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Bald Mountain, Washington Plantation, Maine
Bald Mountain is an outstanding example of an unvegetated mountain summit in western Maine. It is a publicly accessible site that provides excellent exposure of the metamorphic rocks that make up the bedrock of much of this region of Maine and great views of the surrounding mountains. From these exposures, much can be learned about the sedimentary processes that were active as the rocks were deposited and the metamorphic processes that subsequently affected them.
This natural laboratory also affords the opportunity to study the evidence for glaciation in Maine. This evidence includes the broadly rounded surfaces of the bedrock, striations and grooves on the rock surface, and glacial erratics.
Permission: This is a popular site for individuals, families, and groups to hike and enjoy spectacular views. Ownership of this property is not known at this time. Inquire locally for more information. However, the site has generally been accepted to be open to the public.
Location: Bald Mountain is located in Washington Township and is shown on the Mount Blue topographic quadrangle map published by the U.S. Geological Survey. The trailhead is accessed from Route 156.
Access: Ample parking for cars or buses is available in the large pullout along the west side of Route 156. There are no toilet facilities at Bald Mountain.
Group size: Large.
Exposure: The top of Bald Mountain exposes broad expanses of bedrock (commonly called ledge).
Sampling: Do not sample the rocks on this or any other private property without landowner permission.
Directions: From the intersection of Route 2 and Route 156 in Wilton, follow Route 156 north through several turns in Wilton. Continue north on Route 156 for approximately 8 miles until you see the large pullout on the left (west) side of the road. This is approximately one mile south of Hills Pond. If you pass the pond on the left, you have gone too far. Buses will need to turn around before parking. This can generally be done just north of the pullout at a dirt road on the left.
The top of Bald Mountain affords outstanding exposures of the bedrock unit that underlies most of the region. On geologic maps (see Osberg and others, 1985; Pankiwskyj, 1978), this unit is part of the Seboomook Formation, a unit that underlies a large portion of central and northern Maine. The important point here is not the name of the unit but the type of rock and what it tells us about the geologic history of the area.
Another feature that points to the sedimentary origin of the rocks before they were metamorphosed is cross-bedding. Cross-beds are fine layers developed within a single bed. These fine layers are generally not parallel to the base and/or top of the bed and represent rippling of sediment when the bed was formed. Often one set of cross-beds intersects and is truncated by another. Figure 5 shows an example of cross-beds. Another feature that is fairly common in the bedrock of Bald Mountain is folding. Folds develop in response to forces that compressed the rock after the sedimentary layering was deposited. Figure 6 shows an example of folding.
Interpretation of bedrock
The existence of graded bedding, alternating layers of quartzite and schist, and cross-bedding all point to a sedimentary origin for the metamorphic rocks exposed on Bald Mountain. Such features are common in undersea fans where slurries of water and sediment flow down a submarine slope to flatter areas where they slow down and drop sediment. Larger grains generally settle from water faster than smaller grains, resulting in the graded bedding that is so common in these rocks. From this evidence, geologists conclude that the rocks exposed on Bald Mountain were deposited on a submarine slope in an ocean. Each layer of quartzite and schist represents one submarine flow event. It took many events over millions of years to produce the rocks you see exposed on the mountain.
Geologists generally assume that sedimentary layering was deposited in a horizontal position and, with few exceptions, this rule generally holds. On Bald Mountain, layering is tipped up on end. The layering had to be folded from its original horizontal position to its current vertical position. The small folds seen here and there on the mountain are evidence of this episode of folding on a small scale.
Through various means, geologists have determined that the rocks on Bald Mountain are slightly more than 400 million years old. The sedimentary materials were eroded from mountains built when the North American and Eurasian continents collided at that time. The rocks now exposed on Bald Mountain were in the middle crustal level of those ancient mountains and thus subjected to high heat and pressure that metamorphosed clays and other minerals to the micas that are now present in the rocks.
The most recent geological episode to have profoundly affected the rocks of Bald Mountain was the last glaciation. At its greatest extent more than 15,000 years ago, this location was covered by at least several thousand feet of glacial ice. About 13,000 years ago this area became ice free, exposing the landscape you see now.
Glacial erratics are other evidence of the great glacial episode (Figure 9). Erratics are boulders perched on the bedrock that are of a different rock type than the underlying bedrock. They were carried by the glacier from a source some distance away and deposited on the mountain when the glacier melted. A common rock type for erratics in this area is granodiorite (a coarse-grained igneous rock containing abundant plagioclase feldspar and quartz). To the north, the entire basin around Webb Lake is underlain with this rock type. Interestingly, in this area the metamorphic rocks are more resistant to erosion than the granodiorite. This is why the glaciers hollowed out a deep basin in the granodiorite at Webb Lake and left a rim of mountains underlain with metamorphic rocks (Figure 10).
Climbing the trail to the top of Bald Mountain takes about an hour and along the way there are many interesting things to see. Many boulders of quartzite and schist line the trail as it climbs through hardwood forest. It would be useful to point out the characteristics of these rocks to students before they reach the top of the mountain. About two-thirds of the way to the top, note the abrupt change from hardwoods to softwoods at about the point where ledge is exposed in the trail. This is a useful place to have a discussion about the differences in these types of trees and why there is such an abrupt change.
Once out of the woods and onto bare rock, start looking at the layering or bedding. Is it consistently oriented in one direction? Look for graded beds: these generally have light colored quartzite at the base that gradually changes upward to contain more mica. The tops of beds are generally all mica. Can you find beds in which the grading is in the opposite sense to one another? In other words, are there some beds that show the mica-rich top of the bed on the south side and others that show the mica-rich top on the north side? If so, this would be evidence of tight folding that has turned the flat lying sedimentary layers up on end. Look for cross-bedding in the sedimentary layers. These are fine-scaled layers that are at an angle to the overall orientation of the bed. There are often sets of these that cut across one another and are additional indications of the sedimentary nature of the layers.
Several areas on the mountain show folds in the layers. Find a fold and trace an individual layer around it. Does the thickness of an individual bed change in different parts of the fold? If you find several folds, do they have similar shapes and orientations?
There is much evidence for glaciation on the top of the mountain. Generally the rounded sides of the many knobs of bedrock on the mountain face in the direction from which the glaciers advanced and the more abrupt side faces away. Look at several of these features and see if you can convince yourself that they all show the same direction of glacial movement.
The most striking features of glacial origin on the mountain are the abundant glacial striations and grooves. Look at several surfaces that show abundant striations. Are all the striations within one exposure parallel? Which way do they point? Do striations in different exposures point in the same direction? Note that as the glacier flowed around obstacles like Bald Mountain, some sections of the glacier may have locally moved in slightly different directions than others.
Find examples of glacial erratics. These are boulders that are of a different rock type than the ledge on Bald Mountain. How many different types can you find? Where do you think they came from?
References and additional reading:
Caldwell, D.W., 1998, The roadside geology of Maine: Mountain Press Publishing Company, Missoula, MT, 317 p.
Osberg, P.H., Hussey, A.M. II, and Boone, G.M., 1985, Bedrock geologic map of Maine: Maine Geological Survey, 1:500,000-scale map.
Pankiwskyj, K.A., 1978, Reconnaissance bedrock geology of the Dixfield 15-minute quadrangle, Maine: Maine Geological Survey, Open-file Map 78-15, scale 1:62,500.
Text and photos by Robert G. Marvinney.
Originally published on the web as the March 2001 Site of the Month and as a CREST Project field trip.
Last updated on October 6, 2005
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