Acadia National Park - FLOW: Terrain

Terrain--the lay of the land--interacts with precipitation in determining the pattern of flow of water through a watershed. Where that terrain is largely impervious bedrock, as is found on the granite hills of Mount Desert Island, Maine, water seeks the most direct route of descent along the surface. Where the ground is porous with layers of organic and mineral matter, water flows among the particles in the soil, picking up nutrients and carrying them with it in its descent. The elevation, slope, and physical composition of the local terrain influence the amount of nutrients in water flowing through it, and determine the types of vegetation and animate life that water can support in the local watershed.

The principal BEDROCK FORMATIONS on Mount Desert Island include:

GEOLOGICAL INFLUENCES: SHAPERS OF MDI"s TERRAIN

T he terrain of Mount Desert Island features a range of coastal hills running from east-northeast to west-southwest, a range cut by valleys running north-northwest to south-southeast. The striking ridges and valleys of Mount Desert Island have been largely shaped by four primary forces: vulcanism, glaciation, erosion, and deposition.

Vulcanism here refers to the flow of minerals from deep in the earth toward the surface (whether they break the surface or not), as happened in the geologic forerunner of the state of Maine some 350 million years ago as the result of heat generated by friction between colliding tectonic plates. Some of that flow erupted into the atmosphere, spewing new rock into the landscape of the period, but much of it cooled in plutons (subterranean formations of slow-cooling rock) composed of large crystals of quartz, feldspar, mica, and hornblende--a combination which today we call granite, the principal bedrock of the Bar Harbor Hills (and of the valleys between them).

Glaciation is the scouring action of glaciers pressing on and sliding across the landscape. Wave after glacial wave has traversed the land region we call Maine, propelled by the outward flow of ice from its center far to the north, alternately retreating during warmer interglacial periods when melting exceeded the rate of buildup at the core, only to resurge when the climate cooled, grinding away the hardest rock beneath mile-thick sheets of ice trundling across the land. The tempo at which ice sheets alternately advance and retreat is governed by the 100,000-year Milankovitch cycle affecting differences in seasonal temperatures. The alternation of enduring periods featuring hot summers and cold winters, or cool summers and mild winters, brings about the motion of ice sheets during a glacial age. We live on the cooling edge of an interglacial period, in the aftermath of the Laurentian glaciation, the one we know most about because it is closest to us. Human civilization has blossomed in the brief warm period between the last and coming waves of ice. We credit the coming of that last glacier with the sculpting of the Bar Harbor Hills, but there could easily have been more than thirty glacial advances since granite welled up from plutonic regions, each doing its bit to shape the land we know today as Mount Desert Island.

Erosion is the abrading away of earth"s crust by currents of air, ice, or water (whether fresh or salt). Winds, marine waters, glaciers, and streams strip layer after layer of earth from its substrate of bedrock, which, once exposed, breaks up and is sloughed away as well, successively exposing deeper and deeper levels of geological structure. The terrain detailed on modern topographic maps is only temporary; it, too, will crumble and turn to sand. The watershed basins of Mount Desert Island are no accident; they are products of 350 million years of painstaking erosion which has chiseled and polished the landscape of today, flake-by-flake, grain-by-grain. Water is the medium by which watersheds live and die. It brings them forth, sustains them, and carries them away. Wherever we look, we see a landscape seemingly fixed forever, but actually it is flowing with the forces of creation and destruction, change and time.

Erosion often gets a bad press as the relentless weathering-away of the world we know, but without erosion there would be scant life on Earth. Mountains need to crumble into grains of mineral soil in order to provide enough surface area for water to dissolve calcium, potassium, phosphorus, sulfur, and trace amounts of other minerals it then transports to living systems which require them for health and growth. Water is not the only substance flowing through a watershed. Rock (from boulders down to clay), minerals, soils, and nutrients, too, seek the lowest level they can in sloping terrain. Watershed soils are distributed along a gradient from talus at the base of cliffs to fine organic soils in alluvial bottoms and wetlands lower down. Glaciers coursing at intervals through the land hasten the process, grinding and depositing a range of mineral soils as they advance and retreat. But given the vagaries of climate almost everywhere, big rocks are ground into little rocks by wind, ice, and water acting persistently over time, creating a variety of substrates and microclimates favored by diverse vegetation. Somewhere in that range between clay and boulders, particles present enough surface area to hold water by adhesion while also providing tunnels giving access to air, creating ideal habitat for root hairs that need nutrients, water, and air. Soil not only holds water, but an equable temperature as well, smoothing out abrupt shifts in day and nighttime temperatures, insulating delicate roots against extremes of heat and cold. If cliffs did not break off and resolve into boulders and gravel, watersheds would host mainly lichens on the surface, and know nothing of rooted plants drinking water, breathing air, consuming nutrients underground.

Deposition of the end products of erosion in sorted or mixed layers of clay, sand, gravel, cobbles, and boulders puts the finishing touches on the landscape roughed-out by vulcanism, glaciation, and erosion. The hills of Mount Desert Island stand today largely as they were shaped by the most recent glacier, but the valleys between them contain an assortment of deposits left by glacial streams, retreating glaciers, and ocean waves. Glacial till (unsorted glacial debris compacted by the weight of the glacier) and glacio-marine sediments are common in low-lying areas of the park where they frequently serve as the stony treadway of many miles of hiking trails. Balance Rock on South Bubble in Acadia National Park is a fine example of a glacial erratic boulder transported by the last ice sheet.

WATERSHED SOILS

P roperties of Mount Desert Island soils have great significance for the functioning of local watersheds. Steep slopes and shallow depths to bedrock reduce water infiltration and increase stormwater runoff. Soils not only govern the storage and movement of water, but contribute minerals, dissolved organic carbon, and other materials to ground and surface waters through natural decomposition and weathering.

On granite ridges the predominant soil classification on Mount Desert Island is a complex made up of Schoodic, rock-outcrop, and Lyman soils. This soil complex is derived from tills bearing granite and schist components.

The Schoodic, rock-outcrop, and Lyman soil complex includes extensive areas of exposed bedrock where soils are absent, along with areas where soils exist as thin deposits of gravely sandy loam less than 15 cm deep (Schoodic soils) and areas where soils form a black and reddish sandy loam less than 50 cm deep (Lyman soils). This soil complex is described as excessively well drained, with slopes that range from 0 to 100 percent for bare rock, and from 0 to 80 percent in areas with soil. On steep slopes these soils are usually droughty (prone to drought). Lyman and Schoodic soils are spodosols--acidic forest soils characterized by an accumulation of iron, aluminum, and organic matter.

Precipitation drains so rapidly through these mountainous Lyman soils that it has little time to dissolve minerals of subsequent value to vegetation at lower elevations. Lyman soils are considered "low productivity soils" from a forestry perspective.

Valley soils on Mount Desert Island are typically part of the Hermon-Monadnock-Dixfield complex derived from granite and gneiss tills.

They range from excessively to moderately well drained with slopes up to 60 percent. Below elevations of approximately 40 m, these soils are often underlain by glaciomarine silts and clays of the Presumpscot formation. These and similar soils are found, for example, on the western shores of Jordan Pond and Eagle Lake, and in the Kebo Mountain--Sieur de Monts Spring area.

Wetland soils found in such places as Bass Harbor Marsh, Great Meadow, and Fresh Meadow Marsh along Northeast Creek have a low mineral (rock) and high organic (vegetable) content. These often mucky (containing highly decomposed vegetable matter) lowland soils are characteristically wet for large portions of the growing season, and are generally known as hydric soils.

The Soil Conservation Service (SCS) (1987) defines a hydric soil as a soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic (i.e., oxygen depleted) conditions in the upper part. Hydric soils develop under conditions wet enough to support the growth and regeneration of hydrophytic (water loving) vegetation.

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Last update 8/12/00