By AMY CHAPMAN

Kettle bogs, like this one on the east shore of Lower Richardson Lake, formed when chunks of ice separated from the glacier, became buried in glacial debris, and eventually melted, leaving depressions that filled with groundwater.

A recent presentation by Bob Elliott of Bethel on “The Changing Nature of Glaciated Landscapes”explained the origins of many local geological features, including the Greenwood Ice Caves, Buck’s Ledge, Snow and Screw Auger Falls, a hill behind Havoc Fireworks in West Paris and more.

Elliott’s presentation was the final program in the Mahoosuc Land Trust’s 2014 speaker series.

It has been 12,000 years since glacial ice receded from New England, but many of the features of our present-day landscape resulted from the movement of the Pleistocene-era glacier.

Speaking at Gould Academy’s McLaughlin Science Center, Elliott said it is estimated that the vast ice sheet reached a depth of two and a half miles at its center, in the Hudson Bay area, and more than a a mile here in Maine.

One of the most remarkable local remnants of the glacier is a 70-mile-long ridge of gravel and sand, known as an esker, that stretches from northern Oxford County to Cumberland Center.

The “Ancestral Androscoggin Esker,” as it is called by Elliott, a lifelong naturalist and environmental educator, is one of the longest nearly continuous eskers in Maine.

“It’s special in that regard,” he said. “It’s a special work of nature that we even find it, and that it hasn’t been washed away.”

The long, sinuous ridge was created by a river running in a tunnel through glacial ice. The river, which Elliott estimates was at least a quarter the size of the present-day Androscoggin, washed gravelly debris along with it.

“The tunnels are partially filled with sand and gravel, so that when the glacier finally melts, it lowers that ridge of gravel down on top of the land,” he said.

The creation of an esker requires a well-timed combination of geological events.

“The river that created this thing had to be somehow diverted away from it as the ice melted,” Elliott said. Otherwise, he said, it would simply wash the esker away.

The esker begins at Aziscohos Lake and runs along the eastern shore of Lower Richardson Lake, where some parts of it have been covered by water. Parts of the top of the esker emerge from the lake as Hardscrabble and Spirit Islands and the string of long, narrow islands that extends into South Arm.

Some parts of the esker were washed away when the river that formed it broke out of the ice, leaving little evidence in the upper end of the Ellis River valley in Andover, but it reemerges in the lower part of the valley in the form of a sandy, pine-covered ridge that is 60 feet above the valley floor in places.

Since the early 1900s, the esker has been a source of gravel for road-building, and gravel is currently being mined from it for the bridge replacement project at Rumford Point.

A major aquifer located in the esker provides water for the towns through which it runs, and is also the source of the spring in Poland where Poland Spring Water is bottled.

Land features like the “Whale’s Back” in Milton and Day’s Ridge on the Gore Road are actually parts of the 70-mile long esker, Elliott said.

Although much of the esker between Bryant Pond and South Paris was washed away by a break in the ice that formed it, there are numerous places along that stretch of Route 26 where vestiges of it can be seen, including the hill behind Havoc Fireworks. When people sled down this popular sliding hill, they are sledding on the esker, he said.

Further south, in Oxford, Whitney Pond and Hogan Pond are actually a single lake, divided down nearly its entire length by the esker.

In Poland, Route 26 runs along the top of the esker. The former coastline of Maine was just a few miles south of the present location of the town of Poland. There, the river that created the esker also deposited sediment that formed a vast delta.

If you drive south on the turnpike from Gray for about three miles, Elliott said, you drive up a slope that is the front edge of that delta.

“Every time I do that, I get emotional,” he said. “I think, here I am, driving down the front of the delta of the ancient Androscoggin esker river, and gosh, isn’t this wonderful?”

Elliott also gave an overview of the formation of glaciers and discussed several other geological formations common in western Maine that resulted from glacial activity, including kames, kettles, potholes, and glaciated knobs.

Kame terraces run along the sides of a glacier, are made of sand and gravel that washed off the ice, and are usually fairly flat and level.

“Many of our highways on valley edges run along on top of kames,” Elliott said.

Kettles are depressions, usually filled with groundwater, that formed when chunks of glacial ice became buried in the outwash and then melted away. Elliott said there are numerous kettles near the Richardson Lakes and at other locations along the length of the esker.

Potholes formed when rivers which were running at the bottom of the ice layer found their way into small declivities in the surface of the earth and swirled around. The action of the water and the abrasive particles and rocks it carried carved out deep holes over the course of a few hundred years.

Elliott described several local examples of potholes. A series of interconnected potholes and sluiceways can be seen at Snow Falls on Route 26 in West Paris, and also at Screw Auger Falls in Grafton Notch, he said.

He added that the biggest example of a glacial pothole he knows of is also located in Grafton Notch. Known as “the Jail,” it is nearly 35 feet deep and more than 60 feet wide. It has steep granite sides which are difficult to climb, and the bottom is filled with gravel, sand, and other debris.

Land formations called glacial knobs were created when the glacier ground its way over the land, weathering it away down to ridges of bedrock, which were cracked apart along fracture planes. The ice moved slowly up over the north side of the ridges, then, when it reached the top, it plucked away large chunks of cracked bedrock and carried them off.

Glacial knobs are easily identified by a gradual slope on the north side and a steep slope, often a cliff face, on the south side. A familiar local example is Buck’s Ledge, beside North Pond in Woodstock.

The Greenwood Ice Caves are also located on a glacial knob, but instead of carrying away the rock from the south face, the ice separated huge vertical chunks of rock and left them behind when it receded. Eventually, the rock chunks collapsed against each other, forming deep caves that retain ice and snow throughout the summer.

“It got stopped in mid-process, and left these big chunks there,” Elliott said.

On Sept. 27, as part of the Mahoosuc Land Trust’s celebration of the Great Maine Outdoors Weekend, Elliott led a field trip to visit several of the sites he discussed in his presentation. He is also preparing a guidebook for people who wish to take a self-guided tour. Those who would like a digital copy of the guide can request one from Elliott by email at kestrel@megalink.net.