Those are probably glacial erratics - when ever glaciers retreat they dump their load of rocks.
Glaciers are responsible for making the soil here in Illinois (and Iowa) the best in the world.

How does that work? What makes the glacial deposits enrich the soil? Is it the rock content?

Pulverized rock is part of it. Ice age glaciers were awesome creatures, some up to 8000' high.
The glaciers (4 for Illinois) of the Ice Ages formed the soils and terrain of Illinois. The northeastern and central areas of the state are very flat where the last glacier ground up rock and soil in its path. It deposited sand, gravel, dirt, and some rocks from farther north when it melted. Dust and sand also blew onto land surrounding the edges of the glaciers. All these deposits (loess and till primarily) make up the fertile soil of the central area.
Loess covers most of Illinois, in some places reaching a depth of more than 20 feet. It is rich in minerals, contains no rocks, and has a uniform texture that retains moisture - all qualities of good soil for growing crops.
Till - As the ice front receded, leaving an average of a 100-foot thick layer of sediment. As the ice sheet melted and receded, this sediment of clay, sand, gravel, pebbles, and rocks was dropped and packed down over the bedrock. It is called till, which is material directly deposited and covered by series of glacial ice sheets.
Some publications from the Illinois State Geological Survey discuss this well; if I can locate one I'll send you a link.

Thanks. I find it all so interesting.

Many links and photos: https://www.google.com/webhp?s...

Loess - also, name of the rock and mineral club in Springfield, Il.

Here's one -
https://www.isgs.illinois.edu/...

Found the Laurentide ice sheet that covered Long Island:
http://www.wesleyan.edu/ctgeol...

Interesting, nice site. Geology is in my blood.

It has to be better than cholesterol!

Some smooth beach stones and odd pieces of slate come in handy to mark major roots to let me know I do not have vacancies but roots that make digging impossible.

Innovative!

jmorth said:Interesting, nice site. Geology is in my blood.

Thank you for explaining glacier activity so well. I live in Trinity County, CA and have been told that the glaciers of the ice age stopped at about 1000 feet higher elevation than where my property is located. I've tried to do some research to understand the geography of this area, but there's a lot to learn.

Smiles,
Lyn

I've read that the "best" source for gardeners' rock dust is glacial till.

The huge variety of minerals plowed up from all over the far north assures that every micronutrient is present.

At least we don't have to pay for the gasoline or diesel fuel to haul all those boulders, stones, rocks, gravel, grit and sand south.

Here we do pay for gravel, stones, rocks and boulders but not beach stones.

Glaciers are the gizzard of the environment. Once a rock is fully digested it becomes loess, when the work isn't finished it remains a profanity...

Maybe that's why much of the south has such pitiful soil (other than areas adjacent to major rivers, areas called "Delta"). I guess the glaciers never made it here to Mississippi.

I think they only made it to Southern Illinois mid continent wise. Alluvial deposits is what you've got down there.

Well, Alluvial deposits must have been the leftovers then. My yard has perhaps an inch of topsoil and then 20-40 feet of terrible, horrible clay. That's why pine trees seem to flourish down here. Their roots can penetrate that clay. The flooding of our rivers, year after year, decade after decade, century after century has deposited the great, Alluvial soil, and that's why our Delta region has such great farmland. Now that flooding is controlled to a great extent, and no more deposits will be had. Though we are 300 miles north of the coast, lots of seashells are found up here, so eons ago, we were covered by a great ocean.

Once upon a time there was an inland sea up to central Illinois, we're talking millions of years ago. I collect geodes up here and often get fossils from adjacent strata. Most of the fossils I get are of Mississippian age (long, long ago). They are all from creatures.of the sea...crinoids, bivalves, corals and the like.
(an example is the fossil shoe found in a post above)

crinoid head

crinoids

2" high Horn Coral

@frankrichards16 see what you started?

Went on a tangent...