Wayward Plutonic, An Ancient Work of Art EarthCache
Wayward Plutonic, An Ancient Work of Art
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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
“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
"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
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
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.
There are three main types of bedrock, igneous,
sedimentary, and metamorphic,
which are classified according to the processes that formed
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.
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
---- 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
Glacial Boulders in Iowa. Raymond R. Anderson and Jean Cutler
Prior, Iowa Geology 1990, Iowa
(No hints available.)