The cache is traveler from the
ancient past.
What is it, were did come from,
how did it get there? They are not only found in the country side,
they are located in towns as well.
A glacial erratic is a piece of
rock that deviates from the size and type of rock native to the
area in which it rests; the name "erratic" is based on the errant
location of these boulders. These rocks were carried to their
current locations by glacial ice, often over hundreds of
kilometres. Erratics can range in size from pebbles to large
boulders such as Big Rock (16,500 tons) in Alberta.
Geologists identify erratics by studying the rocks
surrounding the position of the erratic and the composition of the
erratic itself. Erratics were once considered evidence of a massive
flood approximately 10,000 years ago, similar to the legendary
floods described in the texts of ancient civilizations throughout
the world. Ancient legends of an epic flood come from many cultures
including Mesoamerican, Sumerian (Epic of Gilgamesh), Hebrew (Old
Testament) and Indian culture. In the 19th century, many scientists
came to favor erratics as evidence for the end of the last glacial
maximum (ice age) 10,000 years ago, rather than a flood. Geologists
have suggested that landslides or rockfalls initially dropped the
rocks on top of glacial ice. The glaciers continued to move,
carrying the rocks with them. When the ice melted, the erratics
were left in their present locations.
"Peculiar," "irregular," and
"uncommon," are words used to describe one class of Iowa rocks --
glacial boulders or "erratics." 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.
When these areas of the state were
settled, farmers were forced to clear fields of the rock obstacles
in order to plow and cultivate. Many of the erratics were used to
build fences and foundations, while others were just piled along
fence rows or into unused field corners where they are seen today.
Clearing farm fields of glacial erratics is a necessary and
frequent chore wherever glacial deposits are cultivated. Over time,
seasonal freezes and thaws work these rocks upward from below the
plow zone to the land surface. Smaller glacial erratics can be
hauled out of the fields; larger ones are frequently blasted apart
by dynamite and the pieces hauled away; while some of the largest
are just left in place and avoided. At the municipal park in Nora
Springs (Floyd County), an adjoining city street actually narrows
to accommodate an erratic protruding into the right-of-way. Glacial
erratics in Iowa are not difficult to identify. The vast majority
are igneous or metamorphic rocks, rather than the usual sedimentary
rocks of sandstone, limestone, dolomite, and shale that constitute
the bedrock under most of Iowa. If you pick up a granite rock,
composed of interlocking crystals of pink feldspar and glassy
quartz, you can be sure it is not native and that it came from
outside the state, most likely carried by glacial
ice.
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.
Transportation by glacial ice,
however, produces some other features unique to this mode of
travel. The most easily observed of these tell-tale signs are
glacial striations, a series of parallel lines or fine grooves
gouged across the beveled faces of erratics or inscribed on the
underlying bedrock surface. .
To receive credit for this
Earthcache, you will need to complete the following three
tasks:
1 Describe to me the way the surface of this erratic
feels. (Is it rough, smooth, etc?) Describe the color and texture
of the minerals in the erratic. Don't Describe it in log. Email me
the answers.
2 Tell what the design on this rock looks like..
3 When logging this Earthcache, please upload a
picture of yourself/team with the erratic in the background and
have your GPS clearly visible. (See Example above)
I'd like to thank Paul Huting of the City of
waterloo allowing me to develop this Earthcache in Valley View
Park
