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Lava flows formed the Columbia River plateau. 
Did you know that the "ridge" of Ridgefield sits on an outcrop of hard, black rock called basalt? About seventeen million years ago, cracks in the earth's surface began spewing molten basaltic lava all across the Northwest. These lava flows, some of the largest on earth, formed the Columbia River plateau - including the "ridge" of Ridgefield! The basalt outcroppings in the refuge are of special interest, because the rock quarried from these areas from 1880 through 1910 was transported by barge to Portland for use as cobble paving stones. top of page Flooding deposited sediments in the Columbia River floodplain and created the land in the refuge. 
Just below the basalt outcropping is the natural Columbia River floodplain. About 12,000 years ago during the last ice age, a huge lake formed in Montana. Known as Lake Missoula, its waters were dammed by ice. As the climate warmed, the ice dam weakened and eventually broke, allowing the water to burst through the ice barrier. This wall of water rushed from the lake down the Columbia River at speeds up to 65 miles per hour, ripping up rock, gravel, sand, silt, and clay. These sediments filled the Columbia River floodplain to as much as 300 feet above sea level. In the Pacific Northwest, beds of gravel and clay left over from this great flood are still found today. However, much of the land in the refuge has also been created since this great flood. Each year the Columbia River and its tributaries flooded the refuge, leaving behind layers of silt, sand, and rock. Today, hydroelectric dams and dikes prevent the Columbia River from flooding across much of the refuge. However, in the Carty Unit, where there are no dikes, the river continues to flood and deposit silt during high water years. These powerful geologic forces created the diverse habitats of the Ridgefield National Wildlife Refuge. 
The Columbia River geology has made it possible for many different types of plants and animals to live in natural communities, or habitats. The Oregon white oak woodlands that thrive on the basalt bluffs are a part of the upland woodland habitat. Below, in the Columbia River floodplain, the soil is wet for much of the year and wetlands, open waters, and riparian woodlands along streams provide habitats for an even greater variety of plants and animals. Try these classroom and field trip activities to learn more about the geology of the refuge. Habitat
Crisscross: Print out this crisscross puzzle, then answer the
clues. Click on the glossary button at the top of this page if you
need help. Learn more about the geology of the Columbia River plateau and floodplains! Clark
County GIS Visit the other learning stations in the Refuge Habitats field trip. |
