western slope of the Portland Hills. These formations, created by lava flows cooling at the surface while their hot interior kept draining, were first identified by R. J. Deacon in 1968, then analyzed by L. R. Squier in 1970; they were studied in greater detail by J. E. Allen and his team in 1974. The Catlin Gabel tubes lie among cinder cones and lava flows from the Pliocene to Pleistocene, and they are the oldest known lava tubes in Oregon, older than the Holocene. The tubes were produced by a small vent at the southern end of the northern segment of the field, extending from its base to the south and then the west. They originated from the uppermost lava flow from a series of eruptions that ran into a valley on the western slope of the Portland Hills. The Catlin Gabel tubes have a width of , with slopes averaging per mile for an average grade of 3 percent. On average, these tubes have a thickness of near their center, with an upper lava unit thickness of that has since been modified by erosion and the deposition of up to of Portland Hills silt. The Catlin Gabel tubes also sit atop of silt from the Troutdale Formation. Running along the tube's arc are five depressions, which were created through the collapsing roofs of the lava tubes within a subsegment that is in length. The characteristics of the tube system are not well documented, since only the collapsed segments are accessible. Some of the channels have been reduced to rubble, and analysis has revealed that they trended northwest, had widths up to and depths no more than , and required special engineering procedures to permit the construction of a 15-story building above them. Oregon vents The following vents are in Oregon: Washington vents The following vents are in Washington: Eruptive history Eruptions at Boring Lava Field occur in a concentrated manner, often in clusters of three to six vents, as at Bobs Mountain and Portland Hills. These types of vents typically produced similar types of magma in relatively short periods of time, and they also frequently show alignment. Vents in the field have generally produced basalt and basaltic andesite, with some andesitic eruptions, including those that produced the Larch Mountain shield volcano. Prior to the 1990s, there was little potassium-argon dating data available for the lava field, and despite the