Natural History: Minnesota's Geology
Minnesota SNAs help us discover how the landscape formed and then nurtured natural communities. This introduction reviews the development of Minnesota's landscape, then introduces the natural communities that have developed here.
Figure 1.1 Geologic timeline.
In this quest for our past, truth proves to be stranger than fiction; Minnesota, icebox of the nation, once sweltered in tropical heat. Minnesota, breadbox to the world, was once the barren scene of titanic mountain-building and volcanic activity. This same country has been covered by occasional seas--slowly advancing, then ebbing. Much later came the ice sheets. To read such a history stretches the human imagination and instills a new perspective on life.
The trail of discovery can lead to unlikely places. Rock exposures, fossils, fault lines, landforms, drainage patterns, and natural communities all provide intriguing evidence of past events. SNAs preserve many of these records. Others occur on lands under private or public ownership. Countless others await us beneath the massive glacial deposits of the landscape.
Geologists have constructed a chronological framework of these clues, summarized in the form of the timeline shown in figure 1. Minnesota's Geology, by Richard Ojakangas and Charles Matsch (Minneapolis: University of Minnesota Press, 1982) presents a highly readable description of Minnesota's geological record. The overview that follows is based on this source.
Early Precambrian (Archean) Minnesota (3,800 - 2,500 million years ago)
Minnesota's oldest rocks--Lower Precambrian or Archean rocks--lie in alternating belts within the Canadian shield, which underlies the northern half of the state and much of the Minnesota River Valley. The belts approximated in Figure 1.2 are of volcanic and sedimentary rocks; granitic rock materials lie in the areas between the belts.
Figure 1.2. Canadian shield with rock belts
Gneiss outcrops along the Minnesota River Valley date back 3,600 million years. Gneiss is a metamorphic rock formed when granite and other rocks were subjected to intense heat and pressure within the earth, causing a chemical and structural change. The Gneiss Outcrops SNA just south of Granite Falls preserves a significant example.
Volcanic and sedimentary rocks, also occurring in the Canadian shield, began their formation 2,700 million years ago, when lava escaped the depths of the earth through rifts in the sea floor. Volcanic formations lie throughout Minnesota's portion of the Canadian Shield, some deep beneath glacial deposits. Volcanic debris--sand, mud, and gravel--released into the nearby seas later settled, forming massive layers of sedimentary rock.
Granite and gneiss rock complexes lie exposed between the volcanic belts throughout the older part of the Canadian Shield. Like volcanic rock, granite formed from magma within the earth but because it cooled so slowly under the earth's surface, it formed large crystals. Burntside Islands SNA has such granite outcrops.
Tectonic activity folded many of these rock formations and formed faults, or slippage planes, during this period. Many of the volcanic rocks have metamorphosed to greenstone. Also during this period of crustal compression, a range of mountains several kilometers high formed in northern Minnesota.
Iron ore and more mountains form (Lower Proterozoic, 2,500 - 1,600 million years ago)
Most of the world's iron ore was formed during Lower Proterozoic time. This started as a time of extensive erosion that leveled the earlier mountain range, releasing dissolved iron and silica into a new sea. It is theorized that the appearance of abundant marine algae at this time began to create large quantities of atmospheric oxygen, which combined with the iron and precipitated in layers of iron formations known as the Mesabi, Gunflint, and Cuyuna iron ranges.
Several other features took shape in Minnesota during tectonic activity from 1,900 to 1,800 million years ago, involving crustal thrusting from the south. The Penokean Mountains emerged, extending from east-central Minnesota through northern Wisconsin and Michigan. The tilted beds of the Thomson formation southwest of Duluth were part of the large folds formed during this time period. Finally a collision of crustal plates produced the Great Lakes Tectonic Zone, extending diagonally across the state from Morris and Alexandria toward Duluth. It is still mildly active today, producing five of the state's last seven earthquakes (all small).
Minnesota's volcanoes (1,110 - 1,090 million years ago)
Late Precambrian (Middle Proterozoic) records in Minnesota again show volcanic and sedimentary activity. The Penokean mountains eroded, producing deposits of quartz sand grains now found in the thick Sioux Quartzite of southwestern Minnesota, dating from about 1,600 million years ago. Later, thinner quartzite formations can be seen as the Nopeming Quartzite in the Duluth area, and the Puckwunge formation, near Grand Portage.
Beginning about 1,100 million years ago, intense volcanic activity took place for the following 20 million years along a great continental rift that stretched from the Detroit area, north through Lake Superior and then south to Kansas. Much of the molten rock never reached the surface, but solidified underground to form dark-colored, coarse-grained gabbro and diabase. Lava that reached the earth's surface cooled in huge overlapping flows, found now along the north shore of Lake Superior mostly as dark-colored basalt. Lake Superior agates formed in the cavities left by gas trapped in the cooling lava. Sugarloaf Point SNA, on the North Shore, protects a prime example of volcanic formations dating from this period.
After the rifting and outpouring of lava suddenly stopped, the rift zone continued to sink. Flowing water deposited thousands of feet of sandstone along this zone extending from Lake Superior to the Hinkley-Kettle River area, and onward to the Twin Cities.
Post-Precambrian activity (544 - 2 million years ago)
Post-Precambrian times (Paleozoic, Mesozoic, and Cenozoic Eras) featured advancing and retreating seas, development of plant and animal life forms, and lots of sediment. The best evidence of this period lies in southeastern Minnesota, where fossils of early marine plant and animal life abound in sandstone, limestone, and shale formations. Major tectonic shifts gave Minnesota an equatorial climate during the Paleozoic Era (544 to 248 million years ago) as seen in the many fossils preserved in these rocks.
Glaciers, from A to Z (2 million years ago to present)
Despite all the geological drama and turmoil up to this time, The present landscape resulted largely from glacial activity during the Quaternary period (2 million years ago to the present). Minnesota's present climate, with its cyclic warm and cold seasons, became established during this time. Glaciers "warehoused" huge quantities of the earth's water supplies, lowering ocean levels and expanding continental boundaries. Plant and animal communities migrated, adapted selectively, or became extinct with the changing climate. Minnesota saw the advance and retreat of several major, successive periods of continental ice sheets.
The gigantic Laurentide Ice Sheet, centered in what is now the Hudson Bay, grew and retreated many times with climatic changes throughout the Ice Age. During colder periods, it extended southward across the upper midwest in what are called glaciations, each named for a geographic area: Nebraskan, Kansan, Illinoian, and Wisconsin.
|Figure 1.3. Minnesota's most recent glacial lobes and paths.
The Wisconsin glaciation, the most recent, had `the last word' as it created most of the surface features we live with today. Beginning about 75,000 years ago, it, too, experienced periodic growth and decay with changing conditions. Its advances produced tongues of ice called lobes, each named for a specific geographic area: Wadena, Rainy, Superior, and Des Moines. Each lobe also experienced periodic growth and decay.
- The Wadena Lobe advanced from the north several times. Its last advances first deposited the Alexandria moraine formed the drumlin fields spanning Otter Tail, Wadena, and Todd counties, and finally formed the Itasca moraine.
- The Rainy and Superior Lobes came out of the northeast and advanced, sometimes with and sometimes independently of the Wadena lobe. Their last advance left a coarse-textured till containing abundant fragments of basalts, gabbro, granite, iron formation, red sandstone, slate, and greenstone strewn across the northeastern half of Minnesota and as far south as the Twin Cities.
- The Des Moines Lobe originated in the northwest and advanced in a southeasterly direction across Minnesota and into Iowa and a offshoot to the northeast across the present Twin Cities area. Its fine-textured till consisted of limestone, shale, and granite fragments, from which developed the fine prairie (now agricultural) soils found in these areas. A little later, in the north, the St. Louis Sublobe moved southeastward, stopping just west of the Superior Basin.
Earlier lobes and glaciations also left their marks, though their effects are generally buried beneath more recent glacial drift. We occasionally glimpse the underlying till that is distinctive of their origin, or the striations on bedrock outcrops that indicate their direction of travel. They echo much earlier times.
Because glaciation is so significant to understanding today's landforms, it warrants a closer look. A glacier is a large body of ice moving slowly across a land surface. It forms when snow accumulates faster than it melts over a long period of time. Under the weight of the ice mass, a sole of ice melts at the bottom and allows the glacier to move by sliding. A glacier also moves by internal, plastic flow. The ice mass shrinks or expands yearly, reflecting the net effect of the year's weather.
Glaciers sculpt the surface of the earth as they expand, cutting through relatively soft materials, picking up pieces of rock debris along the way, and depositing them further on. As the sole picks up various sizes of debris, it acquires texture and abrasive power that varies much like grades of sandpaper. On some bedrock outcrops today, parallel lines scar the surface, indicating that a rock frozen into the glacier's sole passed here long ago. These markings are called striations, while wider, deeper markings are known as grooves. Finely textured particles in the sole produced highly polished rock surfaces, much like fine sandpaper. Striations and grooves are helpful in identifying the direction of the glacial flow that took place on a given site. Gneiss Outcrops and Swedes Forest SNAs are good places to see such markings.
Eventually the glacier deposits its load of rocks and debris in a variety of ways. Glacial deposits, generally, are called drift. However, other terms are more descriptive of the materials and their formation. Glacial debris, unsorted by size or substance, is known as till. Single rocks deposited far from their source are known as erratics. A thin blanket of drift laid down while the glacier is moving is called ground moraine, expressed on the present landscape as low hills and shallow swales. Ground moraine that has been molded into streamlined hills with its long axes parallel to the direction of ice flow is called a drumlin field. An end moraine is an irregular, hilly deposit of till at the ice margin or toe of the ice sheet; Prairie Coteau SNA is a fine example.
|Figure 1.4. Glacial formations.
Often huge masses of ice become stagnant and buried by debris. The ice then slowly melts, and leaves a collapsed pit of debris. This is called a kettle or ice-block, which often becomes a kettle lake when conditions are right. In the prairie, these are called potholes. Boot Lake SNA contains a kettle, or ice-block, lake.
Along the margins of the glacier, wet sediment collects, then settles and slumps, forming hummocks and uneven terrain. A chain of lakes often forms along these glacial margins.
Stagnant ice sometimes forms a melt out depression that fills with outwash deposits when the rest of the ice melts. This leaves a conical hill called a kame. Yellow Bank Hills SNA, in Lac qui Parle county, preserves kames that host a prairie community.
Recurrent melting within the glacier creates streams of meltwater that tunnel through the ice mass. These streams carry debris, which may actually plug a tunnel, resulting in the formation of a new tunnel. Later, when the glacier melts away, this plug is left on the land in a long, molded ridge of sand and gravel called an esker. Ripley Esker SNA, just north of Little Falls, protects a fine example of such a plug. If the stream carries the debris outside the glacier, it may be deposited as outwash, or sorted sand and gravel sediment deposited along the stream bed; Helen Allison Savanna SNA is a good example.
The tundra-like climate of the Ice Ages produced wind that also moved glacial debris; it could pick up the finely textured silt and deposit it downwind in what is called loess. The soils across western and southern Minnesota contain loess, a constituent of prairie soils.
Glacial lakes and rivers
Extensive glacial lakes are another great legacy of the glaciers. All were dammed on one side by the ice sheet and many approached the scale of a sea. Glacial Lake Agassiz was the largest. Fluctuating in size and depth, in left behind a series of beaches that now outline the broad, flat Red River Valley. Others include Glacial Lakes Upham, Aitkin, and Duluth.
From time to time these glacial lakes overflowed, and cut huge river channels. At its highest level, Glacial Lake Agassiz crested a moraine at Brown's Valley and spilled over to become the Glacial River Warren. This torrent fed into the Mississippi River at the Twin Cities. Its bed continues to drain the surrounding uplands, though the water volume of today's Minnesota River is a fraction of the original flow. Consequently, the broad river valley and high stream terraces, remnants from long ago, dwarf today's river. This is also true for today's St. Croix and Mississippi River valleys. Visit the high terrace on Bonanza Prairie SNA near Big Stone Lake to get a sense of the magnitude of the ancient river as you look across this broad lake to the bluffs in North Dakota. Gneiss Outcrops SNA was once the bed of this great river.
The southwestern and southeastern corners of Minnesota escaped the Wisconsin glaciation, though evidence exists that they were subject to earlier glaciations. These landscapes feature more bedrock exposures that escaped a blanket of Wisconsin glacial till, or drift--thus the reference to these areas as driftless. Their rivers and streams are better developed than areas with more recent glaciation, resulting in more efficient drainage systems and more advanced erosion.
|Figure 1.5. Minnesota's glacial lakes and rivers.
The relatively old age of these areas results in landforms and pockets of plant communities that open a window into earlier biological systems. For example, Wykoff Balsam Fir SNA in Fillmore County contains several steep talus slopes with cold air drainage, creating a specialized community usually found well north of Minnesota. Rushford Sand Barrens SNA, in the Driftless Area, contains Paleozoic bedrock exposures covered with sand, deposited when glacial winds from the nearby ice sheet formed the sand savanna for which the SNA is named. The flatness of southwestern Minnesota, with its Blue Mounds quartzite outcrops, is another example of the driftless area that escaped the Wisconsin glaciation.
As described earlier, meltwater from the ice masses first pooled in lakes, then overflowed into rivers, which sought sea level. Though southeastern Minnesota escaped the latest glacial drift deposits, it lay directly in the drainage path to the Gulf of Mexico. Paleozoic limestone formations here were susceptible to the slightly acidic groundwater passing through. Consequently, the limestone gradually dissolved along many of the planes and fractures of these formations.
The cavernous tunnel systems in Fillmore and Olmsted counties are a legacy from this period. When the water volume eventually abated, the caves emptied, and residual seepage began to form stalagmites, stalactites, and other cave structures. Weaker cave ceilings that collapsed created sinkholes, or pits, at the surface. Southeastern Minnesota contains excellent examples of karst topography, which is surface developed by solution and subsidence into underground drainage, sinkholes, and caves.
The ever-changing surface of the earth
Conditions continue to change on the "blue planet"--composition of the atmosphere, orientation of the magnetic poles, adjustments in plate tectonics, and gradual changes in climate are just some of the factors we're only beginning to understand and monitor. The face of the earth has been registering these changes since its formation. Eventually our current landscape will completely erode, just as the ancient mountain ranges did. Minnesota's SNAs capture some of the stages in this natural process--for study, for education, and for appreciation. Go see one soon!