The Geology of Sebago Lake State Park

Geologic History

The surface features of Sebago Lake State Park have been shaped by events of the recent geologic past covering only about the last 20,000 years, but the bedrock (the solid rock that underlies soil, sand, gravel, etc.) of the region is very old. The most abundant type of bedrock in the State Park is light-colored granite. Granite is composed of interlocking grains of the minerals quartz, feldspar, and mica. Streaked through the granite are veins of rock that have a mineral composition similar to granite, but have grains that are much larger. This coarse-grained rock is called pegmatite.

Figure 2

The granite and pegmatite are 200-500 million years old. They were formed deep within the crust of the Earth, during the Paleozoic era (Figure 2) under temperature and pressure sufficiently high to partially melt and liquefy older rocks. At a later date, perhaps 165-200 million years ago, during the Triassic period, the hardened granite and pegmatite were broken and hot, molten rock squeezed into available openings and cooled to form cross-cutting layers called dikes. The dike rock is quite unlike granite and pegmatite. It is black, and so fine-textured that individual mineral grains can barely be seen without magnification, but sparkle like lump sugar. The black dike rock is of several varieties, but most of it is a rock called dolerite. Some of this molten rock reached the surface of the ground in central Massachusetts and Connecticut and also in Nova Scotia during the Triassic period, and flowed out of volcanoes as lava. No volcanic rocks of the Triassic period are known from Maine, but they may have existed and now have been eroded away.

Figure 3

Following the period of dolerite intrusion, Maine, as well as the rest of New England, entered a long period of uneventful, slow erosion. Little by little, grain by grain, rivers carved away the rocks, cut valleys, and gradually reduced mountain ranges to low hills. By 70-130 million years ago, in the Cretaceous period, the present outline of eastern North America was taking shape. Only the eroded stumps of former mountains remained, rising as low hills above a monotonous lowland, across which rivers flowed sluggishly toward the sea (Figure 3a). This lowland was then arched upward by forces within the Earth, and streams began downcutting with renewed vigor, dissecting the up-arched surface into many isolated remnants (Figure 3b). The maximum amount of upwarping was probably less than 6000 feet near the center of the present White Mountains, and decreased toward the present seacoast. The highest peaks of the White Mountains and central Maine and some of the hills nearer the coast of Maine are probably the stumps of the ancient mountains that were carried upward when the entire region was upwarped.

Probably no remnants of the upwarped surface remain on the mountain tops of New England (Figure 3c). However, if you drive any distance northwest of Sebago Lake toward the White Mountains, or southeast toward Portland, you will be aware that the altitudes of the highest summits decrease uniformly toward the southeast all the way from Mount Washington to the seacoast. This regional slope of summit altitudes is the only evidence that remains to suggest that a former plain-like lowland surface was arched upward and dissected by erosion.

The last million years of Earth history have been a time of mountain building and climatic change not typical of the long history of the Earth. The mountain ranges we see today have reached their present form in this last, relatively short, epoch of geologic time. One of the significant events of this last million-year interval, known as the Pleistocene epoch (Figure 2) was repeated expansion of continental-sized ice sheets thousands of feet thick over northern North America, Europe, and parts of the Southern Hemisphere. At least four times during the Pleistocene epoch, snow accumulated over regions in the higher latitudes, compacted, and flowed outward under its own weight as glacier ice, which spread over millions of square miles. As each ice sheet advanced, it scraped away soil and loose surface rocks, and using the broken pieces of rock as tools, ground and polished the underlying bedrock. When climatic conditions changed and the ice sheets disappeared, they left behind heaps of gravel and sand that can be used to interpret the history of ice retreat. Each cycle of glaciation may have lasted 70,000 years or more, and was separated from previous and successive glaciations by warmer interglacial intervals, during which the climate was at least as mild as present. The only large ice caps existing on Earth today cover Greenland and Antarctica, but there is no way of telling whether we now live in an interglacial time or whether the most recent glaciation was the final one. Although Maine was probably covered by all of the successive North American ice sheets, the only record that remains is of the last glaciation, which reached its maximum extent about 20,000 years ago.

At the beginning of the Pleistocene epoch, the region around Sebago Lake had once again been reduced by erosion to a series of worn-down, rounded hills and low mountains separated by the branching valleys of river systems. Successive glaciations scraped away all the deep soil, gouged and steepened the sides of valleys, dammed and diverted rivers, and added much to the scenic beauty of the region. But glacial erosion was more effective in adding detail than in altering the fundamental regional topography. Many hills around Sebago Lake, including the high hill on the west edge of the State Park, have steep slopes facing south and more gentle slopes facing north. This symmetry may be the result of glacial erosion that ground smooth the northern slopes up which the ice moved, but plucked and quarried the southern slopes as the moving ice tended to pull away from them.

Can you imagine an ice sheet covering all of the land around you, sliding and grinding over the place you now stand? We know that Pleistocene glaciers were more than 5,000 feet thick over New England and passed over the highest mountains, for rock fragments not like the local bedrock have been collected from the tops of Mount Washington, Mount Katahdin and many other high peaks. These fragments of rock types foreign to an area are called erratics, and could have been carried up to their present location only by moving ice. You can find many glacial erratics in the loose gravel surface cover of Sebago Lake State Park. An example will be described subsequently.

The final movement of glacier ice over Sebago Lake State Park may have been as recent as 10,000 - 12,000 years ago. As the glacial climate moderated, the annual supply of new snow to centers of glacier accumulation decreased, and the loss of ice by melting and evaporation increased, until finally the ice margin ceased to expand and began to retreat. Movement of glacier ice outward from the center of the pile probably continued for a time even though the outer margin was retreating, but when the first high peaks of buried hills began to appear through the thinning ice, glacier movement must have ceased entirely. Many of the mountains near Sebago Lake State Park rise about one thousand feet above adjacent valley floors, so this measurement may be taken as a minimum thickness for moving glacier ice over this part of Maine.

The final ice to melt in the State Park area was a block a mile or more across that filled the floor of the present Songo River valley. This ice block melted away from the rock hills along the western edge of the State Park; and into the gap sand and gravel were dumped, carried there by streams of meltwater draining from the sediment-filled ice. A flat-topped terrace was built between the rock hills and the remnant ice block. Such ice-marginal features are called kame terraces; they are common throughout New England. The kame terrace in Sebago Lake State Park stands nearly 40 feet higher than the river valley floor that borders it on the east; the road to the Witch Cove camping area is on the surface of the kame terrace most of the length of the park (Figure 1 and Plate 3). The original width of the kame terrace cannot be determined, because later river erosion may have undercut its eastern edge and removed part of the terrace.

Figure 4

With the melting of the last block of ice on the valley floor, the western side of Sebago Lake State Park looked very much as it does today. The valley of the Songo River looked very different, however. The Crooked River, which joins the Songo River near the State Park entrance, drains a large area to the north, and apparently carried great quantities of sand-laden glacial meltwater away from ice melting farther north for a time after the State Park itself was ice-free. During this interval, the river through the State Park was a braiding, shifting, shallow stream, full of sand bars, and subject to seasonal floods of icy water (Figure 4a). A wide river flood plain was formed through the State Park area by successive sheets of sand being deposited along the river course. Finally a sandy plain extended east across the valley floor from the foot of the kame terrace to rock hills on the east side of the valley, and continued north for many miles up the Crooked River valley. When the Crooked River drainage basin became entirely free of ice, the river no longer carried quantities of sand and was able to erode a channel into the sandy plain it had previously built. Instead of a braided, sand-choked stream, the Crooked River and its continuation as the lower Songo River flowed as clear water in a channel that was not constantly being shoaled. Small curves in the river course developed into sweeping meanders as the river cut away its banks. (Figure 4b, Figure 4c, and Figure 4d).

The sand excavated by the river was carried to Sebago Lake and dropped at the river mouth, where currents slackened. A delta has now been built out over half a mile into Sebago Lake by the Songo River. Fishermen and boaters are aware of the shifting, shallow, sandy lake bottom near the river mouth. Currents and waves, generated by wind blowing across the lake surface, have moved some of the sand laterally along the lake shore away from the river mouth. The sandy beaches along the State Park waterfront are composed of sand carried to the lake by the Songo River. The processes of erosion and transportation of sand to the river mouth and subsequent distribution along the beaches of the State Park are continuing today.

Introduction Geologic History Features of Interest Geologic Map (pdf)

Last updated on January 11, 2008

Contents

The Geology of Sebago Lake State Park

Geologic History