We here in Michigan know what a great place Mackinac Island is.
One of Michigan’s favorite vacationing locations, the island
hosts nearly 1,000,000 visitors each year. The island is about 3.5
square miles. The only motorized vehicles on the island are those
for emergency use only. There are bicycles and horse drawn
carriages available to get around by or walking of course. The only
way to the island is by boat there are ferries running from both
Mackinaw City and St. Ignace Michigan. For more information go to
www.mackinacisland.org.
Most that visit the island only get to see the south end of the
island and never realize how this island came to be. This cache
will have you visiting three sections of the island. Though all
three stops are on the south end of the island they will give you a
clear view of this geological crag.
A little history of upper Michigan will explain how Mackinac came
to be the island we know today. It was about 1 billion years ago
when Michigan formed and the Earth's crust fractured from Lake
Superior to Oklahoma. This geomorphic age created mountains
covering the regions now known as northern Wisconsin and Minnesota,
and the Laurentian Mountains were formed in eastern Canada. Over
time these mountains eroded, while occasional volcanic activity
continued. Eventually the fracture stabilized and, over time, the
rock tilted down from north to south.
The region went from fire to ice with the arrival of the glaciers,
which advanced and retreated several times over the last 5 million
years. During the periods of glaciation, giant sheets of ice flowed
across the land, leveling mountains and carving out massive
valleys. The Earth plate’s movement and these glaciers formed
Mackinac Island. Where the glaciers encountered more resistant
bedrock in the north, only the overlying layers were removed. To
the south, the softer sandstones and shales were more affected. As
the glaciers melted and began receding, their leading edges left
behind high ridges. Mackinac would have first appeared about 15,000
years ago.
Mackinac is mainly comprised of the sedimentary rock we know as
limestone. Much of this limestone has had sediment build which
allows the plant life on the island.
Many may know a crag as being a rocky hill or mountain, generally
isolated from other high ground. Well, if you had been here 7,000
years ago that is exactly what you would have seen. By 4,000 years
ago, water surrounded this crag as it does today. Crags are formed
when a glacier or ice-sheet passes over an area that contains a
particularly resilient chunk of rock. The force of the glacier
erodes the surrounding softer material, leaving the rocky block
protruding from the surrounding terrain. Frequently the crag serves
as a partial shelter to softer material in the wake of the glacier,
which remains as a gradual fan or ridge forming a tail.
To complete the EarthCache you will see first hand of
formation of a crag. To do this you will be taking an altimeter
reading in 3 locations. By looking at these changes in the heights
on the island you can see how a crag is shaped.
1. Go to N45 51.078 W084 37.945 here you will find a staircase.
Climb the steps and take your reading at the top.
Then go to N45 50.770 W084 37.084 which is the posted co-ords
remember to get your info for number 2 here.
Next go to N45 51.124 W084 36.837 This is one of my favorite views
on the island. You can get here by either following the road behind
the fort or there is a staircase in front of the fort to get here,
this is my favorite.
Be sure to get your picture for number 3 here.
E-mail me with altimeter readings from these locations.
2. At the posted co-ords, report in your e-mail a clear description
of the glacial till (rock and deposits) left here.
3. When you are visiting N45 51.115 W084 36.805 take and post a
picture of yourself with the Straits of Mackinac.
Be sure to e-mail me within 7 days of logging the cache to
get credit for your work. If the rules of finding an Earthcache and
e-mailing the owner is not followed your log will be deleted
without notice!