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Southern Minnesota is geomorphologically exciting: Glacial outburst floods, knickpoint retreat, and terraces galore!

Today in my flvuial processes class, we’re going to discuss a great paper by Gran et al. on “Landscape evolution, valley excavation, and terrace development following abrupt postglacial baselevel fall.” The paper is set in a landscape I know well – southern Minnesota’s Minnesota River Basin. For my students in northeastern Ohio, this landscape would feel somewhat similar to the area around here, but not quite. Similarly, the glacial history has some degree of overlap (glaciers covering the region and retreating about ~14,000 years ago), but there are some unique elements of each place. Fortunately, I found a series of short video clips that do a nice job of setting the Minnesota River story in its context.

The tributaries to the Minnesota River are still relaxing after all of this geologic excitement. As a result, many of the tributaries exhibit knickpoints or knick zones – steepened areas on the channel profile.

Long profiles of the Le Sueur, Maple, and Big Cobb Rivers showing the presence of a major knick point around 30-35 kilometers upstream from the mouth of the river. Figure from Gran et al reproduced under a CC license, via SERC.

Long profiles of the Le Sueur, Maple, and Big Cobb Rivers showing the presence of a major knick point around 30-35 kilometers upstream from the mouth of the river. Figure from Gran et al reproduced under a CC license, via SERC.

You don’t have to go to the primary literature to learn about the knickpoints and what’s going on in the Minnesota tributaries, thanks to a nice short discussion by Karen Gran. There’s even a huge knickpoint left on the Mississippi River in Minneapolis as a result of the Glacial River Warren and draining of Lake Agassiz. You can read about that one here.

For a great discussion of fixed versus mobile knickpoints, check out this essay by Ben Crosby, who did his PhD work on a particularly knickpoint rich landscape in New Zealand. More good discussion of knickpoint mobility can be found in this blog post by Chuck Bailey. Finally, Steven Schimmrich has a nice series of blog posts on knickpoint retreat at Niagara Falls.

If that’s not enough, knickpoint retreat is often associated with the formation of terraces. There are two types of terraces to come to terms with. Cut-and-fill terraces, where the unconsolidated sediments are actually alluvium, show that the river has had periods of both aggradation and incision (and lateral planation of the valley bottom). In rivers with strath terraces, aggradational periods are either relatively minor or non-existent and the terraces represent alternating periods of incision and lateral planation. Callan Bentley has illustrated each type of terrace below, and you should see his post and comments for good discussion of the intricacies of naming and describing these features.

Terrace types, by Callan Bentley.

Terrace types, by Callan Bentley.

An Ohio Geomystery

Cross-posted at Highly Allochthonous. There are some good comments there.

I had the good fortune of going out in the woods a few days ago with colleagues here at Kent State University. We were in a second growth forest, probably fairly typical for this part of northeastern Ohio. The upland forest had lots of maple trees, and the bottomland forests had cottonwood and sycamore. The forest is underlain by many meters of till (with silicic clasts) and below that are various sedimentary rocks. I was there to take a look at some small streams and wetlands as potential field and teaching sites. Towards the end of our tour, my colleague brought us past this site:

My first view of the geo-mystery

My second view of the geo-mystery

My colleague described the site as the ruins of a “sugar shack”, which I connected with the maple trees to mean that this was the foundation of a small-scale maple syrup or sugar production facility.

But what really caught my eye were the tabular black rocks, which seemed completely out of character for the region.

Close-up of the black rocks. Wading boot for scale.

So, I know what the black rocks are and I have a pretty good idea of why they are there, but I don’t know where they originated. I’d like to hear from our readers what they know or can deduce about these mysterious black rocks of northeastern Ohio, so share your thinking in the comments. I bet together we can get to pretty good story of the human history of these geopuzzling erratics.

A continental divide that runs through a valley

Now that’s pathological.

Parts of the Upper Midwest are disappearing under spring floods. The Red River of the North is at major flood stage, again, and the Minnesota River flood crest is moving downstream. It’s a pretty frequent occurrence in both of these river systems, and in part, flooding is a legacy of the glacial history of the area. The Red River flows to the north along the lake bed of Glacial Lake Agassiz, which is pathologically flat. The Minnesota River flows to the south along the channel of the Glacial River Warren, which was gouged out of the landscape by water draining from Lake Agassiz.

14,000 years ago there was direct connection between what is now the Red River basin and the Minnesota River basin. Today, there’s a continental divide – with the Red flowing toward Hudson Bay and the Minnesota flowing toward the Mississippi and Gulf of Mexico. But what a strange continental divide it is – for it runs through the former outlet of Lake Agassiz, in what is now known as Brown’s Valley or the Traverse Gap. This divide is not so much a high point in the landscape, but a just-not-quite-as-low area. The little community of Brown’s Valley sits between Lake Traverse (flows to the North, forming the headwaters of the Red) and Big Stone Lake (flows to the south, forming the headwaters of the Minnesota).

Here’s what it looks like on Google Earth. Note that I’ve set the terrain to 3x vertical exaggeration, so that you have some hope of seeing the subtle topography of this area.


And here’s a very, very cool oblique photo from Wikipedia. It shows the divide looking from north to south — mostly covered by floodwaters in 2007. It’s not every day you get to see a continental divide covered in water.


Coal, the High Arctic, and the fossil record of climate change

Coals exposed along Stenkul Fiord, southern Ellesmere Island, Canadian Arctic

For more than 55 million years, Ellesmere Island remained in one place while the world around it changed. Fifty-five million years ago, verdant forests grew at 75° North latitude. These wetland forests, [comprised] of species now primarily found in China, grew on an alluvial plain where channels meandered back and forth and periodic floods buried stumps, logs, and leaves intact. Today the forests are preserved as coal seams that outcrop on the edges …[of] modern Ellesmere Island, [where] there are no forests, and the tallest vegetation grows less than 15 cm high. Large parts of the area are polar desert, subject to intensely cold and dark winters and minimal precipitation.

These are the opening lines to my M.S. thesis, in which I contrasted the Paleocene-Eocene and modern hydrological environments of Stenkul Fiord, on southern Ellesmere Island in the Canadian Arctic Archipelago. My thesis goes on to describe a world that no longer exists, except in the fossil record preserved at sites in the High Arctic.  This former world may provide clues as to how polar flora and fauna and their physical environment responded to global mean surface temperatures that were 2-4 degrees warmer than they are today, yet are right in line with the predictions for the end of this century. These clues, recorded in the fossil and stratigraphic record in coal and sediment layers on remote Ellesmere Island, well north of the northernmost civilian settlement in North America, are under attack. The same human demand for energy for that is driving up global temperatures is threatening to erase the very fossils that record polar life under a warmer temperature regime. The government of Canada’s Nunavut territory is currently considering claims by Westar Resources, Inc. to mine the coal beds in one of the most spectacular of all the fossil localities in the High Arctic.

During the Paleocene and Eocene, tropical vegetation extended to 50° N, and broad-leaved evergreens reached 70° N. There was no permanent polar ice, and large parts of the polar regions were covered by forests dominated by cypresses and angiosperms. Fossilized remnants of these forests are found in locations such as Spitsbergen, Greenland, the Yukon, northeastern Asia, and the Canadian Arctic Archipelago. This widespread Arcto-Tertiary forest nearly disappeared as the climate cooled over the past 30 million years and modern temperate forests. Today the last remnants of this flora are preserved in the mountains of China’s Sichuan province.

modern Metasequoia glyptostroboides trunk

Modern Metasequoia glyptostroboides trunk (Image: Wikimedia)

Among the signature trees of the Arcto-Tertiary fossil record is the Metasequoia, a genus which was thought to have gone extinct in the Miocene until an isolated grove of  Metasequoia glyptostroboides, or dawn redwood, was discovered in Sichuan in 1944.  Metasequoia grows to 60 m tall and unlike sequoias, it is deciduous and loses its leaves in the winter.  This would have been quite handy for life in the High Arctic, where in the Paleocene-Eocene winter temperatures might have hovered just above freezing, but would still have been dark for six months of the year.

Metasequoia log, Stenkul Fiord, Ellesmere Island (photo by Anne Jefferson)

Metasequoia log, Stenkul Fiord, Ellesmere Island (photo by Anne Jefferson)

Metasequoia stump, Stenkul Fiord, Ellesmere Island (photo by Anne Jefferson)

Metasequoia stump in its growth position, Stenkul Fiord, Ellesmere Island (photo by Anne Jefferson)

At the site where I worked on Ellesmere Island, there were large Metasequoia logs and tree stumps still rooted in situ in the coal layers. Picking apart the coal layers, I could pull out Metasequoia leaves, twigs, and male and female cones. The siltstones between the coals preserved beautiful fossil impressions of a variety of tree leaves and stems.

An early Eocene tapir fossil from Ellesmere Island (Image courtesy of Jaelyn Eberle)

An early Eocene tapir fossil from Ellesmere Island (Image courtesy of Jaelyn Eberle)

My field site on Stenkul Fiord yielded only plant fossils, and for now, is safe from the development plans of Westar Resources and the Nunavut government. But a bit north at Strathcona Fiord, plants are second fiddle to the best vertebrate fossil locality of the Canadian High Arctic. At Strathcona Fiord,  the fossil record shows that those Eocene forests were inhabited by alligators, giant tortoises, primates, tapirs, and the hippo-like Coryphodon. There have been over 40 papers published on the Eocene fossils of Strathcona Fiord alone. It’s not just the Eocene that makes Strathcona Fiord an amazing fossil locality either. Pliocene layers at Strathcona Fiord have yielded plants, insects, mollusks, fish, frog and mammals such as  black bear, 3-toed horse, beaver,  and badger. It is the only known Pliocene Arctic site with vertebrate remains.

Strathcona Fiord is one of three sites where Westar Resources, Inc. plans to mine the coal. Mining the coal will permanently destroy the embedded fossils and the possibilities for any additional discoveries at this site. The other two Ellesmere Island areas in which Westar has applied for mining permits are the Fosheim and Bache Pennisulas. We don’t know as much about the paleontology of these areas, but the little work that has been done on the Fosheim Peninsula has already discovered Eocene leaf beds and Pliocene fossils.

Paleontologists and geologists around the world are raising their voices in opposition to the proposed coal mining at Strathcona Fiord and the other sites on Ellesmere Island. The Society for Vertebrate Paleontology has issued a press release expressing concern and urging the preservation of the fossils resources. There is also a coordinated letter-writing campaign to the Nunavut Impact Review Board. I’ve just sent a letter to the review board, which I’ve appended below. If you a paleontologist, paleoclimatologist, geologist, Arctic lover, fossil lover, or otherwise moved by the incredible story of alligators and towering trees at 75° N, I urge you join me in writing to the government of Nunavut and encourage them to at least require more study of the localities before mining is approved. Letters can be sent electronically to

To the members of the Nunavut Impact Review Board,

I appreciate the opportunity to write to you concerning the proposed Westar coal project on Ellesmere Island. I am a geologist at the University of North Carolina at Charlotte, and my research focuses on the intersection of hydrology, landscapes, and climate. My graduate M.S. thesis research focused on the paleo-environments of the Eureka Sound Group exposed at Stenkul Fiord on southern Ellesmere Island. I used the coal and sediment layers, and the fossils they contain, to understand variability of hydrological environments that existed in the Arctic 55 million years ago. Today, I work on issues of water and modern climate change, but my perspective was profoundly influenced by the time I spent on Ellesmere Island walking amidst the coal layers and fossilized tree trunks.

The proposed activities by Westar Resources, Inc. could damage or destroy fossil sites that form an important part of Nunavut’s history and environmental legacy. These fossils tell us about the history of Arctic plants and animals, and they are recognized internationally for their scientific importance. They also provide important evidence from a time when Earth, especially the Arctic, was warmer. The fossils of the Ellesmere Island sites proposed for mining by Ellesmere Island provide clues as to how polar flora and fauna and their physical environment responded to global mean surface temperatures that were 2-4 degrees warmer than they are today, yet are right in line with the predictions for the end of this century. Ultimately, I hope that evidence from Nunavut’s fossil record can help us better estimate and prepare for future climate change.

If the fossil sites in the Westar coal project areas are destroyed the evidence is lost forever, therefore I recommend that the Nunavut Impact Review Board advise the Minister, pursuant to article 12.4.4(a) of the Nunavut Land Claim Agreement, that the project proposal requires review under Part 5 or 6. I believe that much more paleontological and paleoclimatic research can be conducted at these sites before any coal is extracted from them and we lose the opportunity to learn all that we can.

I thank you for your consideration, and request that you keep me informed of the results of this screening process.