Whipstock Hill Ups and Downs EarthCache
Hidden : 12/9/2012
Difficulty:
2 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:

This earthcache focuses on a recently exposed fold along Route 279 just west of Bennington, Vermont.  The principle geologic feature is the series of folds here and the cache description discusses the types of folds, the dip (tilt) of the limbs of the fold and the angle of the axial plane.  These are all discussed and illustrated in the cache description.

Located on the south side of the Bennington Bypass (Vermont Route 279) is an outcrop exposing three folds preserved in metamorphic rocks.  These rocks are part of the Early Ordovician (~500 million years old) aged Bascom Formation.   The composition of the Bascom Formation is marble (metamorphosed limestone)

Development of this outcrop began between 1.3 and 1.0 billion years ago with the collision of the micro-continent of Grenvillia and the continent of Laurentia at a convergent plate boundary.  This ancient tectonic event created the Grenville Mountains.  Collision of these two plates caused significant amounts up uplift followed by several hundred million years of erosion. 

Changes in direction of plate movement in this area approximately 300 million years later caused the formation of a divergent plate boundary (rifting) and rifting.  As rifting split this portion of the North American continent, water entered the rift forming the Iapetus Ocean.  As erosion continued, terrestrial sandstones were deposited in the rift, and as the water became shallower, limestone was deposited.  These limestones are now the marbles preserved in the Bascom Formation that form the bedrock of the Vermont Valley, a physiographic feature with steep walls that separates the Taconic Mountains to the west and the Green Mountains to the east. 

The opening of the Iapetus Ocean reached its maximum in the Ordovician and then the basin began to close.  Forces changed from tensional during formation of the Iapetus to compressional as it closed.  As compression took place, subduction was initiated for a second time and an island arc system was created with associated folding and volcanism. 

The island arc that was created was then pushed onto the edge of Laurentia, folding and metamorphosing the limestone into the Bascom Formation marble.  The rocks of the Bascom Formation were also pushed approximately 60 miles to the west along the Maple Hill Fault in the creation of the Taconic and Green Mountains.

Approximately 60 million years later, the micro-continent of Avalonia collided with Laurentia and squeezing the rocks one more time, producing bedrock structures known in Vermont today.
Since uplift in these mountain building events, Vermont has been undergoing erosion ever since.
When rocks undergo compression, a variety of things can happen, including metamorphism, folding and faulting.  At Whipstock Hill, the rocks here began as limestones forming in a warm equatorial ocean.  With continental collision, the parent limestone of the Bascom Formation was metamorphosed into marble.  Additionally, the marble of the Bascom Formation was also folded.

Folding occurs when a bed or a group of beds are subjected to compressional forces and are bent or curved   When compressional forces are applied to rocks several types of folds can occur.  As a rock is folded two limbs are produced (see figure below).  The fold is bisected by an axial plane that divides the fold into two equal halves, much the same as an angle is bisected in geometry (see figure below).


http://www.gg.uwyo.edu/content/laboratory/structure/folding/fold_parts.asp?callNumber=11111


If the limbs of a fold tilt away from the axial plane an anticline is produced.  If the limbs of a fold tilt towards the axial plane a syncline is the result (see figure below).


http://www.nvcc.edu/home/cbentley/gol_135/sideling_hill/readings.htm


Additionally, if the angle of the tilt of the limbs is the same on both sides, the fold is said to be symmetrical (see figure below).  If the angle of the tilt of the limbs is different on either side of the axial plane, the fold is said to be asymmetrical.   Overturned folds are folds in which the axial plane is tilted and beds may also tilt in the same direction on both sides of the axial plane.  Recumbent folds are those with nearly horizontal axial surfaces. 


http://thehiddenwater.wordpress.com/geology/geological-structures/folds/


References used in the development of this earthcache:
Dickas, A.B., 2012.  101 American Geo-Sites You’ve Gotta See.  Missoula, MT:  Mountain Press Publishing Co., 250 p.
http://en.wikipedia.org/wiki/Fold_%28geology%29
http://myweb.facstaff.wwu.edu/talbot/cdgeol/Structure/Fold/Folds1/Recumbent.html

To gain credit for this earthcache, please email the owner the answers to the following (do not put answers in your log):
1. Looking at the fold in the middle of the outcrop, estimate the approximate angle of tilt for the layers in the limb on the west side of this fold.
2. Is it an anticline or a syncline?
3. Is it symmetrical, asymmetrical, overturned or recumbent?
4. Are the other two folds anticlines or synclines
5. Are they symmetrical, asymmetrical, overturned or recumbent?
6. What is the length of this outcrop? (This can be done by using the odometer in your car or by marking a waypoint on your GPSr at one end of the outcrop, walking the length of the outcrop and then using the find option on your GPSr, determine the distance between the two locations).

Additional Hints (No hints available.)



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