It's Ripley's Esker: Believe It or Not!
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The listed coords will bring you to another little geologic gem as a result of our last glacial occupation 12,000 years ago. In front of you lies the Ripley Esker.
Minnesota is well known for its many lakes, most of them formed as a result of glaciation. Many other glacial and fluvioglacial features can be found across the Midwest landscape, including moraines, drumlins, kames and eskers.
Eskers are classic linear, fluvioglacial features and generally show a close correspondence with the most recent direction of regional ice movement. Eskers can form beneath ice which is still active, but most likely formed beneath stagnant or retreating (melting) ice.
Eskers are among the more interesting kinds of glacial deposits found. They were deposited by streams and rivers that flowed on top of the glacier, in cracks in the glacier or, in some cases, in tunnels beneath the ice. These Ice-Age rivers and streams deposited gravel and sand in their ice valleys, just as a modern stream deposits sediment in its valley. The very important difference is that the banks of the esker rivers eventually melted away, leaving the gravel deposits standing as ridges, a complete reversal, or inversion, of the topograpy. What had been a valley became a ridge.
These long narrow, winding ridges are composed of stratified sand and gravel deposited by the subglacial or englacial meltwater stream. Eskers may range from 16 to 160 feet (5 to 50 m) in height, from 160 to 1,600 feet (500 m) in width, and several hundred feet to tens of miles in length. They may occcur unbroken or as detached segments. The sediment is sorted according to grain size, and cross-laminations that show only one flow direction commonly occur.
The Ripley esker is a classic esker; it stands 3 to 18 meters above the surrounding plain, and is about 68 to 76 meters wide. Although the Ripley esker is broken into several segments, it has an overall length of about 11 kilometers.
The narrow, sinuous, steep-sided ridge of sand and gravel that forms the esker is flanked on both sides by a row of small lakes and depressions. A vigorous sub glacial stream eroded a channel bed initially wide enough to contain those flanking depressions. The ice that surrounded the stream was under pressure from the glacier's weight. When the stream's flow diminished, the ice squeezed inward to form a narrower tunnel. The smaller, slower stream in the smaller ice tunnel could not carry as much sediment, and it began to deposit it on the streambed. As the sediments accumulated, they raised the level of the streambed and along with it the stream, which continually melted the ice above it and kept an open passage. This narrow, raised streambed grew to a height that became the ridge one sees today above the surrounding plain. The esker slopes down from the ridge along its length because the sediment slumped after the glacier melted away.
The Ripley esker was formed beneath a tongue-shaped lobe of ice called the Superior lobe, which flowed into this area from the northeast about 20,000 to 15,000 years ago. The Ripley esker was deposited as the Superior lobe receded. The sands and gravels of the esker were derived from the Lake Superior basin and include reddish volcanic and sedimentary rocks as well as Lake Superior agate. The pile of glacial sediment (silt, sand, gravel, and boulders) left at the edge of the Superior lobe at the point of its furthest advance is called the St. Croix moraine. It is the broad swath of high hills that lies to the west of the esker.
The best views of the esker are in the spring and fall, when the trees have no leaves and the prairie grasses are the dominant feature on the top of the esker.
To log this EarthCache:
1. Please e-mail the elevation at the geological marker and the elevation at the top of the esker,
2. Also, please note the average size and type of rocks at the esker; and;
3. Please upload a photo of you and/or your gpsr at the esker.
*****Please note this is a State Natural Area and their rules must be observed at all times.*******
* Most SNAs are open to public use, with noted exceptions that are reserved for research or educational use by written permit only.
* Site development varies widely; signage and parking facilities may or may not exist at individual sites. Some now have interpretive kiosks to help visitors identify key features and processes. Public conveniences, including trails, are the exception.
*Primitive conditions fit program goals, and the visitor who can appreciate this very quality will value these sites. Those who look for the best examples of prehistoric geologic formations or pre-settlement natural communities can most readily find them on SNAs.
*Visitors are encouraged to observe and learn, while protecting the plants, animals, and geological features on the site. Please:
• Leave wildflowers, plants, animals, rocks, and other elements in place to fulfill their life cycle and role in the environment.
• Enjoy the site only on foot, snow shoes, or skis, leaving all vehicles off site.
• Leave the site in as pristine a condition as it was when you arrived.
• In general, pets are not allowed on SNAs in keeping with their purpose to protect and perpetuate, in an undisturbed natural state, the rare and endangered plants and animals that inhabit these preserves.
~~Please enjoy this geological gem. How often do you get to stand in the footprint of a glacier?~~
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