As you drive to this earth cache, you will see no mountains, no
hills, and no waterfalls. You might conclude that this area has no
interesting geologic features, but you would be wrong! This area is
flat as a pancake and how it got this way is fascinating.
Beginning about 2 million years ago, continental glaciers moved
south across North America, covering eastern South Dakota several
times. As each ice sheet advanced, it transported large volumes of
rock debris frozen into the lower layers of ice. If the ice sheet
was very thick and heavy, the glaciers scoured and smoothed off the
terrain. In contrast, where the ice was thin, the glaciers overrode
obstacles rather than planing them. As the ice melted, sediment
called glacial drift was left behind. Geologists believe that the
glaciers were up to 1600 feet of ice high. Since just one cubic
foot of ice weighs about 57 pounds, a glacier 1600 feet thick would
exert 45 tons of pressure on every square foot of ground it covers.
And when it moves-even just a few inches a day-it acts like a
bulldozer, scouring and scraping the land, leveling high places and
filling in low ones, smoothing some surfaces and gouging
others.
Sequence of glacial events in eastern South Dakota
- Big Stone Stage - the youngest stage, about 10,000 years
ago
- Gary Stage
- Altamont Stage
- Bemis Stage
- Toronto till plain
- Brookings till plain
- Older or uncorrelated drift
- Oldest drift
Ipswich, the town this earthcache is located in, is on the
border of the Gary drift sheet and unclassified old drift. Several
buildings in Ipswich are made of glacial rocks, either as rounded
field stones or dressed by skilled masons.
Many years ago, a glacial boulder was found near Mobridge, South
Dakota. This rock was moved to Ipswich and placed in front of the
Marcus Beebe Memorial Library. There are life-sized human hands
etched into the boulder. Archeologists think Native Americans may
have placed their hands in the imprints while praying. To claim
this earth cache, estimate the volume of the rock (Volume = length
X width X height). If you assume a density of 0.07 tons/cubic feet,
calculate the estimated weight in tons of prayer rock. Please
e-mail this information to us and do not post in your log.
Photos are welcome.
Reference: Roadside Geology of South Dakota by John Paul
Gries.
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