Oldenburg Point
overlooks the St. Louis River Valley and is a basinal deposit of
Glacial Lake Duluth. The steeply inclined rocks in the river
channel below are alternating beds of slates and graywackes of the
Thomson Formation, thousands of feet thick formed during Early
Proterozoic Eon times about 1.9 billion years ago.
Slates are rocks formed from original
deposits of mud which are first compacted into shale and
subsequently converted into slate by heat, pressure, and movement
in the earths crust. Graywackes originate as beds of sand with
enough gray and black grains to produce the color. Good example of
the slate can be viewed near the swing bridge.
These original sediments collected on the sea bottom about two
billion years ago and subsequently, by deep-seated earth movements,
were folded, fractured, and tilted to stand at varying angles.
Nearly 50,000 years ago glacial ice, moving over this area,
completely filled the Lake Superior Basin and blocked the natural
drainage through the St. Lawerence River. The meltwaters from the
ice, unable to flow eastward created Glacial Lake Nemadji. At this
stage the water reached Jay Cooke Park with the ancient lakeshore
occurring at the current location of the Munger Bike Trail nearly
10,000 years ago.
Within Jay Cook State Park you will find rocks several billion
years old and sediments only several thousand years old. This
unconformity represents a gap of about 800 million years in
geologic time!
TO LOG THIS CACHE:
1) The glacial lake stood how many feet higher than Lake
Superior?
2) What evidence did the glacial lake leave behind in this park?
Describe the color and texture.
E-mail me the
answer(s)
HERE.
Sources:
- Display: Geological Society of Minnesota and Department of
Conservation State of Minnesota, 1960
- Green, J.C. 1996. Geology on Display: Geology and Scenery of
Minnesota's North Shore State Parks. 69p.
- Sansome, C.J. 1983. Minnesota Underfoot. 224p.