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Rocking Stone Erratic

A cache by El Nimrod Send Message to Owner Message this owner
Hidden : 6/21/2007
Difficulty:
1.5 out of 5
Terrain:
1.5 out of 5

Size: Size: other (other)

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Geocache Description:


The posted coordinates will take you to a remnant of the last Ice Age - the "Rocking Stone", a fine example of a 415 million-year-old Devonian granite glacial "erratic".

The term ice age can refer to all the periods of glaciation during the late Pliocene and Pleistocene, from 2.5 million years ago to 10,000 BCE. In popular usage, "the Ice Age" usually refers to this last cold phase, due to its shaping of some of our Northern Hemisphere landscapes and its influence on our human prehistory.

Quaternary

The Quaternary is a subdivision of geological time (the Quaternary Period) which covers the last 2 million years up to the present day. The Quaternary can be subdivided into two epochs: the Pleistocene (2 million to 10,000 years ago) and the Holocene (10,000 years ago to the present). The Quaternary Period has been one of extraordinary changes in global environment, characterized by cold (glacial) and relatively warm (interglacial) phases.

Causes of Ice Ages

Fluctuations in the amount of insolation (incoming solar radiation) are the most likely cause of large-scale changes in the Earth's climate during the Quaternary. Variations in the intensity and timing of heat from the sun are the most likely cause of the glacial and interglacial cycles.

Three major components of the Earth's orbit about the sun contribute to these climatic changes:

1) Earth's spin on its axis is wobbly, amounting to a variation of up to 23.5 degrees to either side of the axis. The amount of tilt in the Earth's rotation affects the amount of sunlight striking the different parts of the globe - the greater the tilt, the stronger the difference in the seasons. The range of motion in the tilt (left-of-center to right-of-center and back again) takes place over a 41,000 year period.
2) As a result of a wobble in the Earth's spin, the position of the Earth on its elliptical path changes, relative to the time of year. This phenomenon is called the precession of equinoxes, and this cycle takes 23,000 years to complete.
3) The shape of Earth's orbit also changes. At one extreme the orbit is more circular (each season receives the same amount of insolation), and at the other extreme the orbital ellipse is stretched longer (exaggerating the seasonal differences). This eccentricity of Earth's orbit cycles over 100,000 years.

Major glacial events in the Quaternary have coincided when the phases of axial tilt, precession of equinoxes and eccentricity of orbit are all lined up to give the Northern Hemisphere the least amount of summer insolation. The end result is that high latitude winter snows accumulate but do not melt during the summers. When this process continues for centuries, ice sheets begin to form. Conversely, major interglacial periods have occurred when the three factors line up to give the Northern Hemisphere the greatest amount of summer insolation.

Overall, the cycles are characterized by long periods (~100,000 years) of cold climates interspersed with shorter periods (10-15,000 years) of warmer conditions. Four major continental glaciations are recorded in North America. The last (Wisconsin) began ~70,000 years ago, reached its maximum extent ~18,000 years ago, and ended 10,000 years ago. At its peak, ~97% of Canada was covered by ice, with the thickest sheet (~3.3km) occurring over Hudson Bay. We are presently in an interglacial phase that could last another 10,000 years or more.

The Glacial Erratic

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 rocks, which can range in size from pebbles to large boulders weighing many tons. The Rocking Stone is a fine example of 415 million-old Devonian granite. Despite its enormous size, it has been able to be rocked by hand using a nearby wooden fulcrum.

Geologists have suggested that landslides or rock falls initially dropped the rocks on top of the glacial ice. Similarly, the rocks may have been uplifted by the rising ice sheet. The glaciers continued to move, carrying the rocks with it until the ice melted, leaving the erratics in their present location.

To claim credit for this Earthcache:

1) Take a picture of you and your GPSr at the Rocking Stone (optional) and e-mail me the answers to the following:

2) Estimate the weight of the Rocking Stone erratic. You will need to estimate the volume of the erratic and multiply that by the "weight density" for typical granite (use 168 pounds/cubic foot)

Do NOT post your answers on your log, encrypted or otherwise.

DO NOT LOG AS A FIND UNTIL YOU HAVE A PICTURE READY TO POST AT THE TIME OF LOGGING A FIND FOR THIS EC. Logs with no photo of the actual cacher logging the find or failure to answer required questions through e-mail will result in a log deletion without notice. Exceptions will be considered if you contact me first.

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