Field Trip:  Geological stories of regional climate change. (2.5 hours)
 
Topographic map showing field trip stop locations.  You can also find topographic features we will be looking at, including wave-cut scarps, dune ridges, and former offshore bars.

Stop 1:  Sediment from the valley

The sediment in the valley is very fine grained (clay-sized). This is consistent with its origin in a deep lake where surface waves did not stir things up much.  Fine-grained clay settled out in this lake.

Stop 2: First beach scarp and sediment sample
     The two clues to the nature of Lake Agassiz are topography and sediment.  We are at the top of a scarp believed to be an erosional scarp at the edge of former Lake Agassiz.  Similar scarps are seen at the edges of modern lakes, like Lake Michigan shown below.

 
     Several of these beach scarps formed when the lake was at different levels.  This one is the lowest and most recent one, called the Campbell strand line.  Lake Agassiz is thought to have been at this level from about 10800 years ago to about 9300 years ago with a period in between where the lake drained.  As we continue up the road we will come to the Tintah strandline, the Norcross strandline, and finally the Herman strandline.  Each represents a level that the lake stood at long enough to develop a scarp.  The lake level fell because the lake outlet (near Lake Traverse) eroded to progressively lower levels.
     The terrain here is underlain by sandy sediment.  Look for sandy soil and sediment dug up by gophers.  The source of the sand was the beach sand left by the retreating lake.

Stop 3:  Top of Norcross scarp, with sand dunes
Image is looking up toward the scarp, just below the tree.

At this stop, we are at the top of another beach scarp, the Norcross.  The rolling terrain here again represents dunes, formed perhaps from sand blown in from the former beach immediately below us.  Sand dunes often form on the scarp above beaches of modern lakes, such as the Great Lakes.  Imagine a time when the beach of Lake Agassiz was at the base of the scarp below you and the cold, blue lake stretched over 50 miles to the West.   The rise to the East is part of a ridge that may represent an offshore bar that formed at a time when the lake covered the area where you now stand.

Below is a paleogeographic reconstruction of what this area may have looked like when Lake Agassiz was at the Norcross level and the waves lapped at the beach just below you.

Below is a cross-sectional view showing the 4 major strandlines (Campbell, Tintah, Norcross, and Herman), along with ridges formed by sand dunes and ridges that may have been offshore bars during the time of Lake Agassiz.

The image above also shows sediment-sizes in this area.  These data were collected by students in another class.  Each graph shows the distribution of different sizes of particles in the sediment, ranging from fine grained mud on the right to coarse gravel on the left.  The sediment farther offshore is finer (more mud) but as you get closer to the former beach the sediment gets coarser (sandier).  The kind of sediments in a particular location reveal the kind of environment that used to be there.  Beach sand is different from wind blown sand, or river sand.  Sediment in a lagoon is different from glacial till.  Here is a key to typical patterns of sediment from some different environments.

Stop 4:  Herman offshore bar and lagoon
    Below is a paleogeographic reconstruction of the region at the time Lake Agassiz was at Herman level (about 11500 years ago).  Offshore bars existed and a delta or underflow fan probably formed at the mouth of the Buffalo River.  This delta has been mostly obliterated by later wave action along the shore that occurred after sediment volume from the river decreased (once the melting ice retreated to the North and was no longer carried by the Buffalo River).
   (Alternate image of Herman times)
The ridgeline at this stop was an offshore bar at the time Lake Agassiz was at Herman level.  An offshore bar is a ridge of gravel and sand offshore from the beach but where storm waves break and a beach-like environment develops.  The offshore bar shelters more landward water from the strongest of the lake-waves, thereby creating a lagoon environment.  So the area to our east was likely a lagoon during Herman level times (about 11500 years ago).
     This ridge probably represents a high area in the glacially deposited sediments that predated Lake Agassiz.  As the lake fell, this ridge became first an offshore bar and later the edge of the lake.

Stop 5 (with 1/2 mile hike):  Cutbank at Regional Science Center

 

     In the cutbank across the river, we see sediments from past geological episodes in this region.  Common sense suggests that the sediments on the bottom must be the oldest and the ones on the top the youngest, for how could the upper sediments have suspended themselves in mid-air until the lower ones were deposited?  Notice that the blue-gray sediment at the bottom (to the left of the small landslide in the picture) seems to be a mixture of stuff of all different sizes, big boulders mixed in with the fine-grained mud that gives the layer its color.  This type of sediment is said to be poorly sorted.  We don’t find this kind of sediment being deposited in modern rivers, or beaches, or deserts.  We do find it forming beneath glaciers.  Thus, this sediment would seem to have been deposited when this region was covered by glaciers.  It is called glacial till.
      The boundary between this sediment and the more yellowish sediment above it is very irregular and bumpy.  The bumpiness suggests that the surface between these two sediments represents a period of erosion into the glacial till, with the irregular areas representing channels cut into the till by running water.  In addition, the sediment above contains crossbedding and is sorted like river sediments, which are not as poorly sorted as glacial till.
      If the lighting is good, you may be able to make out some faint fingers of blue-gray till within the yellowish river-like sediments (just above and left of the small landslide in the picture).  This suggests that the rivers that eroded the glacial till and deposited the river-like sediments may have existed within the glacier, perhaps as it was melting away.
      A few feet below the top of the cutbank, on the right-hand side, you may be able to spot a roughly horizontal layer of boulders (directly above the landslide in the picture).  Above this layer is a layer of crossbedded sand a couple of feet thick.  This sand contains almost no muddy sediment (unlike rivers or glacial till), and thus is believed to have been deposited on a beach where constant washing by waves washed away any muddy sediment.  Thus, we conclude that this sediment was deposited on a beach of Lake Agassiz, which formerly occupied this region.  The layer of boulders is called lag.  It is the stuff in the underlying sediment that was too large for the waves to wash away.
      Finally, above the crossbedded beach sand is another layer of sand, seen only in a small area at the very top of the hill.  This is believed to be wind-blown sand deposited in an arid period of time after the lake shore retreated away from this area.
      Above all the sediments is a dark layer of soil.

Sediment types tell us about past environments. The till layers are muddy, but also have lots of sand and gravel (poorly sorted).  The sediment worked by rivers running within the ice has had much of the mud washed away and so is better sorted, but has some mud still mixed in with the sand and gravel.  The beach sediments are very well sorted sand, and the wind blown deposits are fine sand and silt, also well sorted.

     The story of change is this:  First came the glaciers, then the glaciers began to melt and rivers formed within the ice.  After the ice was melted, a beach formed in this area.  And finally, after the lake the beach formed on was gone, or had fallen to a lower level, wind blew sand into dunes and sandy flats.  Later, once vegetation became established on the sand, soils developed such as the dark one we see today at the top of the cutbank.

Below is a diagram summarizing these interpretations.
 

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image credits:  Cutbank illustration by Mary and Russ Colson and used for a hiking guide at the MSUM Regional Science Center.  Topo map from a 7.5 minute quadrangle map from the USGS.