This is an EarthCache. There is no physical cache to find. Logging an EarthCache requires that geocachers undertake an educational task relating to the specific Earth Science at the site. If you don't want to answer the questions, don't log this EarthCache.
Prior to logging this cache, click on Message this owner above, and send answers to the following questions:
1. How high are you above the Au Sable River? There are several ways you can determine this:
- Take an altitude reading here. Climb down to the bottom of the dune and take another reading. Do the math. (Heed the warnings if you use this approach.)
- Looking down, estimate the distance.
- Read the Geological Society of America’s sign titled the same as this earthcache.
2. Looking at the face of the dune, what was the direction of the prevailing wind? (Hint: The wind came from this direction to form the dune. Answer can also be found on the sign.)
3. The sign mentions two types of sediment that make up this dune. What are they and how were they deposited? Can you notice the difference on the dune face?
4. Required to log this cache: Please provide a photo of yourself or a personal item in the picture to prove you visited the site. Upload the photo with your log.
Au Sable Dunes
In front of you is an example of a “perched dune.” Perched dunes lie on top of high bluffs. Michigan has many perched dunes, but they are predominantly along the Great Lakes. Two well known areas for perched dunes are at Sleeping Bear Dunes National Lakeshore and Grand Sable Dunes in Pictured Rocks National Lakeshore.
Perched dunes are also known as cliff-top dunes because they can occur on the tops of cliffs. They are uncommon in most parts of the world, because they only develop under unusual geomorphological conditions. Processes by which they may be formed include:
- the dune advances up a pre-existing slope, which is then eroded to form a cliff;
- the dune forms during a period of high sea level, then the sea level drops, exposing a cliff face.
15,000 years ago, this area was under a glacial lake. As the lake drained, what we call the Au Sable River, began cutting into the banks. The river banks were unstable and the sand was easily swept into dunes “perched” on top of the banks.
Michigan was logged extensively in the 1800’s. This area was used as a rollaway or log slide to get harvested logs down to the river during the spring river drive to float the logs out to Lake Huron.
