Bemidji August 2007

Olson Gravel Pit

The Olson Gravel Pit is a gravel mining operation. The deposits of gravel are from a geologic feature known as the "Trail Delta"⁽¹⁾. The delta is the result of discharge waters from the melting of the glacier near the edge of Glacial Lake Aggasiz. A less famous glacial lake, Lake Koochiching drained causing the McIntosh Channel that is outlined in red (or dark gray through the 3D glasses) below. At each end of the channel a delta formed, indicating that discharge happened at both ends, The Trail Delta at the north (named after the town of Trail) and the Fertile Delta at the south (named after the town of Fertile). The photos below will give evidence for the water flow that deposited this gravel.

The maps to the left are best viewed with red and cyan 3D glasses. Doing this will provide a 3D aerial image of this northwest section of Minnesota. The Olson Gravel Pit is part of the Trail Delta on the upper right end (northeast) of the red mark (it will look dark gray through the 3D glasses) on the top map.

This is a small section of a map available at the National Center for Earth-surface Dynamics

They have many other maps and resources available. You should check them out.

The green highlighted areas are beach ridges also known as strandlines or wave cut terraces, outlining the beaches of Glacial Lake Agassiz during different stages in size. Glacial Lake Agassiz formed when the last glacier to cover Minnesota started to receed. The northern outlet for the melt water was clogged by the glacier so the water could not follow the path it does today through Lake Winipeg to Hudson Bay. Instead it flowed through what is now the Minnesota River Valley to the Mississippi and on to the Gulf of Mexico. Go back to the index page to find much more inforation on Lake Agassiz and the River Warren.

This person is holding a shaded relief map of the area. On the map the MacIntosh Channel is visible where coming out from where the color changes from green to rust. The MacIntosh Channel is the remnant of a river channel that had water flowing into Lake Agassiz. The colors indicate the relative elevation and help to mark the boundaries of Lake Agassiz.

The glacier deposited many different sizes of materials. Above the large metamorphic rock boulders that have been moved here from Canada by the glaciers and then deposited by the river. Imagine how fast the water had to be going to move these boulders.

The other photos show abundant evidence of water deposition. There are layers of larger and smaller sized gravel. Looking more closely there are many places where there is cross bedding - a depositional feature only found where deposits are being moved by water or wind.

Some layers have different colors, indicating the probability of different minerals. This means that the material had to come from a different place. Closer inspection shows the flow of the water going in different directions. The current thought of local geologists is that there were two different glacial lakes that fed the river and depending on the situation of the lakes, the glaciers and Lake Agassiz the water flowed different directions in the same river channel.

If you look carefully you should notice that the pattern of rocks is suddenly broken in some spots of some of the phots. The deposits were interrupted. A reasonable explaination is that a large chunk of frozen material got deposited by the river and then later burried. The firs deposit would be the break in the pattern with the pattern starting again on the other side.

1. Minnesota Minerals Education Workshop Fieldtrip Guidebook, by Dr. Tim Kroeger, Professor of Geology, Bemidji State University