Ancient Life Forms EarthCache
Hidden : 6/2/2013
Difficulty:
1.5 out of 5
Terrain:
1.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

You will be looking at a roadcut through a dolomite outcrop.

Warning: The stromatolites are best and safest viewed from the shoulder of the road. There are lots of ticks in the long grass, poisonous wild parsnip plants, and there is lots of evidence of falling rocks.

Logging requirements:
1. find one of the stromatolites and estimate the width of the dome.
2. Estimate the average thickness of the stromatolitic horizon.
3. This is not a requirement, but it would be appreciated if you posted a picture of yourself at the site.


Stromatolites are mound-like communities of prokaryotic bacteria that form in shallow water by trapping floating sedimentary grains. The sediment hardens in successive layers, growing the colony toward the sun. They are the earliest known form of life on earth, fossils have been found that are as old as 3.5 Billion years old and for their first 3 billion years, they were pretty much the only form of life. They are responsible for creating much of the oxygen in the atmosphere, via photosynthesis, which made it possible for more complex forms of life (like us) to evolve.

These particular stromatolites, found in the dolomite outcroppings on the east and west sides of Hwy 52 at GZ, are about 450 Million years old, Ordovician period. This outcropping is part of the Upper Willow River member of the Upper Shakopee formation in the Prarie Du Chien group. During the Ordovician period, this area was covered in a shallow tropical sea called the Hollandale Embayment. It covered south eastern Minnesota, Southern Wisconsin and most of Iowa. The warm salty water made it possible for the stromatolites to thrive.

The seas eventually receded and the stromatolites were covered over by layers of sediment, which became the shakopee formation. This bedrock layer was then covered over by glacial deposits through the various ice ages.

There are 3 stages of stromatolite development:
1. formation of descrete cryptozoan structures.
2. formation of mats(collenia) which link mulptiple cryptozoan structures together to form a lateral hemispheroid.
3. in-filling of void spaces between individual cryptozoan structures.


The stromatolites in this outcrop are all within a visible horizontal layer called the stromatolitic horizon (highlighted below), about half way up the cliff.


The cryptozoan structures are not easily identifiable in this area but the domes(collenia) are still easy to spot.


Sources:
Roadside Geology of Minnesota by Richard W. Ojakangas
Stromatolite Wikipedia page
The Compass: Earth Science Journal of Sigma Gamma Epsilon

Additional Hints (No hints available.)



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