This cache was originally placed by Odyssues2000 and then maintained by CommanderUSN before jones farm took over in November 2023. The original cache details appear below.
An Earthcache is a Learning Experience - instead of a log to sign, you must answer some geology questions to score the smiley. Please send your answers to the cache owner listed at the top of the page. Please do not damage the outcrops and do not trespass on private property. The geological bluff is exposed for viewing along the park's interpretive trail. Please respect the site.
Geology in an Urban Park - Bluff St. Quarry
The history of the land sometimes gets disguised by our use of it. We build our cities over the geology and no longer notice the rocks or the lay of the land. Except, sometimes, they poke out at us. Here in Hoyt Park a bit of a few million years of that geology is exposed in a wall of rock along the interpretive trail. This spot was once a quarry where stone was cut from the bedrock to build some of the historic buildings in Madison. Quarry work exposed this rock face.
Depending on your fitness level and time, there are various places to park.
- Bluff Street - please be aware of parking restrictions and signs. This will be the closest and easiest walk, but parking is restricted.Terrain 1.5
- Parking Lot inside the park - ample parking; site is a short walk down rustic stairs and thru forest. Terrain 2
- Sunset Point - good parking location for a hike through the entire area. Several sets of stairs, but excellent views. Terrain 3
During the Paleozoic Era, the area that is now Wisconsin has been repeatedly submerged under oceans and shallow seas, and waters advanced and retreated from the land several times. The wall in front of you contains layers of millions of years of rock formations laid down during the beginning of the Paleozoic Era, during the Late Cambrian Era, from 541 million years ago to the 485 million years ago. (from "Geological History of Wisconsin Bedrock" by State Geologist James M. Robertson)
Earth during Late Cambrian Era
The above map, from USGS, shows the Earth during the Late Cambrian 514 Million Years Ago. The green landmass in the middle, named Laurentia, will later become Greenland, Canada and some of Northern U.S. Can you find the great lakes and what would become Wisconsin? It helps to tilt your head to the right. If you do, it is under water to the left shore of Laurentia. Most of that time Wisconsin was under water - sometimes a sandy tidal beach or shallow water and sometimes deeper ocean.
Bottom Layer:
The very bottom of the rock wall in front of you is exposed light grey sandstone layer is known as the Jordan Formation or Jordan Sandstone and dates from 541 million years ago to the 485 million years ago. The sandstone was laid down in a shallow tropical sea or lagoon which stretched from what is now Cross Plains through Madison. North of here, Baraboo was a series of islands in this lagoon. The sandstone is believed to be the result of a series of washover fans transported by storms and deposited here.
Narrow Middle Layer:
Later this area was below a shallow sea where the earliest forms of photosynthizing bacterial life, lime-secreting cyanobacteria, built up calcareous mounds and trapped sediment, called stromatolites. That narrow middle, darker grey layer in the rockface in front of you is what's left of the stromatolites - structures laid down by the earliest photosynthizing bacteria in shallow water by binding and cementing sedimentary grains into layered rock structure.
Modern Stromatolites off the coast of Australia
Top Layer:
Like the previous layers, the top section was also laid down during Late Cambrian and under water. The upper part of this cliff is from a rock called dolomite, which is very similar to limestone. It a sedimentary rock composed primarily of the mineral dolomite, CaMg(CO3)2. It is thought to form by the postdepositional alteration of lime mud and limestone by magnesium-rich groundwater. Dolomite and limestone share the same color ranges of white-to-gray and white-to-light brown (although other colors such as red, green, and black are possible). They are approximately the same hardness, and they are both soluble in dilute hydrochloric acid. They are both crushed and cut for use as construction materials and used for their ability to neutralize acids. (Info from Geology.com/rocks/dolomite.shtml ) Detailed information can be found in "Dolomites and Limestones of Wisconsin" http://images.library.wisc.edu/EcoNatRes/EFacs/WiGeologicBulletin/WGB66Econ22/reference/econatres.wgb66econ22.i0010.pdf
What you'll see Here and Questions you must answer:
As you stand at the Earthcache coordinates you will face a rock wall with many layers.
- Q1: Describe any differences in color and texture you see in the three layers.
- Q2: Which layer is most different from the others - how and why?
- Q3: There is a large bolder next to the info sign. Which layer, if any, do you think it's from? If not from the wall, tell me why you think that.
(¯`·._.·(¯`·._.· Geology Layers in Wisconsin ·._.·´¯)·._.·´¯)
Here is an illustration of the various layers of geological strata in Wisconsin. Notice how far down - long ago in time - the Jordan Formation was laid down.
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Basic Guidelines for Geocaching in City of Madison Parks
- You must abide by all Madison Parks Ordinances.
- Geocaching is only allowed in general parklands in the City of Madison. It is prohibited at the following locations: Forest Hill Cemetery, Olbrich Botanical Gardens, all City of Madison conservations parks, all City of Madison golf courses and all areas with identified Indian Mounds.
- All geocachers will be respectful of all public parkland. No digging or penetration of ground surface will be permitted. Trees, scrubs and plants will not be disturbed or altered at any time.