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Although parts of northern Minnesota have bedrock exposed at the surface (especially in the Boundary Waters area and along the short of Lake Superior), the more usual situation here is for the bedrock to be buried very deeply indeed - with, in places, more than a hundred meters of soil overlying it, according to this map.

This doesn't make sense to me; the whole area was buried under an ice sheet during the most recent glaciation, and the hundreds of meters of moving ice would have scraped everything down to bedrock (and scraped off large amounts of the bedrock itself) and prevented any new soil from forming until the ice sheet retreated. Northern Minnesota has only been ice-sheet-free for a blink of an eye, geologically speaking - how did hundreds of meters of soil manage to form in such a short period of time?

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    $\begingroup$ Glaciation also deposits a lot of material that was scraped away elsewhere, in the form of moraines, e.g. ground moraines and terminal moraines. Presumably much of the hundreds of feet of soil you refer to are just gravel and sand deposits, covered by only a few feet of more fertile top soil. $\endgroup$ – njuffa Apr 7 at 16:51
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Biological time scale is not the same as geological time scale, and permanent ice retreated from Minessota around 18.000 years ago.

A search on Google took me to this website, where it says:

The time needed to form a soil depends on the latitude:

  • in environments characterized by a mild climate, it takes 200-400 years to form 1 cm of soil.
  • in wet tropical areas soil formation is faster, as it takes 200 years.
  • in order to accumulate enough substances to make a soil fertile it takes 3000 years.

Source: eniscuola.net

That doesn't mean you need 20.000 years to make 100 cm (one meter) of soil, as said in this blog post.

"Many soils formed after glaciers retreated are only about 10,000 to 20,000 years old, but have thick topsoils."

Source: soilsmatter2011, Wordpress.

The reason why thick soil is formed in that short time scale is, once vegetation is estalished, the process accelerates because tree roots break the rock and contribute to form new soil profiles.

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    $\begingroup$ note that as glaciers retreat they can even kickstart soil formation by dropping out a lot of sediment. Many of the hills in Minnesota are large piles of glacial sediment (moraines) $\endgroup$ – John Apr 8 at 2:30
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As njuffa mentioned I don't believe you are looking at soil 100s of meters deep you are looking at unconsolidated sediments 100s of meters deep.

Depending on the geological process it is possible to deposit 100s of meters of sediments in a very short time. The conditions for deposition of large amounts of sediments are frequently found during the period of time that the glaciers are melting. During this time the sides of the glacier end up forming the glacial fluvial terraces and the toe of the glacier often breaks into pieces of ice that the sediment can build around.

This site from the Minnesota State University describes the majority of the lakes in Minnesota as Kettle Lakes, which suggests that the tail end of the ice age was a time of significant sediment deposition. The North Eastern area would have likely be covered by ice while the South Western portions would have been experiencing the deposition of materials from the rapidly melting glaciers. There is a whole wikipedia site devoted to the glaciation of Minnesota that describes the events relatively well.

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