The Coos Bay dune field is one of ten different sand dune sheets spreading across half of the Oregon coast. How and when did the Coos Bay dunes form?
First, let’s talk about where the sand came from. To understand the dunes, you must first understand the effects of wind and water. Millions of years ago most of Oregon was under water and had a thick, sandy sea floor. Part of this sea floor was pushed up and became the Coast Mountain Range, which is sedimentary rock that was uplifted 12 million years ago. The heavy rain and wind of the Pacific Northwest eroded the sandstone and then rivers carried it to the ocean where the sand is now at least 150 feet deep right off shore. As the waves and high tides carry the sand onto shore, it is dried by the sun and blown inland.
The Coos Bay dune field, directly across the bay in front of you, is the southern end of a 60 mile long dunes sheet that runs north to Florence. They consist of layers of ever-changing sand deposits on top of a hard coastal terrace. The bottom layer is the oldest and most stable---deposited more than 80,000 years ago. The next layer consists of dunes developed between 70,000 and 30,000 years ago. The youngest dune on top began about 5,000 years ago. Dunes advance repeatedly as they slowly progress inland. During the last million years, sea levels, climate, ocean currents, wind directions, and locations of the coastline changed many times. The current eastern boundaries of the dunes in the local area are about 3-4 miles inland.
The natural constantly shifting morphology of the dunes has been disrupted by man. Introduction of European beach grass, which began in the 1930’s, disrupted this movement of sand. The grass was first planted to prevent sand from blocking river channels and roads, but has now spread all along the coast and become a barrier to additional sand movement from the beach. When the sand no longer moves, vegetation easily takes hold. Invading an average of 22 feet of sand a year, plants may entirely cover the dunes during the next 150 years. It's a tough management problem that people are working hard to solve.
While here:
- Estimate the distance of the beach, measured from the ocean to the active foredune.
- Estimate the distance from the beginning of the start of the active foredune to the end of the precipitation ridge.
- Where do you see the most vegetation? Why do you think it’s there?
- Why has the deflation plains formed?
- Finally, to prevent armchair geocaching, find a sign located in this park entitled “Shifting Sands.” On the back is a sticker with a QR code. What is the name of the company, the city the company is located in, and what number is in the blackened circle (hint: it’s between 12 and 12)?