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A brief geological history of the ravines
 

A brief geological history of the ravines

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A short non-reference article published in the Grand Valley Review in 2010.

A short non-reference article published in the Grand Valley Review in 2010.

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    A brief geological history of the ravines A brief geological history of the ravines Document Transcript

    • Patrick M. Colgan A Brief Geologic History of the Ravines The Ravines ravine (ra-vine’) A small valley, usually carved by running water; esp. the narrow excavated valley of a mountain stream. Etymology: French, Patrick M. “mountain torrent.” (p. 419, AGI Dictionary of Geological Terms, Bates Colgan is Associate and Jackson, 1984). Professor of Geology The ravines are a natural laboratory for students and faculty at Grand at Grand Valley Valley. Each semester, the Department of Geology introduces hundreds State University. of students to the geosciences via the ravines, and each year geology stu- dents conduct innovative research projects and ask new questions about the ravines. Here, I summarize the geologic history of the ravines, how humans have impacted the ravines, and what is still not known about the natural history of the ravines. The ravines formed by the episodic erosion of small streams over the last 15,000 years. Water flowing off the upland, upon which Grand Valley State University is built, drains eastward into the Grand River Valley via 11 the ravines. This broad, gently sloping, upland was formed during the Ice Age by a glacier lobe depositing sediment in a large glacial lake. Follow- ing this glaciation, the Grand River began eroding its valley through this landscape. The ravines probably began forming at about the same time, and by about 6,000 years ago they had grown to roughly their present dimensions. During the last 6,000 years the ravines have grown during brief episodes of erosion alternating with long periods of stability. The Back Bone The back bone of the ravines is a broad upland ridge, composed of a thick pile of glacial and lake sediments into which the ravines are cut. This ridge is nearly two miles wide and runs from north to south through eastern Ottawa County for about ten miles. On the topographic map this ridge looks like the back bone of a prehistoric animal, lying with its head to the north where it is bordered by the Grand River and its long tail to the south. The east-west trending interfluves that separate ravines are like vertebrae that make up the back bone of the beast. The ravines are eroded into sediments rather than bedrock, unlike the mountain stream mentioned above. These sediments are generally
    • unconsolidated, not cemented together as bedrock is. The bedrock in this part of Michigan is ancient sandstone and shale, buried by about 100 to 300 feet of glacial sediment left by the ice sheets that advanced into Michigan over the last 2.5 million years. The most recent glaciation of this Ice Age occurred in two phases between about 80,000 and 10,000 years ago. This most recent phase of glaciation is known as the late Wisconsin Glaciation. During this glaciation, massive outlet glaciers of the ice sheet called “lobes” deposited glacial sediments over Michigan and as far south as the Ohio River. The most important lobe of ice in this part of Michigan was the Lake Michigan lobe. The Lake Michigan lobe reached its maxi- mum extent about 21,000 years ago. At this time it extended south into central Illinois. Another glacier lobe extend south through what is now Saginaw Bay. Farther east, the Huron and Erie lobes extended through those basins south into Indiana and Ohio. To the west, other lobes of ice invaded Wisconsin, Minnesota, Iowa, and the Dakotas. Because these glacier lobes eroded the land, they hollowed out great basins that would eventually become our Great Lakes. By about 16,000 years ago, the Lake Michigan lobe occupied the present lake basin and its margin was located just east of the present campus. 12 Gra nd R iver end GVSU Campus moraine Figure 1. A shaded relief image of the topography in eastern Ottawa County derived from digital elevation data (U.S. Geological Survey). Blue and green shades are lowest elevations found near the Grand River, whereas brown and yellow tones are the highest elevations found on the upland ridge upon which GVSU was built.
    • Patrick M. Colgan As the Lake Michigan lobe continued to melt back into Michigan by about 18,000 years ago, a series of lakes formed around the edge of the ice. These lakes are called glacial lakes because they were bordered by the retreating ice sheet. These glacial lakes contained icebergs and glacier sediments were deposited in them as the ice retreated. One of the larg- est of these glacial lakes is known as Glacial Lake Chicago, because the present site of Chicago was where water from the lake drained down into the Illinois and Mississippi Rivers. Sand, silt, and clay were deposited in this lake. When the Lake Michigan lobe advanced, this silt and clay was mixed with local gravel and exotic boulders carried by the ice down from southern Canada. This mixture of boulders, gravel, sand, silt, and clay is called “till.” Much of the University is built on this till. The till may be seen in the foundations of most of the campus buildings and in a few gullies at the heads of the ravines. The till is between ten and fifty feet thick over the campus. Grand Valley students and faculty have mined the silt and clay from the till and fashioned it into fired clay sculptures like those displayed in Henry Hall Atrium (Formation, 1996, Daleene Menning). A Cold, Dark Lake The oldest sediments exposed in the ravines are clay, silt, and sand 13 layers that were deposited in Glacial Lake Chicago. Most of this silt and clay probably settled out of calm cold waters offshore from where a river carried sediment into the lake. The river that delivered this silt and clay may have been an early glacial ver- sion of the Grand River. The silt and clay beds probably took years or decades to build up layers only a few feet thick. The fine sands were probably deposited nearer to the Figure 2. Glacial till exposed shore and the river mouth perhaps in a delta. We know this river flowed in the eroded stream bed in from east to west just like the current Grand River because current ripples the ravine behind the Ravines preserved in sand layers indicate flow to the west and northwest. Some Apartments. The boulder in the upper center of the image of the fine sands exposed in the ravines provide evidence for very rapid is probably granite. The scale deposition, perhaps as much as a few feet in a matter of hours. The evidence is ten centimeters long. for this is a feature called climbing ripples. Normal current ripples form in
    • fine sand that is being transported by a current across the bed of a river or by waves along the shoreline. Climbing ripples form as layers of sand rapidly build up on top of one another as the ripples migrate down cur- rent. In this case, there is so much sand being deposited that the ripples climb on top of one another. 14 Figure 3. Upper image is of ripple cross laminations preserved in sand layers in a ravine along the southern boundary of GVSU campus. These cross laminations indicate that flow was from left to right. The lower image shows climbing ripples preserved in another layer of sand at the same site. Flow direction is also from left to right (both images were taken by Sarah Nagorsen, 2008).
    • Patrick M. Colgan A Glacier Surge As the Lake Michigan lobe wasted back into southern Canada it would periodically surge forward as melt water built up under the glacier. This occurs in many modern glaciers when they melt rapidly. Melt water on the surface of the glacier flows down through cracks (crevasses) and holes (called moulins) in the ice. When water reaches the glacier’s bed it can cause the ice to float. This water greatly reduces the friction near the bed of the glacier, and may cause the ice to surge forward. This has been observed in modern glaciers in Iceland, Greenland, and Alaska, among many other places. During a surge, the edge of the ice may advance several miles in up to a few years. It is likely that the Lake Michigan lobe experienced surges similar to those observed in modern glaciers. As the edge of the ice advanced into Glacial Lake Chicago it would bull-doze up sediment in front of the ice and deposit till along under its margin. Because there are numerous ridges of till called end moraines in West Michigan and around all the Great Lakes, it is likely that the lobes of the ice sheet surged many times between 18,000 and 13,000 years ago. The upland ridge upon which Grand Valley sits is one of these end moraine ridges that probably formed as ice surged into Glacial Lake Chicago. Following a glacier surge the ice would waste back, and icebergs would 15 calve into the lake. Melt water would drain through the ice and discharge from tunnels under the ice. Sediment would be deposited in the lake around the margin of the ice, and glacial till would be deposited under and in front of the ice. The dense, boulder-rich glacial till that underlies most of the campus and overlies the glacial lake sand and clay was the last sediment deposited in the area. The till contains a plethora of different rock types some from as far away as James Bay in Ontario and Labrador. Most of the larger rocks in the till are local bedrock of sandstone, shale, and limestone. Still there are many exotic boulders of granite, basalt, and gneiss. Many of these large boulders can be seen used as landscape boulders and in retaining walls all over campus. The glacial till is very hard and resistant to erosion. It forms a hard cap-rock over the softer glacial lake sediments below. As long as the till is not eroded, the upland landscape is stable. Once erosion in the ravines cuts through the till and reaches the soft lake sediments below, then ero- sion proceeds very rapidly and deep ravines are the end product. This can be seen in the ravine just to the east of the Ravine Apartments. Where the till is still present the stream has not cut down as much as where the till has been removed and erosion then proceeds rapidly.
    • A Barren Landscape Turns Green After glacier ice disappeared from West Michigan, by about 15,000 years ago, the landscape would have been a cold, windswept, and dusty place. There was permafrost or permanently frozen ground at this time. Recently, relict features indicating permafrost have been found on landscapes older than 15,000 in Wisconsin and in eastern Michigan. It is likely that at this time tundra vegetation would have been present (dwarf willow and birch, and many different kinds of sedges). Animals such as mammoth and musk ox were present in the Great Lakes region at this time. The lack of veg- etation at least initially, would have led to dust and sand being blown up out of the river valleys and deposited on top of the glacial sediments. This wind blown silt is called loess. In areas to the south of Michigan along the Missouri and Mississippi Rivers the loess piled up in layers up to 150 feet thick. To the west of our campus on the flat lake bed of the former Glacial Lake Chicago, sand dunes would have migrated across the barren surface. Now most of those dunes are stable under woodland vegetation. Between 10,000 and 13,000 years before present, the climate rapidly warmed enough so that trees and grasses could begin to take a foothold in Michigan. As forests and other vegetation spread, the landscape would have supported ice age animals such as the Mastodon. Pollen records from bogs 16 and lakes in Michigan show the spread of spruce and pine between 13,000 and 10,000 years ago. By 10,000 years ago the climate was probably the same or perhaps even a little warmer than today’s climate. Oak, maple, and hickory began to invade Michigan by this time. Native American peoples populated this landscape and took advantage of its many resources. Birth of the Ravines Most of the erosion that produced the ravines probably occurred very early, just after the local deglaciation, starting approximately 15,000 years ago. There are several lines of evidence for this. First, we know that the Grand River had eroded its channel down to almost its current level by about 9,000 years ago. River terraces above the present Grand River formed before this time. The Grand River may have even eroded even deeper than today because the lake into which it flowed was much lower than the current Lake Michigan. By about 9,000 years ago there is evidence that a very low lake level was reached in the Lake Michigan basin. The earliest version of a non-glacial Lake Michigan is called Lake Chippewa. Lake Chippewa was approximately 300 feet lower than the present Lake Michigan (which is ~578 feet above sea level). Lake Chippewa was low because of a warmer climate and the low elevation of the outlet to the Great Lakes 9,000 years ago. The lake’s outlet was very low because the great mass of the ice sheet had depressed the land, and at 9,000 years ago it had not completely responded to the melting of the ice sheet yet. With
    • Patrick M. Colgan time the eastern outlets of the Great Lakes would rebound and with this land rising, the lakes would rise in level, just as the water level of a bath tub will rise if you raise the drain. Even today, the outlet of Lake Superior at Sault St. Marie is rebounding at a rate of about 20 cm of elevation per century. This rise in the outlet causes the lake level of Lake Superior to slowly rise over time. During the low Chippewa Lake level the rivers in West Michigan responded to the low lake level by cutting deeply down into their channels. The ravines would have been forced to cut down in their channels as well. Another piece of evidence for the depth of the ravines by 6,000 years before present is in ancient wood found buried in the Little Mac Ravine near it confluence with the Grand River. In the fall semester of 2006, geol- ogy students drilled and collected two sediment cores near the mouth of the Little Mac Ravine, approximately 10 feet deep. Near the bottom of one core fossil wood of a log about a half a foot in diameter was penetrated by the core. The wood was sent to a radiocarbon dating lab in Florida and the returned age was 5,720 to 5,920 calendar years old (one sigma uncertainty, Beta-223048). Since the valley had to be there before the sediment and wood could fill the valley, this indicates that the ravines had eroded to a depth greater than their present depth before approximately 6,000 years before present. 17 Sediment Fills the Ravines Figure 4. Students in an environmental geology class taking a core in the Little Mac Ravine in the fall of 2007. A buried log found in a core like this was radiocarbon dated as approximately 5,800 years old. This suggests that the ravines were present and as deep as they are today before approximately 6,000 years ago. Photo by Patrick M. Colgan.
    • The Little Mac Ravine is filled with at least 15 feet of sediment where it meets with the Grand River based on the cores mentioned above. Most of this sediment probably filled the valley as the Grand River responded to the rise in Lake Chippewa Lake Levels after 9,000 years ago. By 6,000 years before present the lake reached a high lake level known as Lake Nipissing. Lake Nipissing was between 20 and 30 feet higher than the present lake level. Much of the Grand River Valley was probably flooded at this time. Some of the ravines would have been flooded or at least have been wetlands at this time. Clay, silt, and sand, along with logs and organic material were deposited in the ravines at this time. As mentioned before, fossil wood of approximately 5,800 years before present, suggests that some of the sediment in the valley is at least that old. Equilibrium As the ravines became heavily wooded during the last 10,000 years, they would have experienced long periods of stability (or equilibrium). As long as vegetation held sediments in place erosion would be held at bay. Events such as droughts and fires would have led to instability and brief periods of erosion. During this time, soils formed on the upland and on the ravine slopes below. Settlement and Development 18 With European settlement the land was cleared first for timber and then for farming. This would have had an immediate destabilizing effect on the ravines. It is likely that upland soils were washed down into the ravines and rapid erosion occurred near the ravine heads. Aerial photographs taken in 1958, just before the GVSU campus was built show active erosion occurring at the heads of the ravines at this time. Many of the ravine heads were filed with field stones and garbage in an attempt to reduce erosion. The ravines were used as a garbage dump at this time. The cleared fields and orchards surrounding the ravine would have been highly susceptible to erosion during spring and summer rainstorms especially in spring when fields were bare and the ground was frozen. The very low infiltration rates of the underlying glacial till would have made runoff and erosion a persistent problem. In many ways the glacial till that underlies the campus is natu- rally as impermeable as is today’s parking lots. Runoff during agricultural use of the campus might have been as great as today’s runoff except that most of the runoff then would have entered the ravines directly overland and at the head of the ravines instead of as today in underground storm water systems. As the campus was established and grew in the 1960s and 1970s there is strong evidence that erosion and sedimentation in the ravines increased greatly. The 1958 aerial photos show that most of the ravine bottoms are heavily wooded. Even aerial photos taken in the late 1970s show the ravine
    • Patrick M. Colgan bottoms as heavily wooded. Today many of the ravine bottoms are filled with dead trees and large open meadows. Many of the trees appear to have died because sediment buried them and killed them as the soils became waterlogged. In the small stream in Little Mac ravine tree stumps can be seen buried by three to five feet of sediment. Logs buried in the sediment are also found. This suggests that since the campus was built sediment moved down off the upland into the ravines filling them in places with three to five feet of sand, silt, and clay. This has had a dramatic effect on the ecology of the ravines. Today, uncontrolled erosion is occurring in many of the ravines, threat- ening the stability and beauty of these natural features. Most of the threat is because of runoff from parking areas and buildings. Storm water from structures and parking lots is focused on the ravines so that today during heavy rains the ravines truly are mountain torrents. Studies of the change in the ravines during the past ten to fifty years have been carried out by Geology Professors John Weber, Patricia Videtich and numerous past geol- ogy students. Ongoing studies by Professor Peter Wampler and students have begun to measure and quantify changes in storm water runoff. Prof. Wampler estimates that since the campus was built that peak runoff in the Little Mac Ravine has increased by more than 200 percent. Some of the most dramatic erosion is occurring in the ravine behind 19 the Ravine Apartments. Observations made by the author and others in the Geology department indicate down cutting of the stream near its head by as much as 10 to 15 feet. This has led to landslides along the sides of the ravines and the toppling of trees some of which are more than seven decades old. This clearly shows that the stability of the ravines has been upset by recent human impacts. This could lead to the ravine in this area becoming an unsightly mess of toppled and dying trees. Along the southern campus boundary, runoff down an old access road and from an adjoining farm property resulted in massive head ward erosion in a ravine tributary between 2003 and 2007. This led to sedimentation in the ravine below and its effects on the ecology of this ravine. This will probably lead to a similar die off of trees in the ravine bottom similar to what happened in Little Mac Ravine. Since the author arrived here in fall 2003, the erosion only seems to be getting worse and with each record rainfall new and irreparable damage to the ravines is occurring. Professor Peter Wampler’s work with the storm water committee has made a good start at developing solutions to the problem of ravine ero- sion and sedimentation. His research group has recognized that a major solution to the problem can only come from controlling and reducing the amount of runoff directed to the ravines. Increasing infiltration in parking areas, building rain gardens, and retention ponds can and hopefully will improve the situation. If runoff is reduced, the ravines have a good chance
    • 20 Figure 5. Photograph of active erosion of a small tributary to a ravine near the southern boundary of GVSU campus. Most of the erosion seen in the image occurred between 2003 and 2006 (Photo by Sarah Nagorsen). of reaching a new equilibrium with human land use changes. Other mea- sures such as planting new trees in the valley bottoms, stabilizing eroding slopes, and protecting eroding trails are other urgently needed steps that need to be taken. One change that could accelerate ravine erosion in the future are the effects of global climate changes on precipitation in this region. Most of us have heard of global warming and are familiar with its effects in the polar regions and on glaciers, but one little known effect has been a overall increase in rainfall in some areas as well as extended drought in other regions. The Great Lakes region has seen only minimal tem- perature changes during the 20th century due to climate change, but the future could bring larger changes in rain and snow fall totals. Larger and more frequent rainfall events have been observed in some regions of the world due to human induced climate change. If we were to receive more extreme or more frequent rainfall events because of climate change this could make our ravines more susceptible to erosion and disequilibrium. Warmer winters could also produce rainfall when in the past we would
    • Patrick M. Colgan have received snow. Warmer and or wetter climates could lead to invasions by new or non-native insect pests and or new tree species changing the makeup of our woodlands in the ravines. Alternative Futures The future of the ravines like all stream valleys is of continued erosion and eventually the denudation of the upland landscape. In tens of thousands of years the ravines will certainly succeed in eroding most of the flat upland topography upon which the campus is built. At this time the now smooth upland will be reduced to a dissected complex of deep valleys and steep ridges. Under natural conditions this would occur episodically and trees and other vegetation would have time to adjust as erosion continued. In the near term, most important to humans, the ravines face two alternative futures. They could reach a new equilibrium with the chang- ing climate and the newly imposed human impacts and become a stable well-managed forest that can be used as a natural laboratory for all. This future will require some careful, thoughtful management by all of the Uni- versity community. Alternatively, the ravines could become an unsightly, chaotic zone of unstable and eroding slopes with toppled, dying trees, and a dense underbrush of non-native and invasive species. The choice is up to the university community and those who manage the university property. Hopefully, patient management by Grand Valley will choose the 21 former instead of the latter. We probably need to stop taking the ravines for granted, assuming that they will always be a beautiful natural resource, and start managing them with a purpose. More Questions The geological and environmental history of the ravines remains poorly understood in many ways. Future student projects will hopefully shed light on these questions. How deeply were the ravines eroded at the end of the Ice Age? Cores taken from Little Mac Ravine have not penetrated the deepest parts of the filled ravines. The deepest cores go down more than 15 feet and the bottom has not been reached. What is the oldest sediment preserved in the ravines? So far the oldest radiocarbon age on fossil wood is approximately 5,800 years old, but sediment twice that age could be found in the ravines if our understanding of their formation is correct. How was this sediment deposited? Was it deposited in a lake that backed up into the Grand River Valley, or was it deposited in small streams eroding, and then filling the ravines? Other questions pertain to the building of the upland itself into which the ravines are cut. How big and how deep was the lake into which glacial lake sediments were deposited? When and exactly how were these sediments formed? In summary, the ravines are a wonderful laboratory for our students, where all of the long geological history of West Michigan is laid out for
    • those interested enough to inquire and learn about it. The ravines are also a natural resource that currently needs some help and management in order to retain their beauty and uniqueness. 22