In later Tertiary and early Quaternary time scores of volcanoes erupted in the Portland area, and in adjoining areas, particularly to the east. The products of this episode of volcanism have been names the Boring lava by Treasher (1942) for their occurrence near the town of Boring, Oregon. They are basaltic flows and pyroclastic rocks of local origin, mostly near their source vents. The rocks of late Pliocene to late (?) Pleistocene age here mapped (in this publication) as Boring lava include those that have been referred to in part to both the Rhododendron and Cascan formations by Hodge (1938, p.877-879 and 884-885), and to the Troutdale formation by Lowry and Baldwin (1952,p.10).
The Boring lava is exposed in eruptive vents, many of which still have their initial cone shape, and in dissected lava plains. The Boring lava is exposed only on higher topographic levels and prominences, but probably it is present locally also beneath the mantle of Quaternary sediments that covers much of the area. Reconnaissance of adjoining areas to the north, west, and south indicates that the Boring volcanism did not extend more than 6 miles to the north or south of the report area and was nearly limited to the area on the west. The extent of equivalent rocks in the Cascaade Mountains to the east is unknown, but is probably great. With the exception of the lava plain south and east of Oregon City and the eroded margin of the lava plain south of the Columbia River and east of the Sandy River, all the Boring lava is locally found around a single vent source or a small complex of vents. The Boring vents in the northeastern part of the map area ... (Web note: map area of this 1963 report) -- are the symmetrical cone of Green Mountain, the volcanic center of Prune Hill, lesser vents at Brunner Hill, and the multiple vent areas of Mount Norway, Nichols Hill, and Bear Prairie near the Washougal River. In the central part of the area are the Chamberlain Hill vent, the many eruptive centers in the Mount Scott area and in the hills near the town of Boring, and the smaller volcanic centers such as Rocky Butte, Mount Tabor, Kelly Butte, and Powell Butte in or near the city of Portland. The tongue of lava near the east edge of the area, between Deep Creek and Eagle Creek, probably came from Lenhart Butte, about 4 miles to the east. The lava exposed east of the Sandy River are the marginal rocks of a lava plain extending eastward that probably originated in such cones as Ross Mountain, Pepper Mountain, Larch Mountain, and Lookout Point. Boring lava underlies much of the surface in the southern part of the area between the Clackamas and the Willamette Rivers. With the exception of the multiple vents between Abernethy Creek and Clear Creek, most of these lavas constitute a vast plain and are thought to have originated in Highland Butte and associated smaller vents. Highland Butte is located about a mile south of the area in sec.9,T.4S,R.3E. The Boring lava west of the Willamette River lies along a northwestward-trending belt on the west side of the Tualatin Mountains and their southern extensions, which form the east side of the Tualatin Valley. The two southern areas of Boring lava in this belt are the eruptive centers north and south of Oswego Lake; the southern one is near Hazelia and the northern one is Mount Sylvania. The largest area of Boring lava west of the Willamette River extends from the crest of the Tualatin Mountains westward to the floor of the Tualatin Valley and lies between Bronson Creek and the upper part of Beaverton Creek, just south of the Portland area. That area probably contains at least four source vents, including Swede Hill, Elk Point, and two vents north and south of Cornell Road on the crest of the hills. The area of Boring lava north of Bronson Creek is probably an extension of the larger area, separated from it by erosion.
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.
The Boring lava is usually thicker at or near a vent than it is farther away. For example, a well more than 400 feet deep on Prune Hill, in the NE1/4 sec.8,T.1N.,R.3E., was still in Boring lava at the bottom. Likewise, more than 500 feet of Boring lava was penetrated in a well drilled for water at the Willamette National Cemetery on the east side of Mount Scott, in the NE1/4NE1/4 sec.27,T.1S.,R.2E. About 2 miles to the southeast, however, in the saddle in the NE1/4SW1/4 sec.36,T.1S.,R.2E., the lava is little more than 100 feet thick. Similarly, the Boring lava is about 300 feet thick near Cornell Road on the west side of the Tualatin Mountains in the NE1/4NE1/4 sec.35,T.1N.,R.1W., but a half a mile west along Cornell Road, in the NW1/4SW1/4 sec.35,T.1N.,R.1W., 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, in the SE1/4 sec.2,T.1S.,R.1W., the Boring lava is about 200 feet thick. The thickness of the Boring lava along the west side of Mount Sylvania, just east of the Willamette meridian, is about 150 feet. In an area a mile or two south of Oregon City the Boring lava is about 100 feet thick, and this may be 10 miles from the source of the lava. 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. The Boring lava is probably between 100 and 200 feet thick in most places except at or near the vent. This generalization probably is true in most cases but there will be many exceptions, both greater and less than the indicated thickness.
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 in NW1/4SE1/4 sec.33,T.1S.,R.1E., 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.
Tuff breccia, ash, and tuff in the Boring lava are known at only one locality. A Boring vent just northeast of Carver, in the NW1/4 sec.18,T.2S.,R.3E., apparently was explosive in the early stages of eruption, but later became quietly effusive. More than 300 feet of tuff-breccia, with a minor interstratified tuff about 10 feet thick, is overlain by lava at this locality. The tuff breccia on the east side of the hills dips 15 degrees to 20 degrees to the northeast, but that on the southwest side dips steeply to the southeast. Loosely consolidated sandy ash beds are associated with the tuff breccia near the base of the exposure on the east side. To the west, below highway level opposite the Carver school, 10 to 20 feet of scoriaceous lava, which is capped by 15 to 20 feet of the pyroclastic rocks, are exposed to within 15 feet of the Clackamas River flood plain.
Although the tuff breccia and tuff are known only at the Carver vent, 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.
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.
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.
The Boring lava is characteristically a light-gray olivine basalt with a pilotaxitic to diktytaxitic texture. Light-brown altered olivine phenocrysts, altered in part or entirely to iddingsite, are characteristic of the rock. The plagioclase is labradorite.
Vugs in Boring lava in the Multnomah Count prison quarry on the east side of Rocky Butte, in the NE1/4SE1/4 sec.28,T.1N.,R.2E., contain an assemblage of minerals that were identified optically by E. J. Young of the U. S. Geological Survey. The sample (USGS serial no.251789) contained several well-crystallized vug minerals of which five were identified. Pale-brown hairlike prismatic crystals were identified as pargasite (optically positive hornblende). They are several millimeters in length, some are twinned, and all have perfect prismatic cleavage. Tiny equant dark-green pyroxene crystals were identified as hedenbergite (70 percent hedenbergite, 30 percent diopside). White ball-like aggregates, slightly more than a millimeter in diameter, are colorless plates of tridymite that have wedgelike edges. Brown granular wedge-shaped crystals of olivine (80 percent fosterite, 20 percent fayalite) are associated with the tridymite. The olivine within the basalt is optically identical with the olivine in the vug. Tiny hexagonal black metallic plates of hematite with beveled edges also were identified from the vug. The assemblage of minerals in the vug is unusual and the association of free silica in the form of tridymite with the low-silication mineral olivine is unexplained.
The surface of much of the Boring lava has been weathered to depths of 25 feet or more. The upper 5 to 15 feet commonly is a red clayey soil that retains none of the original character of the parent rock. Spheroidal weathering is common in the lava and is evident in exposures along the roads at the bluff south of Oregon City, in SW1/4 sec.31,T.2S.,R.2E., and along the highway in the NE1/4SW1/4 sec.32,T.3S.,R.3E.
Most of the Boring lava was erupted onto a surface of erosion on the Troutdale formation. Locally, however, it may lie directly on older rocks, such as the Skamania volcanic series. Such a relationship probably occurs on the north side of Bear Prairie, but the contact is not exposed. At other localities the Boring lava is probably in direct contact with either the Columbia River basalt or the Rhododendron formation. Locally, the surface of the underlying strata was deeply weathered prior to the extrusion of the Boring lava. The Boring is overlain only by Quaternary surficial deposits thought to be of late Pliocene or early Pleistocene age (Walters Hill formation). The preservation of the breached cinder cone southwest of Gresham at an altitude lower than adjacent early (?) Pleistocene alluvial deposits (Springwater formation) suggest that this cone is younger than early Pleistocene. Furthermore, Boring lava at the Carver vent has flowed into a Clackamas valley cut deeper than the late (?) Pleistocene floor. The Boring lava, therefore, is of late Pliocene to late (?) Pleistocene age.
The Boring lava probably is the product of local and discontinuous volcanic activity. Scores of small volcanoes contributed to the formation, but they probably were not in eruption simultaneously. The lava apparently was viscous, because most of the flows did not move far from their source vent. However, the relatively minor amounts of pyroclastic rocks in the Boring lava indicate that explosive eruptions apparently were rare.
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