7. PRSR 04-11-2007 - Laserfiche WebLink

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A Brief Look at the Natural History of the Study Area The particular resource elements present in any area and their patterns in the landscape are the result of historical processes, including climate, hydrology, plant and animal migrations and interactions, and human decisions and activities. This section very briefly describes the role these interactions have played in determining the present day composition of natural communities and landscapes in the area. Landscape History of the Study Area Bedrock Geology and Glacial Landscapes The landscape history of the area includes bedrock geology. The vast majority of the region is mantled by debris left by glaciers, but the bedrock that is closest to the surface in Elk River is of the Cambrian System, some 5-600 million years ago. The topography of the area was most heavily influenced by the last period of glaciation (the "Wisconsin Stage"), which ended in east-central Minnesota about 10,000 years ago. During this event, glaciers sculpted the landscape and left behind a variety of deposits, including drift/till and outwash composed of sand and gravel, and windblown deposits of very fine sands. During the last glaciation, the Superior Lobe was the first ice sheet to advance into from the region from the northeast, and finally retreat about 20,000 years ago. With each advance and retreat, the melting ice sheet deposited immense piles of sand and gravel along its margins. Outwash deposits from the Superior lobe are found in the north-central part of the city, along Highway 169 and in a small area just southeast of downtown. Throughout the vast majority of the city, Superior Lobe deposits are covered by those of the Grantsburg sublobe of the Des Moines Lobe glacier. The Des Moines Lobe entered the area from the northwest, giving rise to the Grantsburg Sublobe that moved to the northeast, through the Twin Cities and into western Wisconsin. The Des Moines lobe left a variety of deposits that dominate the landscape in the area, including rugged terrain from till, outwash, and more subtle terrain from lake deposited sands. The topography, soils, and pattern of streams, lakes and wetlands that resulted from glacial activity greatly influenced the pattern of vegetation and plant communities that developed later in the area. Existing plant communities such as Oak Savannas and Prairies are well-adapted to droughty soils, and developed in areas where the landscape allowed periodic fires. Areas with a steep, rugged terrain burned less frequently and supported oak woodland communities, while wetlands were found in depressions and valleys with denser, hydric soils. After the glaciers Immediately after the glaciers melted, spruce trees and tundra colonized the periglacial environment. This was later followed by pine barrens and mixed boreal forests with a bracken fern-dominated ground layer. As the climate of the region warmed dramatically Top of the World Park Addition Natural Area Management Plan 1