The Hillsboro Formation

Pleistocene Epoch

Glencoe Swale is located in an area that was formed during the Pleistocene Epoch when a series of floods, known as the Missoula Floods, scoured out areas of Washington, eroded the Columbia Gorge, and flooded the Willamette Valley. The scope of geological dynamics created by the Ice Age Floods was studied extensively by Doyle C. Wilson from the Department of Geology at Portland State University. He wrote his doctoral dissertation about the geologic history of the Tualatin basin,
“the core from the 334m deep hole…drilled by Oregon Department of Geology and Mineral Industry at the Portland-Hillsboro Airport, provides the primary information for the study…” (Oregon Geology, Vol.60, 1998).
Hillsboro Airport property is where the headwaters for Glencoe Swale originate. Wilson’s study provides important information about the geologic history of the area.


In his dissertation, Wilson provided in-depth descriptions supported by geological evidence to explain differences in sediments that resulted from flooding in the Willamette Valley from those that were deposited in the Tualatin Basin. His descriptions indicate that Glencoe Swale is located in the Hillsboro formation. The sand, silt, and clay suggests there was earthquake activity in this part of the Tualatin Valley 12,000 to 15,000 years ago… a long time after the silts were deposited here during the  Ice Age Floods ( in the Columbia Basin, as much as 1 to 2 million years ago). Wilson’s research also used sediments from the core samples to determine the age of the Hillsboro Formation. Interestingly, pollen found in the Hillsboro Formation characterized the Pleistocene interval with open spruce, fir, and pine forests. It seems that the rich, fertile soils that were deposited here during the Ice Age floods have supported rich forest lands for a long time; and continue to support the rich agricultural lands of today.

Modern Geologic Hazards

Modern geologic hazards in Hillsboro are noted on the Oregon Department of Geology website. Oregon HazVu map for the Glencoe Swale area indicates this geologic location:
  • could be prone to very strong shaking during an earthquake;
  • contains moderate to high earthquake soft soil;
  • is in a 100 flood plain;
  • the silt-sand-clay layer deposited during the Ice Age floods will wiggle and undulate if an earthquake should occur in the Hillsboro Foundation.

-Resources –

Oregon: A Geologic History

Oregon HazVu