<br />12 <br /> <br />certerra.com 4660 Main Street, Suite 100, Springfield, Oregon 97478 T: 541.393.6340 <br />2.0 SUBSURFACE CONDITIONS <br /> <br />2.1 Mapped Geology and Soils <br /> <br />The subject site lies within the Willamette Valley Geomorphic Province, east of the Coast Range and west of the <br />Cascade Mountains Geomorphic Provinces. The Willamette Valley Province is regional lowland that extends from <br />just south of Eugene, Oregon to Vancouver, British Columbia. Within Oregon, this narrow alluvial plain is <br />approximately 130 miles long and ranges from approximately 20 to 40 miles wide (Orr and Orr, 1996). The province <br />is drained by the Willamette River, the longest north-flowing river in North America. Compressional forces attendant <br />with uplift of the Cascade and Coast Range Mountain Ranges during the Miocene and Pliocene epochs <br />(approximately 4 to 20 million years ago) depressed the Willamette Valley. The bedrock lithology of the Willamette <br />Valley in the vicinity of the subject site consists of the late Eocene aged (approximately 35 million years ago) Eugene <br />Formation, a generally well consolidated to lithified, tuffaceous near-shore marine sedimentary rock that was gently <br />folded during the geosynclinal compressional period described above (Yeats et al., 1991). <br /> <br />Prior to deformation, low energy streams and lakes present within the southern Willamette Valley during the Pliocene <br />epoch covered the Eugene Formation with fluvial and lacustrine deposits of silts and clays to various depths. With <br />the rapid uplift of the Cascade Mountains in the Pliocene epoch, steepened stream gradients resulted in increased <br />erosion of the Cascades and rapid deposition of thick gravel layers that incised the soft fluvial and lacustrine deposits <br />overlying the Eugene Formation. Locally, fining upward sequences of rhythmite deposits from the Pleistocene aged <br />(approximately 2.6 million years ago) Missoula Floods are preserved that record up to 30 advance and retreat cycles <br />of Lake Allison (Waitt, 1985), which filled the Willamette Valley to a depth of approximately 350 feet with each flooding <br />event (Allen et al., 1986). <br /> <br />According to the Web Soil Survey's Soil Map - Lane County Area, Oregon, available at <br />https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx), the native soils on the approximate west half of <br />the site consist of Steiwer loam (soil unit No. 125F). Steiwer loam soils are a low hill landform, typically found on <br />sites with slopes of 20 to 50 percent. The parent material is described as colluvium and residuum derived from <br />igneous or sedimentary rock. The typical depth to a restrictive feature is 20 to 40 inches to paralithic bedrock, <br />and depth to a water table is more than 80 inches . This soil unit is typically well drained and is mapped with no <br />frequency of ponding or flooding. The typical soil profile for Steiwer loam is, 0 to 16 inches – loam, 16 to 29 inches <br />– clay loam, and from 29 to 39 inches – weathered bedrock. <br /> <br />The native soils on the approximate east half of the site consist of Dixonville-Philomath-Hazelair complex (soil <br />unit No. 43C). Dixonville-Philomath-Hazelair complex soils are a hill landform, typically found on sites with slopes <br />of 3 to 12 percent. The parent material is described as colluvium and residuum derived from basalt. The typical <br />depth to a restrictive feature is 20 to 40 inches to paralithic bedrock, and depth to a water table is more than 80 <br />inches. This soil unit is typically well drained and is mapped with no frequency of ponding or flooding. The typical <br />soil profile for Dixonville-Philomath-Hazelair complex is, 0 to 14 inches – silty clay loam, 14 to 26 inches – silty clay, <br />and from 26 to 36 inches – weathered bedrock.