55 million years ago, Volcanos Arise in the Western Pacific:
55 million years ago, the west coast of America ended at roughly where the Cascade Mountains are now located. Note: Back then, the Cascade Mountains had not yet been formed.
Sea spouts (volcanos) rose in the northwestern Pacific Ocean. Present-day geologists gave this landmass the name “Siletzia”. The sea spouts were likely geologically similar to those that originally formed Hawaii.
Those sea spouts were attached to a slab of earth’s crust known as the Farallon tectonic plate. Over time, that plate moved slowly east, driven by shifts of “a few centimeters per year” Miller, Marla. Oregon Rocks! P. 1. Meanwhile, the west coast of America moved slowly west.
As Siletzia was of volcanic origin, the moving land consisted “almost entirely of basalt”. Ibid. Miller, P. 5 and The Geology of the Portland and Oregon Basin
50-45 million years ago, A New West Coast Appears:
After roughly five million years, the land masses of Siletzia and continental America began to collide. “The Siletzia basalt stretches from Oregon to British Columbia, and it varies from 10 to 32 kilometers thick.” Nerdfighteria.info. The Ancient Island That Transformed Washington .
In other words, Siletzia became the new west coast of Oregon, Washington, and part of Canada and southern Alaska. It made up most of what now is the land west of the Cascade mountain range. For more about this, watch the first eight minutes of Nick Zenter’s “Siletzia Fireworks in the Pacific Northwest”.
Bridlemile’s Geological History
16 to 15 million years ago: Basalt Lava Covers the Ground
The map above shows the area covered by the Columbia Gorge Flood Basalt – including Bridlemile.
Modified from a map in Alt, David and Hyndman, Donald. Northwest Exposures. Mountain Press. Missoula, Mt. 1995. P. 243.
17 million years ago to 6 million years ago, huge volcanic eruptions broke out in what is now eastern Washington and Oregon. The resulting lava (aka the “Columbia River Flood Basalt”) poured out from eastern Washington and Oregon. Much of it flowed down the Columbia River basin west to the Pacific Ocean. In all, it covered over 81,000 square miles of land with over 50,000 cubic miles of basalt.
Washington Dept. of Natural Resources - Washington 100. Columbia River Flood Basalt Volcanism.
The land covered included what is now Portland, including Bridlemile.
“It is believed that the fronts of the lava flows were several stories (approximately 30 meters) high as they flowed from the eruption center at speeds up to three miles per hour.” Idaho State University. Digital Geology of Idaho - Columbia River basalts.
“Approximately 94 percent of the lava erupted by 14.5 million years ago.” Miller, Marli. Oregon Rocks. P. 6.
“Flows of the Columbia River Basalt Group are exposed primarily in the Portland Hills and include the 16 to 15 million-year-old Grande Ronde Basalt.” Miller, Marli. Roadside Geology of Oregon. P. 51.
“Flood-basalt flows of the Columbia River Basalt Group form the bulk of the Portland West Hills.” USGS. A tunnel runs through it -- an inside view of the Tualatin Mountains, Oregon. 2011. Lower-left part of the map.
“Columbia River Basalt is the bedrock which forms most of the Portland Hills. … The Columbia River Basalt is composed of numerous lava flows lying one sheet upon another. The entire mass of basalts has been uplifted to form the Portland Hills.”
Redfern, Roger. “Environmental Geology of the Marquam Hill Area. Portland State University Masters Thesis. 1973. P. 6.
https://pdxscholar.library.pdx.edu/open_access_etds/2124/
After covering Portland, the lava flow continued on to the Pacific Ocean. A prominent remnant of these eruptions is Haystack Rock in Cannon Beach.
Haystack Rock, 2024-08-19
7 to 8 Million Years Ago: The West Hills Appear
The Portland West Hills (aka the Tualatin Mountains), including Bridlemile, rose about roughly 7 to 8 million years ago.
“about eight million years ago the tectonic forces that pushed up the North Cascades also folded these basaltic flows (from the Columbia River Basalt Group) into the Tualatin Mountains, the Coastal Range and the present coast line.” ForestHiker.com.
The Geology of the West Hills
“The Tualatin Mountain Range begins at the Coast Range near Scappoose, follows the Willamette River south as far as the Canby delta, including the gap at Willamette Falls. They were formed from volcanic lava flows about 7 million years ago. For thousands of years, the lava flows came down the Columbia River Gorge from the east near where the Oregon, Idaho and Washington borders meet. The lava flows, known as Columbia River Basalt, created peaks ranging at least as high as 1200 feet above sea level such as Council Crest.” Addington. Yvonne. Tualatin Mountain Range – Lost in Oregon. 2018-10-01. Tualatin Life. 2018-10-01.
Nick Zentner in his Portland Hills Geology video, described how the land’s clockwise rotation helped push up the basalt to form the Portland West Hills.
1.2 Million to 120,000 Years Ago: “Boring” Volcanos Erupt Near Bridlemile
Above: Boring lava (likely), to the side of Highway 26, just west of the Sylvan exit. 2024-08-15.
Bridlemile lies on the north part of the land that includes a series of small volcanos that erupted between 2.7 million and 56,000 years ago. These are known as the Boring volcanos.
Over time, these volcanos erupted from south to north. That is, the Boring vents become younger to the northwest across the region. Wells et al. Geologic Map of Greater Portland. P. 17. As Bridlemile was on the north end of the area, it likely was hit by the more recent of these volcanos, meaning the ones from 1.2 million to 120,000 years ago.
Commonly mentioned volcanic events near Bridlemile include those on Swede Hill north of Barnes Road and near what is now St. Vincents Hospital and at Elks Point between Barnes Road and Highway 26.
“Construction of the TriMet Light Rail tunnel through the Portland Hills exposed a cross section of a 1-m.y. Boring cinder cone built on the west flank of the Portland Hills and subsequently faulted and tilted westward by growth of the Portland Hills Anticline (Walsh and others, 2011).” Wells et al. Geologic Map of Greater Portland P. 16.
“Probably the most interesting feature of Cedar Mill geology is the existence of lava tubes near our eastern areas. These were formed by the eruptions of three small volcanoes, located near where Highway 26 goes into the tunnel. One of these volcanoes is called Elk Point—the others are unnamed. They are relatively young, having formed approximately 244,000 years ago. These light grey lava flows are known as Boring Lava, not for their lack of interest, but because it was first identified near Boring, Oregon.
A lava tube is a tunnel formed when the surface of a lava flow cools and solidifies while the still-molten interior flows through and drains away. One of these lava tubes was found during excavation for St. Vincent’s Hospital in the late ‘60s. It took 6000 truckloads of concrete to fill up the collapsed lava tube to create a stable foundation for the hospital.” Brice, Virginia Bruce. The Ground Beneath Our Feet. Cedar Mill News. 2017-01.
“Boring Lava overlies the Columbia River Basalt in the Council Crest area. Distribution of these lava flows and relief suggest that Council Crest is the probably site of the source vent for the Boring Lava found in the study area.”
Redfern, Roger. Environmental Geology of the Marquam Hill Area. Portland State University Masters Thesis. 1973. P. 6.
“locally along the west slope of the Tualatin Mountains between 200 and 300 feet of clay separates the Boring lava and the Columbia River basalt.” Trimble, Donald E. Geology of Portland, Oregon and Adjacent Areas. 1963. P. 30.
“Boring lava is usually thicker at or near a vent than it is further away. … Boring lava is about 300 feet thick near Cornell Road on the west side of the Tualatin Mountains … but a half mile west along Cornell Road … it is less than 200 feet thick, and near Cedar Mill it is between 100 and 150 feet thick. Near the intersection of Barnes Road and the Sunset Highway … the Boring lava is about 200 feet thick. … The Boring lava is probably between 100 and 200 feet thick in most places except at or near the vent.” Trimble, Donald E. Geology of Portland, Oregon and Adjacent Areas. 1963. P. 38.
“The Boring Lava … occurs along the west edge of the Tualatin Mountains generally above 200 feet elevation … The Boring Lava is principally a gray, olivine basalt having an expanded texture. It is weathered from the surface to depths as much as 15 feet and along joint fractures. The jointing is blocky and an abundance of large, spheroidally weathered boulders occurs at the surface. Associated with the lava adjacent to the vents are pyroclastics composed of lapilli tuff, breccia, and cinders.
The Boring Lava within the project area ranges from a few feet in thickness in the Cedar Hills district to more than 150 feet as indicated by water wells near the Springville Road.” Schlicker, Herbert, Deacon, Robert. Engineering Geology of the Tualatin Valley Region. 1967. P. 23.
20,000 To 5,000 Years Ago: Floods Swamp Bridlemile.
Above photo: Bridlemile school as it might look if the floods from 15,000 years ago had instead happened in current times.
15,000 to 20,000 years ago, the huge Missoula ice-age floods originated near Missoula, Montana, covered eastern Washington and rushed down the Columbia River and into the Willamette Valley. Water rose to up to 400 feet, which is roughly the height of classroom ceilings at Bridlemile School.
“The Missoula Floods, when a huge ice-dammed lake in Montana burst through its dam and the floodwaters charged across eastern Washington, down the Columbia River, and into the Pacific Ocean. … Water inundated what is now downtown Portland to a depth greater than 350 feet. …The Missoula Floods consisted of more than forty floods. …The floodwaters created two sizable but short-lived lakes in Oregon, where the water backed up behind narrow stretches of the channel. One of these lakes formed behind Kalama Gap downriver of Portland, and filled the Willamette Valley all the way south to Eugene.” Miller, Marli. Roadside Geology of Oregon. PP. 28-29.
Temporary Lake Allison.
“After exiting the Columbia River Gorge, mega-floods immediately spread out into the Portland Basin. Further downstream outburst floodwaters backed up behind another hydraulic constriction at Kalama Gap, forcing floodwaters south into the Willamette Valley via two flood channels at Lake Oswego and Oregon City (arrows). Soon after the flow of water drained out through these same narrow conduits. The backwater rose to a maximum of 400 ft. elevation creating Lake Allison, which likely only lasted a couple of weeks or less before all floodwaters had drained out to the Pacific.”
Bjornstad, Bruce Norma. Ice Age Floodscapes of the Pacific Northwest: A Photographic Exploration. Springer. 2021.
14,000 Years Ago: Windblown Silt Covers Bridlemile
Above: Portland Hills silt as displayed in a core sample at the TriMet’s Washington Park light-rail train station. 2023-09-21.
“A yellow-brown clay commonly referred to as Portland Hills Silt that was deposited here during the last ice-age, about 14,000 years ago.” ForestHiker.com. The Geology of the West Hills.
“Accumulated in the Portland hills as a windblown deposit called loess during the ice ages of the Pleistocene Epoch. The loess, which ranges in thickness from 1 to 100 feet (0.3 – 30 m), is relatively strong when dry but becomes weak when saturated with water.” Miller, Marli. Roadside Geology of Oregon. P. 51.
“Loess (Ql) was deposited by strong east winds blowing down the Columbia Gorge during glacial episodes. The loess consists of massive, micaceous, quartzofeldspathic silt and very fine sand forming a widespread mantle on uplands (Lentz, 1981; Lawes, 1997). To depict the bedrock geology, we have not shown the full loess extent, which is ubiquitous west of the Gorge. Thickness decreases westward from 40 m in the Portland Hills to a meter or less in the Coast Range (Lawes, 1997). The upper few meters is pale gray to tan silt, commonly overlying firm, red-brown silt with pisolitic iron-oxide concretions forming the lower part of the loess mantle. Some young loess may be windblown Missoula flood deposits, whereas other loess may be as old as early Pleistocene. Older loess appears to be interbedded with 1.1-Ma lavas of the Boring Volcanic Field in the Portland Hills. (Walsh and others, 2011).” Wells et al. Pamphlet to Accompany Geologic Map of Greater Portland. PP. 18-19.
“Portland Hills Silt: A blanket of silty sediment overlies other geologic units above elevations of approximately 200 feet throughout most of the area. Thickness of the silt averages about 15 feet in the study area, and thicknesses in excess of 42 feet area known in the eastern portion of the area. The silts are uniform, geologically structureless, yellow-brown to buff sandy silt and clayey silt with occasional well-rounded pebbles. These silts have been called Quaternary loess by Trimble (1963), Portland Hills Silt by Lowry and Baldwin (1952), and Upland Silt by Schlicker and Deacon (1967); Portland Hills Silt is probably the most widely used name for this unit. … Samples from power auger holes indicate that the silts increase in clay content with increasing depth.” Redfern, Roger. Environmental Geology of the Marquam Hill Area. Portland State University Masters Thesis. 1973. PP. 8 – 9.
“Silt overlies the older rocks in the uplands at elevations generally from 250 feet to the summits. It caps ridges, spurs, and flatter parts of the Tualatin Mountains, Cooper and Bull Mountains, and the Chehalem Mountains, where it overlies variously the Columbia River Basalt, Helvetia Formation, Troutdale Formation, and Boring Lava. It is extensive east of Portland and Oregon City (Trimble, 1963).
The silt is consistently uniform in appearance, texture, and mineralogy over its entire outcrop area. It consists typically of massive and structureless, yellow-brown to buff, micaceous sandy silt and clayey silt with occasional well-rounded basalt pebbles. The silt and sand fractions of the deposit are composed principally of quartz and feldspars.
Composition of the siIt was determined by Lowry and Baldwin (1952) as containing muscovite, biotite, feldspar, quartz, augite, hypersthene, hornblende, magnetite, tourmaline, garnet, apatite, tremolite, volcanic glass, sponge spicules, and diatoms. X-ray analysis of the siIt reported by Trimble (1963) indicated the clay minerals are kaolinite, illite, and perhaps montmorillonite and chlorite.”
… The Willamette Silt within the project area ranges from a few feet to about 50 feet in thickness. In the Tualatin Valley it is generally 20 to 50 feet thick and thins rapidly along the margins of the valley.” Oregon Department of Geology and Mineral Industries.
Engineering Geology of the Tualatin Valley Region. Bulletin 60. 1967, P. 30.
“On the lower slopes of the West Hills of Portland below the 400-foot elevation contour, most of the fine-grained soils, called the Portland Hills Silt, were eroded away. This soil is composed of wind-blown loess (similar in composition, origin and particle size to the Palouse Formation of eastern Washington) that had been deposited over the hills after being blown out of the Columbia River floodplain.
… Above the 400-foot elevation contour of the West Hills of Portland …, one finds one finds a deep, silt-rich soil called the Portland Hills Silt. This soil (Loess) was mainly created from particles blown off the floodplains of the Columbia River over the past 2.6 million years. The soils range in depth from five feet to over one hundred fifty feet thick. … Most of this airborne silt probably appeared following each of the Missoula Floods.”
Allen, John, Burns, Marjorie, and Burns, Scott. Catalysms on the Columbia: The Great Missoula Floods. Ooligan Press. Portland, Oregon. 2009. PP. 163, 168.