An esker is a serpentine ridge of gravelly and sandy drift, believed to have formed within ice-walled tunnels by streams which flowed within and under glaciers. They tended to form around the time of the glacial maximum when the glacier was slow and sluggish. After the retaining ice walls melted away, stream deposits remained as long winding ridges. Water can flow uphill if it is under pressure in an enclosed pipe, such as a natural tunnel in ice.
Eskers may also form above glaciers by accumulation of sediment in supraglacial channels, in crevasses, in linear zones between stagnant blocks, or in narrow embayments at glacier margins. Eskers form near the terminal zone of glaciers, where the ice is not moving as fast and is relatively thin.
Plastic flow and melting of the basal ice determines the size and shape of the subglacial tunnel. This in turn determines the shape, composition and structure of an esker. Eskers may exist as a single channel, or may be part of a branching system with tributary eskers. They are not often found as continuous ridges, but have gaps that separate the winding segments. The ridge crests of eskers are not usually level for very long, and are generally knobby. Eskers may be broad-crested or sharp-crested with steep sides. They can reach hundreds of kilometers in length and are generally 20–30 meters in height.
The path of an esker is governed by its water pressure in relation to the overlying ice. Generally the pressure of the ice was at such a point that it would allow eskers to run in the direction of glacial flow, but force them into the lowest possible points such as valleys or river beds, which may deviate from the direct path of the glacier. This process is what produces the wide eskers upon which roads and highways can be built. Less pressure, occurring in areas closer to the glacial maximum, can cause ice to melt over the stream flow and create steep-walled, sharply-arched tunnels.
The concentration of rock debris in the ice and the rate at which sediment is delivered to the tunnel by melting and from upstream transport determines the amount of sediment in an esker. The sediment generally consists of coarse-grained, water-laid sand and gravel, although gravelly loam may be found where the rock debris is rich in clay. This sediment is stratified and sorted, and usually consists of pebble/cobble-sized material with occasional boulders. Bedding may be irregular but is almost always present, and cross-bedding is common.
Welcome to the Miller Esker Natural Area. The segment of the Esker here, is owned by the Western Pennsylvania Conservancy, and is open to the public. It is part of the longest segment of the esker, at 3 miles. This particular glacial deposit took shape during the peak of the Kent glacier about 23,000 years ago. The Miller Esker, also known as the Jacksville Esker and/or the West Liberty Hogback. It is probably the best remaining example of an esker in Pennsylvania. It extends back about 6 miles from the glacial margin in two separate segments and remnants of the esker have been detected over nine miles. The esker was originally a continuous ridge throughout its length, but its center has been eroded by Slippery Rock Creek.
At the end of the esker, at the intersection of West Liberty and Moore Roads about a ½ mile toward West Liberty from the site marker, is a large body of sand and gravel that was deposited as a delta where the subglacial stream that formed the esker flowed from the glacier into the glacier-dammed Lake Edmund.
Eskers vary in shape and size. Most are sinuous, with a height of a few to several tens of yards. The longest continue for several miles but most are shorter or discontinuous. Eskers can be broad and flat-topped, or have a single crest or split into parallel ridges. A beaded esker has broad hillocks strung out at intervals along its length. Eskers may locally rise up-slope, a clear indication of water moving under pressure in a pipe-like conduit.
Its configuration gives clues to its formation and the thickness of the glacier. The winding ridge of sand and gravel was deposited as meltwater dropped the sediment inside a tunnel. Upstream, the esker was more sinuous, and probably originated inside a tunnel at the base of the thicker ice. Toward front of the glacier, the esker exhibits more straight line portions, which indicates a thinner glacier where fractures reached its base and controlled the orientation of the tunnel. At the end of the esker lie glacial deposits of sand and gravel. These deposits are the result of the melting glacier dumping sediment as a delta into a glacial lake. A big thanks to the Pennsylvania Department of Conservancy and Natural Resources for the information on the esker.
In order to get credit for this Earthcache, please email me the answers to the following questions:
1. What is the elevation at the base of the esker?
2. While at the top of the esker, what is the elevation and estimated width at the base of the esker (the highest point off West Liberty Road)?
3. What is the elevation difference of the Esker?
4. Is this esker sinuous or beaded?
5. REQUIRED: post a picture of yourself or a personal item at the esker. Please upload your photo(s) with your "found it" log.
Be sure to e-mail me within 7 days of logging the cache to get credit for your work. I will only contact you if there are issues with your answers. If the rules of finding an Earthcache and e-mailing the owner is not followed your log will be deleted without notice.