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Siuslaw National Forest |
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Siuslaw National Forest (541) 750-7000 TTY for hearing & speech impaired: |
Geology of the DunesThe Oregon Dunes are like no other dunes in the world. Desert-like landscapes, lakes, rivers, ocean, and forest blend together, creating diverse ecosystems of plants and animals Managed by the USDA Forest Service, this is the only part of the Oregon coast covered by extensive sand dunes. The sand dunes were formed by wind, water, and time. Why Sand?The sand in the Oregon Dunes is from the Coast Mountain Range, which is sedimentary rock that was uplifted 12 million years ago. As rock was moved downstream by rivers, it tumbled and abraded itself into sand. The present shoreline stabilized 6,000 years ago. Tides, wave action and strong coastal winds moved sand up to 2.5 miles inland for thousands of years. This area of dune development rests on a gently-sloping terrace of solid marine sandstone called the Coos Bay Dune Sheet. This low rock surface stretches 56 miles from Heceta Head to Cape Arago and contrasts with steep headlands found on most of the Oregon coastline which prevent inland movement of sand. WindWinds are a major influence in dune formation. Summer winds blow steadily from the north and northwest at 12-16 miles per hour. Mountain barriers near the coast deflect wind currents, sculpting the sand info many different shapes. In winter, winds are generally lighter; however, they can exceed 100 miles per hour during intense winter storms. These winds blow from the south and southwest moving large amounts of sand. Seasonal changes in wind direction reshape dune sculptures and ridges. WaterWater influences dune formation. Strong ocean currents flowing north in winter and south in summer hold sediment from rivers near the shore. Currents, tides and wave action dredge sand from the ocean floor and deposit it on the beaches where the wind takes over. Sand absorbs and stores a large part of the annual rainfall. Where winds have removed sand down to the water table, plants have flourished. In the wet winter, the rising water table creates marshy areas with standing water several feet deep. With the upward pressure of water, the sand grains become more saturated and may float, resulting in quicksand. Look for quicksand in low, unvegetated areas between the dunes. Shaping the SandWind and water are the two strongest forces shaping dune formation. Summer winds blow steadily from the north and northwest at 12 to 16 miles per hour. Mountain barriers near the coast deflect wind currents, sculpting the sand into many different shapes. In winter, winds vary more; however, they can exceed 100 miles per hour during intense winter storms. These winds bow from the south and southwest, moving large amounts of sand. Seasonal changes in wind direction reshape dune sculptures and ridges. Strong ocean currents flowing north in winter and south in summer hold sediment from rivers near the shore. Currents, tides and wave action dredge sand from the ocean floor and deposit it on the beaches where the wind takes over. Sand absorbs and stores a large part of the annual rainfall. Where winds have removed sand down to the water table, freshwater plants have flourished. In the wet winter, the rising water table creates marshy areas with standing water several feet deep. With the upward pressure of water, the sand grains become saturated and may float, resulting in quicksand. Geology Everywhere You LookThe Oregon Dunes are the largest expanse of coastal sand dunes in North America. Spend some time in the dunes, you are sure to see geology in action! Here, the desert-like landscape blends with lakes, rivers, ocean and
forest, creating a diverse ecosystem filled with plants and animal. Day
to day, season to season this ecosystem can change dramatically. From the Shore to the Forests - Take A Closer Look!Beaches Foredunes Hummocks Deflation plains Transverse dunes Tree islands Oblique dunes Oblique dunes move constantly, making it impossible for vegetation to grow on them. The west side of an oblique dune consists of a long, sloping ridge. The east side is usually higher and steeper, forming a precipitation ridge. In winter, the southwesterly winds create freestanding sculptures call yardangs. These unusual carvings can reach 15 feet high. The Umpqua Dunes trail provides access to the best example of these dunes. Parabola dunes Transition forests Estuaries Coastal Lakes Over the years, several lakes have been buried by sand or dried up.
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USDA Forest Service, Siuslaw National Forest |