Friday, August 10, 2012

The Riddle of Ferris Rock

Smugglers Cove, Valcour Island
44 32.05 N 073 24.32 W

Being on Valcour Island and walking the trails almost always leads me to wonder about the natural wonders of this area, both visible and invisible.   Today, I'm drawn to the puzzle of Ferris Rock.

Long time blog readers may remember that Ferris Rock is a hazard that we shun.  It is a single shallow spot isolated in an otherwise deep section of the lake.  If we tried to sail over it, we would run aground.   I've complained before that no matter where we go, Ferris Rock seems to always be in front of us posing a hazard.

But today I'm not writing about the hazard of Ferris Rock, but rather how it manages to be there.   Based on the depth soundings on the chart, is must be shaped like a nearly vertical chimney, 120 feet tall, and 300 feet wide at the top.   I try to visualize what it would look like if the water were drained from the lake and it stood alone.  I come up with a picture resembling the famous hoodoos of Bryce Canyon Utah.  

I could wonder how a chimney like Ferris could form.  In the US West there are numerous chimney-like rock structures.  But today I'm trying to figure out how it could survive the glaciers?   It seems only reasonable that this chimney of rock would have been broken off and carried away by the mile-high glacier that covered this region.  Right?

Ferris Rock is not unique.  On Champlain I can immediately think of several similar structures.  Colchester Reer, Appletree Shoal, Rock Dunder and Diamond Island to name a few.   I have also sailed on Lake Malaren in Sweden.  Malaren is similar in size to Champlain, and it too was under mile-high glaciers 10,000 years ago.   Malaren is full of hazards resembling Ferris Rock.  On the charts they look like hoodoos -- very tall very thin chimneys of rock standing tall.

If my imagination is correct that these are chimneys of base rock that withstood the passage of the glaciers, then I'm stuck for an explanation.  It doesn't seem possible.

Reluctantly, I suppose that a more likely explanation is that my imagination is wrong.   These rocks are not as tall and thin as I imagine.  They are wider than they are tall.   They could be enormous boulders left behind as the glaciers melted and receded  

The shorelines around here are covered with small rounded rocks and small boulders that are left by melting glaciers.  I believe the technical word is moraine.    Could it be that the moraine has billions of rocks an inch or two in diameter, plus a small fraction of larger rocks a foot or two in diameter, plus a handful of truly gigantic boulders like Ferris Rock perhaps 1000 feet in diameter?  

A geologist should be able to answer the question simply by looking at rock samples.  Are these rocks part of the local baserock, or are they alien rocks brought here from far away?

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