Unit 4 - Soil Forming Factors

Chapter 5 - Parent Materials for Soil Formation

Parent materials (PM) do not have to be rock. In most cases in Minnesota the parent material of a soil has been transported by wind, water or ice. A geology map of Minnesota shows the different parent materials. This map is very similar to the state soil map. Click on the map image to see the Surficial Geology Map of Minnesota. For more information about Geology Maps, go to Minn. Glg. Survey

Residual Parent Materials

In order for a parent material to be residual, the soil must develop directly from the bedrock. There are only a few areas in Minnesota where this could occur. One is in southeast Minnesota, where glacial activity was minimal. In this area the St. Peter Sandstone is often at the surface and a sandy soil profile develops in this soft rock. St. Peter Sandstone

In northeastern Minnesota the bedrock of basalt is often close to the surface and a thin soil can develop directly from the basalt bedrock. basalt & Resid soil
In this photo ( {basalt rock}) of basalt near Taylors Falls, MN. a reddish soil can be found in a crack of the rock. Basalt weathers slowly and is high in iron, which imparts a red color to the soil. In some areas limestone or granite can also be a parent material for residual soils.

Transported Parent Materials

Transported parent materials are divided into four groups based on the method of transportation. Most of the parent materials in Minnesota were transported by an agent to their present location. The agents of transportation will influence the characteristics of the PM.

1) Water

Water is an effective means of moving earth materials. In fact, it is the main agent responsible for the leveling of mountains and filling of valleys. Water-deposited materials often have layers of different size material. This occurs over time, as the speed of the water transporting the material changes. Fast-moving water deposits large stones and gravel, while slow-moving water deposits sands and silts. Parent materials deposited by moving water are called alluvium. alluvium

An alluvial soil{alluvial soil}will have many different strata of water deposits. This alluvial soil has 3 distinct strata. Each layer will have a different composition, depending on the speed of the water that deposited the layer.

Alluvial deposits that occur where the stream frequently floods over them are called flood plains. The flood plain of the White Water River, seen below, is used for row crops in Winona County, near the town of Elba. Where the stream has cut its valley deeper into the landscape, the alluvium that is now above the flood plain is called old alluvium and the landscape position is called a terrace. Terraces in this picture are not very high and are adjacent to the flood plain in the forest. Many of the larger rivers in Minnesota have terraces. In older landscapes out west it is common to see three or four levels of terraces along the major rivers. See block diagram for observation of terraces and floodplains See Soil Forming Factors

The Anoka Sand Plain is a large alluvial deposit of the Mississippi River. Since the Mississippi was being fed at that time by glacial meltwaters the alluvial sediments in this area are considered to be glacial outwash. This outwash plain was created as the Grantsburg Sublobe retreated to the southwest and the Mississippi River gradually moved back to its current position. While the river was gradually retreating to the southwest it kept depositing a fine sand which became the parent material for the Zimmerman soil.
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2) Wind

Wind deposited sand=eolian sand

Wind can also be an effective transporter of parent materials. If the particles are sand size, then dune sand deposits are formed. Sand dunes are extensive around portions of the Great Lakes, and in central Minnesota. Sandy deposits are extensive throughout the central U.S.A. and occur throughout Minnesota with the largest areas north of the Twin Cities and in Wadena County.

Sand Dune areas are common in Minnesota especially adjacent to the Anoka Sand Plain which is just north of the Twin Cities. Map sand dunes The Sand Dune State Forest is located in the northeastern Anoka Sand Plain.

Wind deposited silt= loess

When silt sized particles are blown and deposited by the wind the material is called "loess" (pronounced lus). Loess deposits are extensive in southeastern Minnesota and can occur in any portion of the state. Loess is an extensive parent material in the central U.S.A. and in China where the loess blew off the desert of Mongolia. Minnesota also has extensive loess deposits in the southeast and southwest corners of the state.

3) Gravity

Material deposited at the foot of a steep slope is called colluvium. This material is brought downslope by gravity. The rocks in colluvial deposits are not rounded, since the abrasive actions of rolling around in a stream did not take place. This colluvial deposit is in SE MN (just below the hill that is in the "terrace " photo). In Minnesota colluvial deposits occur in the unglaciated southeast corner of the state. This close up of some Colluvial deposits colluvial deposits show a mixture of silt and rock fragments. The rocks have sharp angles because they have not been smoothed by water.

4) Ice

Ice is the most important agent for the parent material for the soils that have developed in Minnesota. Ice transported materials cover 80% of the state. Much of the Northeastern U.S.A. has soils that developed from glacial deposits. See these maps for a look at the extent of glaciation in the USA North America or seeNorthern Hemisphere glacial extent or Northern Midwest glacial extent or Ice Sheets of US. Some one to two million years ago, at the beginning of the Pleistocene epoch, the climate in the northern part of the continent changed. Winters lengthened, and snowfall increased and did not entirely melt in the short, cool summers. Glaciers advanced and retreated four or five times. Between ice invasions, moderate climates like the present prevailed, and soils developed in Minnesota. In fact, we are probably in an interglacial period right now. The last ice retreated from Minnesota about 12,000 years ago. Soils have since been developing in the transported materials left by the glacier.

Go to Glacier animations for animated views of a glacier.

If you are interested in looking at the current ice cover of planet earth, go to Glaciers

Glaciers act like giant bulldozers and scrape the earth. Materials deposited because of glacial activity are called Glacial Drift. Look at these diagrams to see how glaciers accomplish this.-Glacial Landform -

This picture has the common depositional landforms left by a retreating glacier Common Landforms from a Glacier

Glaciers can remove the soil in place and deposit new material which is a mixture of rocks, sand, silt, and clay. This specific kind of glacial drift is called Glacial Till.Superior Lobe Till

Moraines

As continental glaciers moved across Minnesota, they left behind a unique hilly terrain, the most prominent features of which are called moraines. Moraines are made up of unsorted material (glacial till) the glacier gouged out of the earth as it slid across the landscape. When the till is piled at the end of the ice sheet it is called a terminal moraine; when it is behind this terminal moraine it is called ground moraine. Both of these features are readily seen in the Twin Cities area. A good example of a moraine is at I-694 and Silver Lake Road. In the Twin Cities area the Superior Lobe advanced first, and a later advance of the Des Moines Lobe ( Grantsburg Sublobe) rode over the red till of the Superior Lobe.

Both the red and tan tills are evident in this roadcut. Des Moines lobe till (brown) is the upper till and and Superior Lobe (red) is the lower till in this road cut. The separation line is just above the head of the person in the dark jacket. A roadcut of a moraine just north of Mankato shows numerous glacial tills as evidenced by the different color of each till. The bottom gray till is evidence of the first glacial advance across Minnesota and is over 500,000 years old. Till Strata

Loess

The Parent Material map {short description of image}shows the extensive loess or wind-blown silt deposits in SE and SW Minnesota. Many of the river valleys were not vegetated after deglaciation and when the silty alluvial materials dried they were easily picked up and moved about by the wind. Loess is also located in other areas of the midwest -midwest loess map. Loess is a common parent material throughout the world. Loess is easily eroded by water and roadcuts are more stable in loess if they are vertical rather than sloped.
See this Loess Exposure in Mississippi or this one in Missouri

Lacustrine

In areas where the melt water from the glacier could not escape, large glacial lakes developed. The materials deposited by the slow moving water are high in silt and clay. These lake bed deposits are called Lacustrine. See this cut of a lacustrine deposit. One extensive area is the Red River Valley in northwestern Minnesota,(Red River Valley) which previously was glacial Lake Agassiz, as seen on the Parent Material map or in this Minnesota Glacial Lakes . Around 14,000 years ago, the last ice sheet receded and what is now the Red River Valley became an immense glacial lake, Lake Agassiz, covering more than 100,000 square miles. When it drained into Hudson Bay and disappeared, it left fertile black, clay soils potent with minerals and organic matter. Today, here on the western edge of Minnesota, towering shelterbelts turn farmsteads into wooded castles surrounded by oceans of waving wheat. Information on Glacial Lake Agassiz

{Glacial Lake Duluth}

An interesting note is that this large glacial lake was named for the person who convinced the world that glaciers did occur. See Louis Agassiz. The area is very flat, as you might expect the bottom of a lake to be, and the soils are very high in clay because the coarser sand and silt deposits were dropped near the shore. The extent of this lake was impressive -see Lake Agassiz Extent). Another lacustrine area is, just south of Mankato and the area is called "Glacial Lake Minnesota." Glacial Lakes Aitkin and Upham are in Aitkin County in northern Minnesota. Glacial Lake Duluth was an extension of Lake Superior.

Minnesota is the land of "10,000 Lakes" because the glacier left many depressions that later filled with water. The following diagrams show how this is possible. Formation of Lakes in a glacial landscape and formation of Lake Mille Lacs.

Outwash

In areas where the meltwaters were moving very fast away from the melting ice, deposits of sand and gravel occurred. These deposits are called "outwash" and look very similar to alluvial deposits. Extensive outwash deposits are located just north of Minneapolis on the Anoka Sand Plain. This extensive area of sandy soils is well suited to urban development. An esker is a low hill that was formed by an under-ice stream which deposited very coarse materials because the water was moving very fast, esker . A kame is also formed under the ice but it was a large hole in the ice so the resulting landform is a hill, KAME . This kame in Dakota County was having the gravel removed for building roads. The hill of the St. Paul Campus is a kame as evidenced when the basement for Borlaug Hall was dug and the sandy outwash was removed for free by the excavating company. See the Borlaug Excavation and the soil under the street. This diagram show how the outwash plain forms under the ice. {glacial landforms}

Geologists call soil parent material "surficial geology deposits". Go to Northern Minnesota Surficial Geology.

More than one parent material

One thing to keep in mind when dealing with glacial deposits is the fact that the ice advanced and retreated more than once. This will often complicate our present day interpretation of glacial landscapes. It is not uncommon to find a soil developing from more than one parent material. It is possible to have loess over till, loess over bedrock, till over bedrock, or lacustrine over till. In each case the soil has developed in more than one parent material. This situation where there is more than one parent material in the soil profile is called: Lithologic Discontinuity
Till over SandstoneLoess over Till

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