Ice Age Floods National Geologic Trail
Ice Age Floods National Geologic Trail
Six miles west of McMinnville, Oregon, a 90-ton boulder of banded argillite sits on a gentle hilltop 300 feet above the Willamette Valley floor -- 500 miles from its birthplace in the mountains of northern Idaho. This is the Bellevue Erratic, the largest glacial erratic found in...
Narrated audio coming soon · full transcript below
This boulder is in the wrong place, and that is exactly what makes it priceless. It is argillite, a rock that forms nowhere near the Willamette Valley. Its nearest source is the mountains of northwestern Montana and southern Canada, more than 400 miles away.
It did not roll here, and no river could shove a boulder this size uphill into these foothills. It floated. When the ice dam in Montana failed and the floods raced for the Pacific, they carried icebergs broken off the dam, and frozen inside this one was a chunk of Montana mountain. The floodwater backed up into the Willamette Valley as a temporary lake, the iceberg drifted in like a ship, and when the water drained it set its cargo gently down right here.
The elevation is the tell. This rock rests hundreds of feet above the valley floor, which means the floodwater stood at least that deep, this far inland, more than 400 miles from where it started.
Why it matters: a single out-of-place boulder is both a depth gauge and a delivery receipt. It records how high the water rose and how far the ice carried its load.
Bring this stop into the classroom: NPS “Investigating Ice Age Floods” Teacher’s Guide →
Six miles west of McMinnville, Oregon, a 90-ton boulder of banded argillite sits on a gentle hilltop 300 feet above the Willamette Valley floor -- 500 miles from its birthplace in the mountains of northern Idaho. This is the Bellevue Erratic, the largest glacial erratic found in the Willamette Valley, and its journey is one of the most remarkable stories in geology. The boulder was frozen inside an iceberg that calved from the Purcell Trench ice dam and floated on the Missoula Floods all the way down the Columbia River corridor. When the floodwaters backed up behind the Kalama Gap chokepoint and formed temporary Lake Allison in the Willamette Valley, the iceberg drifted south until it melted and dropped its cargo on this hillside. The rock's composition -- 1.5-billion-year-old Belt Supergroup argillite -- can be traced precisely back to its source near the ice dam, providing irrefutable evidence of the floods' reach. A short uphill hike leads to the erratic, where interpretive signs help visitors grasp the vast landscape of floodwater that once filled this valley to 400 feet elevation. Touch this ancient stone and you are touching a piece of Idaho that rode an iceberg across four states.
Erratic Rock State Natural Site (also called the Bellevue Erratic) is a 4.4-acre Oregon State Park unit off OR-18 between Sheridan and McMinnville, Yamhill County. It is day-use only with a small parking area and a 0.2-mile paved trail up to the boulder; no fee. Year-round.
The boulder is the largest known glacial erratic in the Willamette Valley — argillite of approximately 600 Ma sea-floor origin, sourced from the Belt-Purcell Supergroup in southeast British Columbia. It was rafted into the Willamette Valley on an iceberg floating on temporary Lake Allison, the slackwater pond that formed when Missoula floodwaters back-flooded the valley behind the constricted Kalama Gap on the lower Columbia. Lake Allison stood at roughly 400 ft elevation, and the iceberg grounded on the present hillside as the lake drained. The original boulder mass was probably about 90 short tons; what remains today is roughly 40 short tons after a lightning strike and weathering. The trip from its source to its resting place is over 500 miles.
The most-cited age window remains 12,000–17,000 years ago in popular materials; Balbas et al. 2017 narrows the dominant floods to 18.2 ± 1.5 ka and 15.6–14.7 ka. No site-specific provenance research published since 2017. No updates found since 2017.
Within the Willamette Valley chapter's interpretive territory. State Parks panel on site; no IAFI-branded panel.
Year-round; mild grade, accessible. Pair with the Wapato Lake National Wildlife Refuge and Yamhill Valley wineries for a half-day. A small overlook gives a sense of the Lake Allison elevation across the valley.
Every site along the trail will receive the full Terrain360 capture treatment: ground-level 360° panoramas, drone aerial imagery, and photogrammetry-based 3D models that visitors can spin in their browser. This page reserves the slots; the imagery flows in as field capture completes.
Ground-level 360° panorama, every step along the feature, captured by Terrain360 field crews.
Drone flyovers reveal the geometry of catastrophe — ripple marks, gravel bars, and scour patterns invisible from the ground.
Photogrammetry and Gaussian-splat models let visitors rotate, measure, and inspect features in detail-page WebGL viewers.
Ice Age Floods National Geologic Trail
Six miles west of McMinnville just off of Hwy 18 sits a 90-ton rock, the Bellevue Erratic, that was floated as much as 500 miles in an iceberg by way of the Columbia River during the Ice Age Floods.
The largest iceberg erratic found in the Willamette Valley, it’s originally from the Northern Rocky Mountains.
When the iceberg in which it was encased melted, the rock was left behind at the 300 foot elevation level.
A short uphill hike leads visitors to the Erratic Rock State Natural Site, where they can look out across the vast landscape and imagine the huge amount of water that filled the Willamette Valley during the Ice Age Floods.
The Bellevue erratic rests upon a gentle hill 150 ft above the floor of Oregon’s bucolic Willamette Valley.
The erratic floated in on an iceberg during a Missoula mega-flood when floodwaters backed up to 400 ft elevation behind the Kalama Gap choke-point, forming temporary Lake Allison.
This is akin to Wallula Gap forming terminal Lake Lewis and Lake Condon on the southern/downstream side of the Gap, and the erratics found at ~1200ft asl there.
The erratic Bellevue boulder consists of banded argillite, which can be traced back 400 miles or more to the 1.5 BILLION-year-old Belt Supergroup that lay beneath the ice dam for glacial Lake Missoula in northern Idaho!
This is the source of pinkish Missoula flood sediments, as well.
The discovery that erratic rocks were found at or below the 400 foot elevation in the Willamette Valley indicated that the water inundated this region from Portland down to Eugene up to 400 feet above present day sea level!
The Portland region owes its rich agriculture, beautiful geography, and many handsome erratics to a series of massive Ice Age Floods that burst from an ice dam, the last of them about 15,000 years ago with a few smaller Glacial Lake Columbia mega-flood pulses finishing off around 14 kya.
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