Glaciers
Glaciers are essentially gigantic layers of ice
hundreds of metres thick and move very slowly downhill like a slow
motion river. A mass this huge has a profound impact on the
environment and the local geology. For example:
~The ice is so heavy that it deflects (bends) the Earth's crust
downwards.
~When the ice melts, the crust can suddenly bounce back and
cause earthquakes in places that are nowhere near a fault.
~So much water is frozen in the glacier that sea levels drop
worldwide (that’s how people walked from Siberia to North
America 10,000 years ago).
~The frozen ice carries rocks ranging from the size of sand to
the size of houses (known as glacial erratics) hundreds of
kilometres.
~These rocks in the ice act like sand paper and carve deep
gouges and/or polish rock faces smooth.
~At one time most of North America was covered in these massive
layers of ice. Even the highest peaks in the San Juan archipelago
were under ice.
~There were two glacial periods in relatively recent history,
the Vashon and Admiralty glaciers.
~The rocks carried by the glacier that were left behind when the
glacier melted are known as glacial till. Some locations in the San
Juans are covered in a thick layer of glacial till and deep
excavations/drilling are required to reach all the way to the
bedrock.
Gravel quarries exist where there is an abundance
of glacial till, such as Bald Hill nearby. Gravel is used in many
applications ranging from road beds to playgrounds.
The feasibly of drilling a well depends what the
underlying layers of soil are composed of. Glacial till is ideal
since the porous spaces between the rocks hold water in an
aquifer.
Argyle Lagoon and Sandy Hook
A sand bar attached to a solid, rocky island is
known as a Tombolo.
As currents and waves washed against the edge of
Bald Hill, particles were washed off and were carried by the water
down the shore and built up to form a sand spit. Argyle Lagoon is
unique due to the existence of Little Island. If not for this
“anchor island”, the lagoon would have totally been
enclosed by the bar. The illustrations below show how this feature
formed over many years.
Stage
1
Stage
2
Stage
3
Bear in mind that Argyle Lagoon is not in its
natural state. If an aggressive storm hits, the sandbar could be
washed away entirely as part of the natural process. Human
involvement has included a paved road and stabilization on the
outer surface.
To Log This Earthcache
In order to log this cache you must message me the
answers to the questions below through my geocaching.com profile.
There is no need to await my response to log but I am required to
delete a log if see a log but receive no answers in a reasonable
amount of time.
1.) Look to the east bluffs at the base of Bald Hill.
Estimate the height of the bluffs.
2.) Calculate the approximate volume of the Jackson's
Beach spit assuming it is 10 feet deep. Use the approximate length
from San Juan Island to the washrooms.
Volume = Length x Width x 10 feet
3.) Take a stroll to the north tip of the spit where the
ocean flows in/out of Argyle Lagoon. Note the time and direction of
the water flow.
4.) Optional-Take a picture of yourself and your GPS and
the most picturesque location you can find on the spit.
References
Roy Davidson , McLellan. "QUATERNARY SYSTEM."
THE GEOLOGY OF THE SAN JUAN ISLANDS. Version 1. University
of Washington Publications in Geology, n.d. Web. 25 Feb. 2011.
<www.nps.gov/history/history/online_books/geology/publications/state/wa/uw-1927-2/sec3d.htm>.