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Geologic features and processes in Voyageurs reflect the two widely separated time periods that formed the present landscape: the older, 2.5 billion year old Precambrian (Archean) and the Pleistocene Epoch ice age. The ancient rocks form part of a large area of basement rocks called the Canadian Shield, which is part of the North American craton.
The Archean rocks at Voyageurs are associated with a mountain- building episode called the Kenoran (Algoman) orogeny. The Kenoran orogeny is the earliest datable orogeny in North America (Harris et al. 1995). Both igneous and metamorphic rocks are exposed in Voyageurs. Schists and gneisses, the products of metamorphism, are exposed in the west and central portions of the park (Rainy Lake and Kabetogama Lake) while granite, an igneous rock, is exposed on the east end at Namakan Lake and Sand Point Lake. Immediately south of the park and extending to the Vermillion Fault is a broad transition zone that contains bands of migmatite, a rock type that has characteristics of both igneous and metamorphic processes.
The advance and retreat of Pleistocene glaciers as recently as 10,000 years ago left an indelible imprint on the rock in Voyageurs. Characteristic features of continental glaciation observed in Voyageurs range from scratches and grooves on bedrock to the chain- like pattern of lakes that formed from glacial scouring.
Precambrian Features and Processes
In Precambrian time deep crustal processes transformed sedimentary deposits into dense, recrystallized, and often foliated, metamorphic rocks. High pressures and temperatures at depth preferentially align mineral crystals, especially mica, in metamorphic rocks. Depending on the degree of pressure and temperature and the original composition of the sediments, this crystal alignment imparts a foliated or schistose character to the rocks. Submarine mudstones are converted to various schists. Siltstones, sandstones, and conglomerates are metamorphosed into metasiltstones, metasandstones (quartzites), and metaconglomerates. At Voyageurs a result of these metamorphic processes is the biotite schist that dominates the landscape.
The Rainy Lake- Seine River fault zone, which separates the Quetico subprovince from the more northern Wabigoon subprovince, is a major, northeast- southwest trending strike- slip fault zone, trending N 80� E, that cuts through the northwestern part of Voyageurs and extends westward to International Falls. The fault has transported the rocks in the greenstone belt a considerable distance from their place of origin. The greenstone belt is 1.25- 2 miles (2- 3.2 km) wide and includes Big American Island, Dryweed Island, and the smaller islands between Dryweed Island and the Kabetogama Peninsula. At the Seven Sisters Islands further to the west, greenstone interfingers with pods of anorthositic gabbro.
Greenstone belts are not as highly metamorphosed as the other Archean rocks in Voyageurs National Park. They are of special interest to economic geologists because greenstone belts are often host rocks for metallic mineral deposits. All of the known metallic mineral occurrences in Voyageurs National Park and the surrounding area have been found in the metasedimentary and metavolcanic rock units in the Rainy Lake greenstone belt. Greenstone belts form unique elongated downwarps of mixed metasedimentary and metavolcanic rocks. The green color comes from minerals such as chlorite, epidote, and actinolite that formed during metamorphism.
Pillow lavas in the greenstone belts indicate that they were extruded in a marine seafloor environment. Most of the pillow structures in the Voyageurs area have been destroyed by metamorphism, but a few "pillows" up to three feet long (1 m) have been found on a small islet off Steamboat Island (north of Cranberry Bay).
Some of the poorly sorted mixtures of clay, mica, and quartz (graywackes) are derived from the decomposition of pyroclastic debris (tuff and ash) suggesting that some explosive volcanic activity occurred in the area during Precambrian time. Lapilli tuff can be found near Dog Island. Other clastic material has characteristics of rapid deposition by marine currents, possibly in an ocean trench (Harris et al. 1995). The finegrained sediments lithified into mudstones or siltstones that later metamorphosed to chlorite- schist, a green, foliated rock. The coarse silt to sand- sized sedimentary part of the greenstone belt formed a feldspar- rich sandstone, or arkose, that metamorphosed to metaarkose.
Interspersed with these rocks and the pillow basalts are seafloor hot- spring- derived chemical sedimentary layers high in silica and iron carbonate that may locally contain metallic sulfides and precious metals. These layers became banded iron formations, a principal source of iron in the Mesabi Range of northeastern Minnesota. Amorphous metallic sulfides and minor gold and silver are often found along with silica and iron in these banded iron formations. These minerals probably formed in the Precambrian by much the same geologic processes that are active on modern day seafloors. Hot spring vents on today's seafloors often have chimneylike mineral walls built up around them. These vents are referred to as black smokers. Minerals associated with sediments deposited around black smokers include copper, zinc, lead, nickel, manganese, gold, and silver.
The granitic rocks in Voyageurs are part of the Vermillion Granitic Complex and vary widely in composition. Rock types include granite, biotite granite, granodiorite, and quartz diorite (Davis et al. 1994). The Lac La Croix biotite granite batholith is the largest and youngest igneous intrusion in the area. It also comprises the principal rock type in the majority of the Boundary Waters Canoe Area Wilderness. Age dates on these granitic rocks show them to be almost as old as the schists, ranging in age from 2,690 million years before present (B.P.) to 2,640 million years B.P.
During a period of very slow cooling in the late stage of granitic emplacement, numerous thin, tabular- to irregular- shaped bodies of very coarse- grained crystalline rocks solidified. These coarse- grained igneous rocks are pegmatites. Pegmatites are recognized by their coarse- grained texture, pink coloration (potassium feldspar), and a composition similar to granite but often with rare minerals, some of gem quality.
Precambrian Fault Systems
The Kabetogama- Kenora Dike Swarm formed about 2,200 to 2,100 million years B.P., in the Proterozoic Eon, when crustal bulging opened a northwest- trending fracture system in the area. Dark, intrusive rocks including gabbro, quartz diorite, and quartz diabase filled the fractures. The Kabetogama- Kenora Dike Swarm in Voyageurs is typical of the tabular dikes that formed in the long, linear fractures. They can be traced for several miles in length, while seldom exceeding 300 feet (91 m) in width. A mafic dike cuts the bedrock at the Kabetogama Visitor Center. Magnetite (magnetic iron oxide) is present in some of the dikes, allowing them to be mapped by means of surface or aerial magnetic surveys, even in areas of deep soil cover (Davis et al. 1994).
The regional faults that cross almost perpendicular to the Kabetogama- Kenoran Dike Swarm are a secondary set of faults subparallel to the subprovince boundary structures. These faults cannot be traced for as great a distance and do not appear to have significantly displaced major rock units as those in the Kabetogama- Kenora Dike Swarm. The most notable segments of this fault system are the two faults within Voyageurs that are responsible for the two Chains of Lakes. The westernmost of these two faults controls the alignment of Locator, War Club, Quill, and Loiten Lakes while the second major structure is expressed by topographic lows containing Oslo, Brown, and Beast Lakes, and the western extension of Mica Bay (Davis et al. 1994).
The third set of regional structures form a grid- like fracture network in the granite. There is a very strong north- south fracture set and a similar, linear, but somewhat less well- defined east- west fracture set. The fractures are probably related to the cooling of the batholith.
Three phases of Precambrian deformation are recorded
in the rocks at Voyageurs. The deformation folded the
bedrock and can be recognized by the following (Day
Quaternary Features and Processes
Because the Voyageurs area is well within the major advance of the continental glaciers, very few morainal deposits exist. The prominent landforms are glacially scoured surfaces and extremely thin residual soils. Four types of glacial deposits are found in Voyageurs: lakebed sediments, outwash gravels, peat, and till. Glacial outwash and till (glacial debris) deposits are usually less than 100 feet (30 m) thick (Davis et al. 1994; Harris et al. 1995). Glacial debris and outwash deposits left by retreating glaciers thicken westward to the extensive lake bed deposits of postglacial Lake Agassiz. Lake Agassiz was a vast, inland body of water that covered hundreds of square miles north and south of the international boundary for several thousand years (Teller and Clayton 1983; Harris et al. 1995). Four of the five major phases of Lake Agassiz are recorded in the Voyageurs region (Teller and Clayton 1983; Davis et al. 1994).
Holocene peat deposits formed in hollows or depressions such as old lake basins, bogs, and swamps.
Deposits of unsorted glacial debris, or till, formed ground moraines and end moraines. Ground moraine debris was deposited under the ice sheet and interspersed with lakebed sediments forming an undulating surface known as "swell- and- swale" topography. End moraine till left hummocky ridges along the front of the ice sheet. One of these till deposits, left by the last, glacial retreat can be seen along the southwest shore of Rainy Lake.
Glacial erratics are abundant in the Voyageurs area. These rocks of various compositions were transported from the north by glaciers and deposited randomly over the landscape when the glaciers melted. They range in size from pebbles to large boulders. For example, a large, light- colored granitic rock approximately 10 feet (3 m) in diameter is visible in Cranberry Bay where it rests on dark colored schist bedrock. Also, migmatite erratic sits on chlorite schist of the greenstone belt along the shoreline of Skipperrock Island in Rainy Lake.
Glacial striations and glacial grooves appear as parallel, linear scratches or furrows on bedrock and erratics. Striations form from the scraping and scratching of quartz grains and hard, sharp rock fragments frozen into the bottom of the ice scratching across rock surfaces as glaciers move. Striations are generally oriented in the direction of glacial movement. In Voyageurs National Park, striations point south, south by southwest, and southwest (Harris et al. 1995). Cross- striations on some outcrops indicate more than one direction of ice movement.
Repeated abrasion by glacial movement has smoothed and polished many of the bedrock exposures in the park. On highly resistant outcrops in Voyageurs, glacial polish still appears glossy and fresh or only slightly weathered.
Ellsworth Rock Garden
Ellsworth was also a sculptor and, with his wife, collected thousands of rocks of all sizes from all over the area, often transporting them across the lake. The Ellsworths used the rocks to terrace the hillside near their home and within the terraces, they erected stone sculptures of a variety of forms. For example, a stone duck is perched on the side of a hill overlooking the lake, a deer stands behind a tree, a flat slab of rock supported by three triangular rocks forms a table, and stone monoliths stand in the woods.
At its peak, the Ellsworth Rock Garden contained over 200 sculptures. About 80 of these have been restored and in 2001, Lamp, Rynearson and Associates, Inc. worked with historical architects to retrace and locate as many features as possible using GIS.
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