Alluvial Deposits:

"During glacial maxima, sea level was as much as 400 feet (120 meters) lower than at present. The Columbia River in response deeply incised its bed, flowing through a narrow valley about two kilometers west of the map area (Ridgefield Quad). The Lewis and East Fork Lewis Rivers presumably also adjusted to lower base level at these times. Late in the last glacial period, huge glacier-outburst floods from Glacial Lake Missoula coursed down the Columbia River valley and hydraulically ponded in the Portland Basin; silt and fine sand that settled out of this temporary lake now cover much of the map area. As sea level rose during the late Pleistocene and Holocene, the late Pleistocene valleys gradually filled with sediment."

Cataclysmic-flood deposits:

"During the last glacial maximum in late Pleistocene time, an ice dam impounded Glacial Lake Missoula in western Montana. The dam failed repeatedly, releasing floods that coursed down the Columbia River and into the Portland Basin. The sediment-laden floodwaters were hydraulically constricted by the narrow reach of the Columbia River valley north of (downstream from) the Ridgefield quadrangle. The constriction caused temporary ponding in the Portland Basin and tribuatry valleys to levels as high as 400 feet (120 meters). Radiocarbon ages, paleomagnetic measurements, and tephrochronologic data indicate that the last glacial episode of floods occurred chiefly between about 17,000 and 13,000 ¹⁴C years B.P. Similar episodes of cataclysmic flooding probably occurred earlier in the Quaternary.

During each flood, the suspended load of fine sand and silt settled out of the temporarily ponded floodwaters. In the northern Portland Basin, multiple floods collectively built up deposits of laminated micaceous sediments as thick as 100 feet (30 meters). These slack-water deposits (Qfs), which grade almost imperceptively northward from fine sand to silt, now mantle the entire surface of the Ridgefield quadrangle south of the East Fork Lewis River. Partly eroded flood deposits lie north of the river between 200 and 300 feet (60 and 75 meters) elevation, and local unmapped patches of micaceous silt are found at elevations up to 400 feet (120 meters)."

Late Pleistocene and Holocene alluvial deposits:

"Remnants of a terrace with surface elevations of 110 to 140 feet (33 to 43 meters) lie along the south bank of the Lewis River directly downstream from its confluence with the East Fork Lewis River. The terrace deposits (Qtd) consist of unconsolidated, poorly sorted sandy pebble gravel and lithic sand overlain by micaceous silt of Missoula-floods origin (Qfs). Terrace-gravel clasts are chiefly Tertiary volcanic rocks eroded from the Cascade Range, indicating that the terrace sediment was carried by the Lewis River rather than the Columbia."


Source:    R.C. Evarts, 2004, "Geologic Map of the Ridgefield Quadrangle, Clark and Cowlitz Counties, Washington", USGS Scientific Investications Map 2844.