The Geology of Sebago Lake State Park

Features of Geologic Interest in the State Park Area

The geologic history of Sebago Lake State Park as set forth here has been deciphered from evidence available for all to examine. A series of selected features will now be described in detail, so that visitors to the State Park area can see for themselves how Earth history is recorded in rock and landscape.

Bedrock Types

Figure 5

A good place to examine the bedrock of the State Park area is along the waterfront, at the floor of the rocky hill between Witch Cove Beach and Naples Beach. At the west end of Naples Beach, below the pump house, is an exposure of bedrock (Figure 5) that may have been ground smooth by glacial erosion but is now pitted by weathering. Notice the bands of pegmatite streaking the rock surface in wavy patterns. Even without a magnifying glass, you will be able to see how the mineral grains in the coarse pegmatite veins form an interlocking network.

The colorless or gray, glassy-looking mineral in pegmatite and granite is quartz. The pink mineral is feldspar; it breaks along flat planes that reflect light like little mirrors. The third common mineral in granite and pegmatite is mica, which splits easily into thin sheets that may be black or transparent. In addition to these three common minerals, the granite and pegmatite contain much garnet, in the form of sand-size, pink or red glassy grains. In a few places you may be able to find garnets as large as a fingertip, but they will shatter and crumble if an attempt is made to remove them from the rock.

Figure 6

At the West end of Witch Cove Beach at dike of dark igneous rock cuts the granite. The edges of the dike are nearly vertical (Figure 6) and the dike is about 20 feet thick. Examine the margins of the dike where it is in contact with granite. These margins are finer-grained than the central part of the dike. We say that the margins are chilled, because the molten rock cooled more quickly in contact with the granite than in the interior of the dike, and crystals did not have time to form. Notice how sharply the dike cuts across pegmatite veins in the granite. We know the dike is younger than the granite and pegmatite because of this cross-cutting relationship. You can find many other examples of dikes cutting granite in the State Park area. Some dikes are only a few inches wide, but they always cut sharply across the "grain" of the granite and pegmatite.

A Glacial Erratic

Figure 7

Drive or walk to the large gravel pit near the entrance to the State Park campground, at the Thompson Point Road fork. Standing in the center of the floor of this pit is a large boulder of dark gray rock streaked with veins of white quarts (Figure 7). The boulder, measuring about 5x10x12 feet and estimated to weigh at least 27 tons, was uncovered by the removal of sand and gravel from the pit. The boulder is of a metamorphic rock type called argillite, but no large masses of argillite are known in the bedrock of the State Park area. Therefore, this great block is probably a glacial erratic, or foreign stone. We do not know its place of origin, but it presumably came from the northwest, the direction from which the last glacier moved over this part of Maine. A search of any exposure of loose gravel or any rock pile in the State Park will reveal many other smaller erratics.

Till

The sediment abandoned by melting glacier ice is called drift; it includes material deposited directly from the ice and material washed out of the ice and deposited by meltwater streams. Till is the name given to glacial drift that was deposited directly from ice without being washed by running water.

Till is widespread over the rock hills of the State Park area, but it is patchy and thin, and not well exposed. A good exposure of till may be seen in the trail up to the State Park reservoir, across the campground road from the theatre parking lot. As you climb the trail to the reservoir, notice the variety of rock types underfoot. You should be able to identify many pieces of the local granite, pegmatite and dolerite, but you will find many erratics too. Notice also the wide range in size of the rock fragments in the till. Large boulders, pebbles, sand, and fine clay-size particles are all mixed together. The outstanding characteristic of till is lack of sorting by size. It is not the type of deposit formed by a river, which sorts its load according to size and leaves boulders and gravel in its mountain headwaters, sand in its central portion and mud at its mouth.

Figure 8

Till also contains stones with flattened surfaces, or facets, and some of the stones are marked with the distinctive scratches (striations) and grooves (Figure 8) that are also found on some glaciated bedrock surfaces. Some of these stones may have been the tools that gouged into the rock floor beneath the glacier. As they were pushed over the underlying rock, or against each other, the stones were ground flat, then turned and beveled at another angle. The lack of sorting and the shape of the stones in till are two kinds of evidence that indicate till is a form of glacial drift that was deposited directly by melting ice without subsequent transportation by running water. Less than 10% of the stones in most till deposits are faceted and striated, so be patient if you wish to find an example on the trail to the reservoir. Granite and pegmatite stones are too coarse-grained to be susceptible to faceting; they were more often broken into sharp fragments or crushed to sand by glacial action. Look for fine grained, dark rocks as the most likely to be faceted and striated.

A Glacial Kettle Hole

Walk east on the road from Naples Beach campground, through the old entrance gate and past the ranger's cabin. As you walk up the road, you will be on the kame terrace that fringes the hills on the west side of the State Park. On the north edge of the road, about 500 feet east of the ranger's cabin, you will notice a large, basin-shaped hole in the ground, grown full of trees. This hole, or kettle, is about 200 feet across at the top, nearly circular, and about 30 feet deep. It has no valley entering or leaving it, so it could not have been cut by running water. The grass- and tree-covered sides of the kettle slope inward at an angle of about 25 degrees, approximately the angle of slope at which a bank of loose sand and gravel will cease to slide.

Figure 9

This kettle formed during the building of the kame terrace along the west side of the valley. A block of melting ice, approximately the size of the kettle, was buried here in sand and gravel that washed into a gap between till-strewn rocky hills on the west and the melting ice that filled the center of the valley. Protected and insulated by its cover, the buried ice block melted slowly, as more sand and gravel were washed over it. After the kame terrace had been built, the ice block completely melted, and the overlying layers of sediment collapsed into the hole, leaving a neat, round kettle (Figure 9).

Figure 10

Many of the smaller lakes in Maine are kettle lakes, formed as outlined above but now filled with water. Many kettle lakes have neither inlet nor outlet but are fed by springs, and drain by seepage through permeable gravel beds. Sebago Lake is not a kettle lake, but owes it origin in part to a dam of glacial drift that fills a preglacial outlet near Sebago Lake Station at the south end of Lower Bay. The Otter Ponds, which lie on both sides of the Maine Central Railroad tracks half a mile east of Sebago Lake Station, are kettle lakes in this dam of drift (Figure 10). They are 15-20 feet lower than Sebago Lake, and are fed from it by seepage.

The Songo River Valley

The Songo River valley in the State Park area has a two-chapter history, as outlined previously. During deglaciation it received quantities of stream-transported sand, and the valley floor was a flat, sandy flood plain over which the river spread in shallow, shifting, "braided" channels. Later, in postglacial time, the river began to cut into the sandy valley floor and changed from a braiding to a meandering habit. The evidence for such a history is presented below.

As you drive toward Songo Beach from the checking station, you are on the surface of the old flood plain. Notice how flat the ground is on both sides of the road. Stop at the first place the Songo River swings near the Songo Beach Road, and look at the sandy river bank. Notice first that the river is actually increasing its sharpness of curvature by undercutting the bank at your feet and migrating away from the low slope on the opposite side. By this manner, the river forms great loops that are eventually cut through at a narrow place and become abandoned. The swamps along the Songo River are a series of such former channels.

Notice also the height of the river bank at your feet. The present surface of the Songo River is very nearly at lake level. The old flood-plain surface here, at the north end of the State Park, is at least 15 feet above river level. As you drive south toward the beach, you will see that the flood-plain surface slopes downward, and is about 5 feet above river level at the boat launching ramp, and only about 2 feet above river level at the Songo Beach parking area. The flood-plain surface has a southward slope through the State Park of about 10 feet per mile. Such a slope may seem gentle, but it is steep compared to the very slight grade of the present river. At the time the flood plain was being built, the river required this steep downstream slope in order to move its load of sand.

Figure 11

If you have a small shovel or other digging tool, climb part way down one of the river banks and scrape the loose sand away so that you can examine the original layering, or stratification, in the sand that forms the flood plain (Figure 11). Notice how the sand forms layers a few feet long that slope in many directions and end abruptly against each other. This kind of stratification is typical of sand deposited in bars in a shallow, shifting river channel.

Examine a handful of the sand from the river bank. It is composed of quartz, feldspar and mica, as is the granite and pegmatite from which it was derived. Notice the uniform size of the sand grains; no gravel or large stones are present. Notice also the angularity of the sand. If sand is carried a long way in a river the grains are worn smooth and round. Thus we know that this sand has not traveled a great distance.

Here is the evidence for the history of the Songo River Valley: (1) sharp, angular sand derived from a granite source but not carried more than a few tens of miles; (2) a river flood plain built of this sand, having a steep downstream slope, and (3) the present river not building, but eroding the older deposit, flowing slowly and meandering from valley side to valley side. Do you see how the history of a river valley can be learned by interpreting such evidence?

Beaches

Having examined the sand in the banks of the Songo River, you will have no difficulty in recognizing the origin of the sand on the beaches of Sebago Lake State Park. The mineral composition and the size and angularity of the sand on the beaches show that it has been derived from the river banks. Many people comment that the beach sand in the park feels sharper, because the beach sand in the State Park is subject to less stirring and abrasion by wave action than ocean-beach sand.

A sandy delta has been built nearly half a mile into Sebago Lake at the mouth of the Songo River. The delta represents the sand removed from the old flood plain since the river began to excavate its meandering channel. But how does sand carried to the lake at the river mouth become spread along the two-mile State Park waterfront? An observation of shoreline activity in the lake will answer that question.

On a day when the wind is causing waves to strike the beach at an angle, float a chip of wood near the waterline and watch the path of the chip as it is carried by the waves. As each wave breaks, the chip is carried toward the high water line, but not straight up the beach. It is also carried laterally a few inches, depending on the direction force of the wind-driven waves. As the wave withdraws, the chip is carried back down the beach. Thus the chip follows a saw-toothed path, angling up the beach on the crest of each wave, then sliding straight back down. Now dig a small hole at the waterline and watch the movement of the sand grains that fill it. You will see the same pattern followed by the chip of wood. Each grain of sand carried by the waves . . . and there is enough sand in a cupful of turbulent shallow water to cover the bottom of the cup . . . follows the path of the chip, migrating up and down on the beach but also moving laterally downwind. This is the process by which sand is constantly spread from the river mouth to the State Park beaches. The sand never stops migrating; the process of nourishing the beaches is probably as active today as it has been in the past. Under the influence of winds blowing up the lake from the southwest, Songo Beach receives new sand; if the wind is from the southeast, Naples Beach and Witch Cove Beach are supplied.

As the river sand is moved along the beaches, it becomes better sorted. Small flakes of mica, being very flat, are kept in suspension longer by wave action and collect in sheltered places on the bottom. Garnet, the dark pink or red mineral that is scattered in small grains through granite, is slightly heavier than the other minerals in the sand and is abandoned near the waterline as the lighter-colored and lighter-weight grains move on. It is possible to skim a layer of pure red garnet sand nearly an inch thick from some parts of the beaches in the State Park. Unfortunately, garnet sand has only minor commercial value as a non-skid deck and floor surfacing. A hole dug nearly anywhere in moist beach sand will show alternating layers of red garnet sand, black mica, and white or pale pink quartz and feldspar sand, illustrating the variable competence of the lake waves in storm and calm conditions.