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Wayward Plutonic, An Ancient Work of Art EarthCache

Hidden : 12/22/2010
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Geocache Description:

Smack dab in the middle of Iowa sits a wayward traveler, a magnificent work of art that was sculpted as it travelled from regions north to a location nearby, then finally to its present inconspicuous, yet honorable location.

“Geologists define erratics as stones or boulders that have been carried from their place of origin by a glacier and then left stranded by melting ice on bedrock of a different composition. In Iowa, glacial erratics are commonly observed where glacial deposits occur at the land surface, primarily in the north-central and northeastern parts of the state. In western and southern Iowa, erratics generally lie buried beneath wind-deposited silts (loess) that cover the glacial materials. In these areas, erratics generally are restricted to valleys, where streams have eroded through the loess and into the underlying glacial deposits.

The erratics seen in north-central Iowa are the most recent to arrive in the state. They are found on the Des Moines Lobe, the region last covered by glacial ice 14,000 years ago. The ice sheet entered Iowa from Minnesota and moved southward between what is now Mason City and Spencer, advancing as far as the capital city of Des Moines. This ice melted away about 12,500 years ago. Northeastern Iowa also has a significant concentration of boulders across the landscape, and the greatest number of exceptionally large erratics. This region, known as the Iowan Surface, was once much like southern Iowa, with loess deposits mantling steeply rolling terrain composed of glacial materials deposited in Iowa over 500,000 years ago. About 20,000 years ago, extremely cold climatic conditions led to erosional beveling of this area and removal of much of the finer-grained glacial materials, thus concentrating the larger pebbles and boulders at the land surface.

Most glacial erratics appear worn and rounded, and sometimes include beveled or faceted surfaces. During the course of their journey, the rocks were jostled against other erratics or scraped against the underlying bedrock, rounding off corners and planing smooth surfaces, eventually producing their characteristic appearance. Glacial transport also caused some boulders to fracture, producing fresh angular edges. Rocks carried by rivers also undergo abrasion and become rounded in the process. In fact, most of the igneous and metamorphic rocks in Iowa's river valleys were originally transported into the general area by glaciers, then eroded from the glacial deposits and moved some additional distance by a river.” Raymond R. Anderson and Jean Cutler Prior

This Awesome Erratic

"As a final tribute, I would like to say that I have seen an uncounted number of glacial erratics, but I have never seen one that had so many interesting features as this one does." Charles S. Gwynne, Professor of Geology, Iowa State University,1970.

"Gwynne's Erratic," as it is called by many, is a truly amazing specimen, with its dark-grey granite transected by bold, purplish raised bands. To set the record straight, the natural resting place for this magnificent boulder was Story County, less than a mile from its current location, just north of the Iowa State University campus. This boulder has probably never been in Linn County, and if it was, which is doubtful based on the direction of glacial movement, it hasn't been there for at least tens of thousands of years. Click here to read more about how this awesome erratic found its way to its present location.

Based on published information, we know where Gwynne's Erratic was discovered and how it came to its present location.

But, where did it come from?

While we can not know specifically where a glacial erratic came from, we can make a general conclusion based on the bedrock found in areas over which the glaciers flowed on their way to the area where the erratic was deposited. Below is a map which shows the geography and age of bedrock formations in north-central United States. Also included on the map are the location of Gwynne's Erratic in central Iowa and the generalized limits of two of the most important glacial episodes, the Pre-Wisconsin and Late Wisconsin glacial sheets. The Pre-Wisconsin glacial event was the earlier of the two (650,000 to 150,000 years ago), but as you would suspect, the formation of bedrock (500 to 74 million years ago in IA) took place well before the glaciers that would later shape the land and transport rocks and boulders of one bedrock type to geographical locations where the bedrock is different.

Wayward Plutonic Map

There are three main types of bedrock, igneous, sedimentary, and metamorphic, which are classified according to the processes that formed them.

Igneous rocks are formed when molten magma cools and are divided into two main categories: Plutonic rock and Volcanic. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth's crust (example granite), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta (examples pumice and basalt) .

Sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65% (mudstone, shale and siltstone); sandstones 20 to 25% and carbonate rocks 10 to 15% (limestone and dolostone).

Metamorphic rocks are formed by subjecting any rock type (including previously formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals.

Bedrock Types

Schematic adapted from K.A. Lemke,

Looking at the map, you can see that the bedrock of the entire state of Iowa is made up of sedimentary rock of a few different age origins, mainly limestone and dolomite. Gwynne's Erratic consists of two types of granite, which is classified as plutonic rock. Therefore the glacial origin of this erratic is from a northern region of plutonic bedrock.


To claim credit for this Earth Cache, you must email me with answers to the following questions:

1. Using the map and the information above, which two states are the most likely provenances for Gwynne's Erratic and why?

2. Based on the limits of the Pre-Wisconsin and Late Wisconsin glacial sheets, which of these two generalized glacial events were most likely the transport for this erratic? Can either one of the glacial events be ruled out as the possible transporter?

3. To prove that you visited Gwynne's Erratic, measure and email me with the horizontal circumference of the boulder at its widest point
---- or ---- post a picture of yourself with the erratic.


When you log the cache, please upload a photo of you and/or your GPS near this awesome glacial sculpture.

Do Not post the boulder measurements with your log.

*** Parking can be difficult to find on the Iowa State University campus, especially on weekdays. There are a limited number of spaces with parking meters about 400 feet northwest of the earth cache (Stange Rd, south of University Blvd). On weekends and university holidays, you may be able to park close to the cache on Osborn Dr. During the week, this street is gated with limited access and fines for those without permits. The cache owner is NOT responsible for any parking violations. Please obey all parking signs, or the Parking Gestapo will catch you.

References and Additional Information:

Inside Iowa State article: Most Awesome Erratic

Glacial Boulders in Iowa. Raymond R. Anderson and Jean Cutler Prior, Iowa Geology 1990, Iowa Erratics

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