Monday, April 4, 2011
Look what Canada and Minnesota Gave Us?
Wander around any creek, stream or river bed in the 42N area (and about 91W) then look at the exposed assortment of rock. Here in Iowa the rocks which exist just below the best soil in the world is mostly limestone, dolomite and other assorted sedimentary based concentrations. But take a close look at the photo for the rest of the story.
The pebbles seen here from the Otter Creek hike this past weekend are fairly typical of what can be found around the state. About half of this photo consist of assorted eroded sedimentary rock. The other half is made up of non-native metamorphic and igneous stones. The pebble shapes indicate likely water erosion (perhaps from the creek) but probably from another source - glaciers!
The last advance of glaciers 12,000 to 14,000 years ago picked up all sorts of native rock from Canada and Minnesota then headed south. Otter Creek is located on the Iowan Surface area which lies just east of a direct hit from the Des Moines Lobe advance. Glaciers did deposit lots of well rounded granites which consisted of transported basement material from the Canadian Shield.
Long story short - thanks to glaciers we have varied landscapes here in the Hawkeye state and untold tons of glacial gravel. The stone later provided a resource for natives to fashion tools. Still later both the limestone and granites were harvested and processed into early concrete for buildings and roads. But one of the best features from glaciation was the formation of topography that eventually yielded richly organic soils that we use today for farmland. Know that.