about 20 feet wide cuts about 50 feet of conglomerate of the Troutdale formation, whose bedding has been arched adjacent to the feeder. The lava fed by this vent flowed down a preexisting slope and thickened beyond the point of emission. Local eruption on a sloping Troutdale surface also is represented by the Boring lava on Powell Butte, in NW1/4 sec.13,T.1S.,R.2E., on the hill south of Foster Road, in the south half of sec.13,T.1S.,R.2E., and on Mount Talbert, in sec.3,T.2S.,R.2E. On Mount Talbert a small vent apparently erupted near the eastern crest and a single flow poured southeastward down a previously carved slope. The eruption at Rocky Butte, in east Portland, must have buried a small hill of Troutdale formation, because conglomerate is exposed at an altitude of about 500 feet, just above the road parapet below the tunnel on the west side of the butte. These occurrences may represent eruptions on an irregular post-Troutdale topography, which was subsequently eroded. -- Trimble, 1963

• Visit A Volcano - Powell Butte

• Prune Hill (West): 555 feet elevation -- Allen, 1975
• Prune Hill (East): top of hill in Troutdale Formation, 610 feet elevation -- Allen, 1975

• Cindery pyroclastic rocks of the Boring lava are present at several localities. The structure of the cinder cone on Mount Tabor is visible in an excavation that exposed the throat and the dipping cinder beds on the flanks. The cinder cone is the southwest patch of Boring lava shown on Mount Tabor. The small area of Boring lava northeast of the cone is flow rock that probably was extruded near the base of the cone in a manner similar to flows emanating from "bocas" at Parícutin in Mexico (Krauskopf, 1948,p.716). The peculiar topographic feature in SE1/4 sec.18,T.1S.,R.3E., about 2 miles southwest of Gresham, is a breached cinder cone. Weathered cinders are exposed along a small creek draining the crater through the breached wall on the west side. Cindery deposits are exposed also along Skyline Boulevard on the west side of the Tualatin Mountains, between Cornell Road and Thompson Road in sec.25,T.2N.,R.3E., on the small hill east of Brunner Hill in sec.24,T.2N.,R.3E., and on Prune Hill, near the road intersection in the SE1/4NE1/4 sec8.T.1N.,R.3E. -- Trimble, 1963

• The bluff along the south side of Prune Hill, in the SE1/4 sec.8, and the SW1/4 sec.9, T.1N.,R.3E., is red scoriaceous lava that is quarried for ornamental stone (rock walls, rock gardens). This scoriaceous phase is probably vent material of the Prune Hill volcano. Thick jointed flows are exposed a mile to the west in a quarry. -- Trimble, 1963

• Although the pyroclastic rocks at the Carver vent and the scoriaceous rock at Prune Hill are thought to be closely associated with their respective vents, and Troutdale pebbles are common in Boring lava, the only place that a feeder column for the Boring lava can be seen cutting Troutdale sedimentary rocks is in an abandoned quarry on the west end of Kelly Butte. Here a column of basalt about 20 feet wide cuts about 50 feet of conglomerate of the Troutdale formation, whose bedding has been arched adjacent to the feeder. The lava fed by this vent flowed down a preexisting slope and thickened beyond the point of emission. Local eruption on a sloping Troutdale surface also is represented by the Boring lava on Powell Butte, in NW1/4 sec.13,T.1S.,R.2E., on the hill south of Foster Road, in the south half of sec.13,T.1S.,R.2E., and on Mount Talbert, in sec.3,T.2S.,R.2E. On Mount Talbert a small vent apparently erupted near the eastern crest and a single flow poured southeastward down a previously carved slope. The eruption at Rocky Butte, in east Portland, must have buried a small hill of Troutdale formation, because conglomerate is exposed at an altitude of about 500 feet, just above the road parapet below the tunnel on the west side of the butte. These occurrences may represent eruptions on an irregular post-Troutdale topography, which was subsequently eroded. -- Trimble, 1963

[Map,160K,InlineGIF]
Rocky Butte, 1:25,000
-- Basemap courtesy TopoZone, 2003

• Rocky Butte: 2 vents, 612 feet elevation -- Allen, 1975

• Rocky Butte: 609.0 feet elevation -- Portland Office of Transportation Website, 2002

• Boring lava is characteristically a light-gray phyric olivine basalt. A specimen from Rocky Butte is predominantly labradorite, with phenocrysts of olivine, mostly altered to iddingsite. The volcanoes locally contain scoria, cinders, tuff, tuff breccia, and ash. Weathering may extend to depths of 8 meters (25 feet) or more, the upper 2-5 meters (5-15 feet) commonly being a red clayey soil. ... The best view of the volcanic field is from the summit in Rocky Butte Park ... where massive cliffs of flood-scoured lava form the northeast face of the Butte. -- Allen, 1990, IN: Wood and Kienle

• Allen (1975) located more than 30 Boring vents within 21 kilometers of here and more than 90 vents (about 50 of which he classified as "certain") within 32 kilometers of Troutdale (5 kilometers northeast of Gresham). The Boring in this area is between about 1.3 million years old (Rocky Butte, 3 kilometers northeast of here) and 2.1 million years old (in bluffs near Oregon City, 18 kilometers south of here), judging from unpublished K-Ar ages (Sherrod, oral commun., 1988) and a K-Ar age of 1.56 +/- 0.2 million years on a flow at Bear Prairie 29 kilometers east-northeast of here (Tolan and Beeson, 1984). -- Swanson, 1989

• Although the pyroclastic rocks at the Carver vent and the scoriaceous rock at Prune Hill are thought to be closely associated with their respective vents, and Troutdale pebbles are common in Boring lava, the only place that a feeder column for the Boring lava can be seen cutting Troutdale sedimentary rocks is in an abandoned quarry on the west end of Kelly Butte. Here a column of basalt about 20 feet wide cuts about 50 feet of conglomerate of the Troutdale formation, whose bedding has been arched adjacent to the feeder. The lava fed by this vent flowed down a preexisting slope and thickened beyond the point of emission. Local eruption on a sloping Troutdale surface also is represented by the Boring lava on Powell Butte, in NW1/4 sec.13,T.1S.,R.2E., on the hill south of Foster Road, in the south half of sec.13,T.1S.,R.2E., and on Mount Talbert, in sec.3,T.2S.,R.2E. On Mount Talbert a small vent apparently erupted near the eastern crest and a single flow poured southeastward down a previously carved slope. The eruption at Rocky Butte, in east Portland, must have buried a small hill of Troutdale formation, because conglomerate is exposed at an altitude of about 500 feet, just above the road parapet below the tunnel on the west side of the butte. These occurrences may represent eruptions on an irregular post-Troutdale topography, which was subsequently eroded. -- Trimble, 1963

• Visit A Volcano - Rocky Butte

• Ross Mountain: 1,380 feet elevation -- Allen, 1975

• Scout Camp: 3 vents, 945 feet elevation -- Allen, 1975

• Swede Hill: 995 feet elevation -- Allen, 1975

• TV Hill: 1,275 feet elevation -- Allen, 1975

• Walker Peak: 2,450 feet elevation -- Allen, 1975

[Map,144K,InlineGIF]
Powell Butte and Vicinity, 1:100,000
-- Powell Butte, Mount Scott, Mount Talbert, Willamette National Cemetery
-- Basemap courtesy TopoZone, 2003

• Willamette National Cemetery: 910 feet elevation -- Allen, 1975

The primary geologic formation underlying the East Buttes ... is Columbia River basalt. This formation is composed of lavas which erupted from volcanic vents east of the Cascades 17.6 million years ago and which flooded much of the Columbia River basin in one of the largest lava floods on earth. The Columbia River basalt is locally overlain by up to 1,500 feet of sandstone and gravel deposits known as the Troutdale Formation. This formation has two distinct compositions: the lower facies consists of gravels containing quartzite, schists and granites which tie it to the ancestral Columbia River, the upper facies is primarily sandstone of basaltic origin presumably eroded from the Cascades. The deposition of these sands and gravels began ten million years ago and ceased nearly two million years ago (Price 1987).

Near the end of the Troutdale deposition until only a few hundred thousand years ago, a group of shield and cinder cone volcanoes erupted across the lower Willamette Valley. The Boring Volcanoes, as they are collectively known, are comprised mainly of high-aluminia basalts, but locally contain ash, cinders and other materials. These basalts are similar to those of Mount Hood and other Cascade mountains and the Boring volcanism is believed to be tied to the uplift of the High Cascades. The Boring lavas were viscous and did not flow far from their source vents with explosive eruptions being rare. Three of the cinder cone volcanoes are local landmarks ... Rocky Butte, Kelly Butte and Mount Tabor. At Rocky Butte, an intrusive body of Boring lava has been exposed by erosion and uplift. Thickness of the lava ranges from over 600 feet at a vent to less than 50 feet for individual flows away from the vent. Age of the lava is reported to be 1.33 million years (Swanson 1986).

During the early part of the Pleistocene period (beginning 1.6 million years ago), extensive erosion occurred in the lower Willamette Valley lowlands, scouring the lowlands and leaving the prominent volcanoes. Treasher (1942) notes that the Clackamas River once had a course east and north of Mount Scott and nearby hills. He surmises that the Clackamas and Columbia Rivers "shifted back and forth in various channels as they cut down to their present level and must have swept past the sides of these three buttes [Mount Tabor, Rocky and Kelly]." The rocky masses of Rocky and Kelly Buttes were resistant to the erosive forces of the rivers, but evidence of deep cuts in the sides of the buttes can be found. Unlike these two buttes, Mount Tabor is composed mostly of sand and gravel. Treasher speculates that a combination of factors, including deflection of the rivers by Mount Scott and Kelly Butte and the presence of erosion-resistant lava on the lower slopes, enabled Tabor to withstand the erosive forces.

The most spectacular geologic event of recent times, the series of catastrophic floods known as the Missoula Floods, is most directly responsible for the creation of the East Portland terraces. Advancing glacial ice had blocked the Clark Fork River valley in western Montana forming Lake Missoula--a lake 250 miles long and 2,000 feet deep. Repeatedly, between 16,000 and 12,000 years ago, the glacial dam failed causing some of the largest floods known on earth. The flood waters spilled across Idaho and eastern Washington, surged down the Columbia River and through the Gorge, and met head-on with the Boring volcanoes. Rocky Butte in particular stood in the immediate path of the flood waters and its facing slope was cut into a nearly vertical bluff. With the exception of the Boring volcanoes, the entire east side of Portland was submerged under up to 400 ft. of water. The East Portland terraces were formed primarily through deposition of unconsolidated sand and gravel from the flood waters and the short-lived lake in the Portland Basin.

As many as five distinct terraces are now evident in east Portland. Perhaps the best example of the first terrace (at 150 feet mean sea level) is the Overlook Bluff, discussed later in this report. Other terrace levels can be observed along NE Glisan Street and other east-west streets in the area. Evidence of erosion during and after the time of the Missoula Floods can be seen in several deep swaths cut into the depositional surfaces and bedrock. One such swath passes from Rocky Butte and Mount Tabor to the southwest toward Lake Oswego. The most easily recognized example of this erosion is Sullivan's Gulch, a resource site covered later in this report.

Portland Area Volcanoes and Volcanic History - Menu

Return to:
[Boring Lava Field Menu] ...
[Portland Area Volcanoes and Volcanics Menu] ...
[Oregon Volcanoes and Volcanics Menu] ...
[Washington State Volcanoes and Volcanics Menu] ...