Pineo Ridge Glacial Advance & Delta, Eastern Maine
A Field Trip to the Pineo Ridge
Moraine/Delta Complex, Eastern Maine

Boulders abound on the moraine surfaces.

Colby students peer out from behind a particularly large moraine boulder.

Kettles result from melting of blocks of buried glacial ice.
The delta surface is one of the most extensive flat areas in Maine.

The front of the delta bears a wave-cut bench that reveals enormous boulders from an underlying moraine.
Giant foreset beds are exposed in this old sand pit, shown in 1995.
Delta foreset sands interfinger with the bottomset beds of the Presumpscot Formation.
Periodic burning keeps weeds down in the commercial blueberry areas.

The Pineo Ridge area near Cherryfield and Columbia Falls, in Washington County (eastern Maine), provides an excellent field experience for Geology students in studying glacial phenomena. The Pineo Ridge glacial advance, about 12,500 years ago, produced a world-class glacio-marine delta where the glacial ice advanced directly into the margin of the Atlantic Ocean. Though this area is now well above modern sea level, at the time of the Pineo Ridge advance it was depressed by some 400-500 feet, as a result of the Earth's adjustment to the tremendous weight of the late Pleistocene Laurentide ice sheet that had recently covered this area to a depth of perhaps as much as three kilometers.

Boulders abound on the surface of the Pineo Ridge moraine in particular. This and the associated delta are perhaps most famous in Maine as "The Blueberry Barrens", since together they produce about 95% of the entire U. S. crop of low-bush blueberries each year.

In a number of places, the rapid discharge of meltwater and debris buried blocks of the glacial ice. When these blocks of ice subsequently melted, they left ponds and lakes called kettles on the landscape, ranging in size from small kettleholes, like the one shown here, to the 1-mile-diameter Schoodic Lake. Radiocarbon dating of the basal organic sediments in these ponds and lakes helps to provide minimum ages for the melting of the ice.

Where the ice terminated in the margin of the Atlantic Ocean, a broad delta was built up, producing what is some of the most extensive flat land in the state of Maine today. One can walk here in a straight line for 8 miles without rising or falling more than 20 feet from a flat surface, which was why one of the original baselines for surveying of this part of the United States was established here. Subsequent isostatic rebound raised the region back up above sea level even as the ice was melting.

Unobstructed grand views such as those shown in some of these photos are becoming less and less common on the Barrens, as commercial blueberry growers are planting hedgerows of red pine to break the winds and provide shelter for the solitary blueberry bees that are the most efficient pollinators of this tasty crop.

The gently sloping front of the delta bears evidence of the short-lived effectiveness of waves in eroding this enormous pile of sand and gravel. Near here, a fairly clear wave-cut notch can be seen in the delta front, and is dotted with boulders up to 4 meters in diameter; these large boulders were once awash as waves crashed upon the beach.

The foreset beds of the Pineo Ridge delta, exposed in a large sand pit, are explored by a group of Colby students in 1995. These units provide excellent sources for clean sand, and are readily excavated by contractors and others. This also means that the exposures available for study change continually. Note the nesting cavities for cliff swallows at the top of this exposure! The last time we visited this site, it had unfortunately been filled in and completely bulldozed over - a common fate for great exposures of loose Quaternary sediments.

At the base of the Pineo Ridge delta, the foreset beds of the delta can be found interfingered with the fine-grained silts of the marine environment, the sediments of the Presumpscot Formation. These silts here represent the bottomset beds of the delta, and were the sediments that were carried in suspension by the glacial meltwater. Radiocarbon dating of rare shells from the Presumpscot silts as well as of basal organics in local kettle ponds, as noted above, has helped to date the timing of glacial retreat from this part of coastal Maine. Recent (2000-2001) cosmogenic nuclide dating of exposed moraine boulders by Tom Davis (Bentley College) and Paul Biermann (University of Vermont) has confirmed the chronology of events here.

Local blueberry growers use a variety of techniques to keep weeds down and keep trees from invading the Blueberry Barrens. In the photo at the bottom left, a burner is being used in early spring to kill plants other than the blueberry bushes (which are remarkably resilient to this treatment) in late spring. This field will be allowed to remain fallow for one season, to give it a chance to recover from the burning. Since the geologic substrates here are all sands and gravels, this means of weed control is potentially less hazardous than the use of herbicides, residues of which would readily infiltrate into the subsurface and enter the groundwater with melting snow and rains.

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Last modified 25 April, 2001