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Geologic History During and After Glaciation
One to two million years ago global temperatures cooled at few degrees to usher in the Pleistocene epoch, during which time huge masses of ice built up on the northern continents. At least four times in this epoch such masses of ice - continental glaciers - formed and spread southward covering more than half of North America and Eurasia. During each glaciation, the level of the earth's oceans lowered by as much as 450 feet (140 meters) due to the water frozen on land as glacial ice.
In Maine we see only the effects of the last glacial ice sheet which covered our area between about 20,000 and 13,000 years ago. The advance of this ice mass and its subsequent pulsing retreat is recorded in the surficial sediments - sand, gravel, and clay - found blanketing the bedrock of much of the region. It is not possible for much of this record to be understood from observations within the area of the two state parks inasmuch as there are neither natural nor artificial exposures of the surficial sediments. We have to look at a much broader area of the state in order to piece together how the last continental glacier advanced over this area, how thick the ice was, how the ice sheet retreated, and how relative sea level changed in response to the melting of the glacier.
Glacial Features and Deposits
The till with the high clay and silt content was deposited during ice advance (see Table 3). This variety of till, which is very compact and impermeable, is referred to as lodgement till. In places, the base of the advancing ice sheet picked up more rock debris and soil than it was capable of transporting and consequently had to deposit this overload even as the ice moved on its way. This material was subjected to the weight of a great thickness of ice above it, and also to the shearing action of the moving ice. In the Portland area, lodgement till generally forms only a thin irregular veneer on top of bedrock.
When the ice sheet had made its furthest advance to the south it covered the entire state of Maine, probably extending out to the area of Georges Banks at the outer edge of the Gulf of Maine. How thick the ice sheet may have been over our area is not exactly known. We do know that it overtopped all mountain peaks of the White and Longfellow Mountains. On top of Mount Washington in New Hampshire, elevation 6,288 feet (approximately 1950 meters), boulders can be found of rock types unlike those which make up the bedrock foundation of the mountain, but which occur in ledges of areas to the north. These boulders, called glacial erratics, were deposited from the glacial ice as it melted. Thus we know in central New Hampshire and Maine that the top of the ice sheet had an elevation of perhaps 7,000 feet (2100 meters). It is not unreasonable to suspect that it was at least a few thousand feet thick in the Portland area. The weight of the ice that existed over this area depressed the crust of the earth by about 787 feet (240 meters). This was to have a great effect upon the way the ice sheet disappeared from this area, and the types of deposits left during and after deglaciation.
About 13,000 years ago, global temperatures warmed and the ice sheet began to diminish. The ice front quickly receded by melting, releasing tremendous quantities of meltwater and the load of rock flour, sand, pebbles, cobbles, and boulders the ice had been carrying. Meltwater streams in front of the melting ice sheet deposited copious amounts of sand and fine gravel which was very well sorted and bedded, called outwash. Outwash deposits are present in the vicinity of the two state parks (Figure 14). Both the outwash and glacial gravels deposited near and around glacial ice are extensively used for fill and highway construction because they are free of the finest sediment particles, silt and clay, and therefore do not retain water. They drain well and pack down firmly, forming ideal surfaces for highway and building construction.
Close to the retreating ice front, in quiet waters away from the mouths of meltwater streams, fine silt and clay - rock flour of the glacial load - were laid down over the previously deposited glacial sediments. We see this silt and clay, locally with abundant marine shells, throughout much of coastal Maine up to elevations of 70 meters (220 feet) above present sea level, and at even higher elevations further inland. This marine clay, as it is most commonly called, is named the Presumpscot Formation. How did the Presumpscot Formation get to be at such high elevations? When glacial ice covered this area, its weight was sufficient to depress the land considerably. As the ice melted, two events happened to change relative sea level: (1) meltwater was released to the ocean to raise sea level, and (2) with the ice gone from the area, the earth's crust began to rise back to its original level; it began to rebound. At first, sea level rose eustatically (i.e., by the addition of meltwater to the ocean) at about the same pace as crustal rebound, thus resulting in little relative change of sea level, and also maintaining the coastline close to the retreating ice front. In time, however, the rate of crustal rebound exceeded the rate of eustatic sea level rise, and consequently the land began to emerge from the ocean waters. When crustal rebound was completed about 8,000 to 10,000 years ago, the Presumpscot Formation could be found at elevations as high as about 220 feet (70 meters) above sea level in the Portland area.
Sea Level Changes and Beach Formation
Glacial geologists suggest that sea level stood as much as 65 to 100 feet (20-30 meters) below its present level 8,000 years ago when the crust had completed its rebound. Between 8,000 and 5,000 years before the present, sea level rose rapidly as the ice sheets quickly melted, and by 5,000 years ago had risen to a level perhaps 8 to 12 feet (3-4 meters) below its present level. Since then, sea level has risen at a much slower rate, and it is during this time that the present beaches have evolved. Prior to 5,000 years ago, the rate of sea level rise was probably too rapid to allow beach-forming processes time to effectively operate. However, with the lower rate of rise after that time, beaches began to form several hundred feet further offshore than their present position, and 8 to 12 feet (3-4 meters) lower. They started as offshore barrier islands and as spits attached to rocky headlands. Behind these beaches shallow lagoons gradually filled with silt and sand, and vegetation began to grow to form the extensive salt marshes we see today.
These beaches were gradually forced landward over the marsh deposits as sea level rose to its present position. This migration will continue if sea level continues to rise (a lesson we should heed well as we contemplate shoreline development projects).
Referring to Figure 14, you will note that the sand of Crescent Beach fringes a zone of swamp deposits representing the remnants of a formerly larger salt marsh. At the eastern edge of the beach, sand has retreated completely across the former marsh and now rests against glacial outwash deposits. At both its eastern and western ends, Crescent Beach is anchored to ledges of the adjacent headlands.
Last updated on January 16, 2008
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