The Geological Formation of the Blue Hole
Typically, a river or in this case Mill Creek, flows over a large step in the rocks which may have been formed by a fault line. Over a period of years, the edges of this shelf will gradually break away and the waterfall will steadily retreat upstream, creating a gorge of recession. Often, the rock stratum just below the more resistant shelf will be of a softer type, meaning undercutting, due to splashback, will occur here to form a shallow cave-like formation known as a rock shelter or plunge pool under and behind the waterfall. Eventually, the outcropping, more resistant cap rock will collapse under pressure to add blocks of rock to the base of the waterfall. These blocks of rock are then broken down into smaller boulders by attrition as they collide with each other, and they also erode the base of the waterfall by abrasion, creating a deep plunge pool.
Mill Creek becomes wider and more shallow just above waterfalls due to flowing over the rock shelf, and there is a deep pool just below the waterfall because of the kinetic energy of the water hitting the bottom.
At the Blue hole, you will see what the Mill Creek has done which results in all of this beauty.
There are several types of waterfalls:
BLOCK: the water descends from a relatively wide stream or river
CASCADE: the water descends a series of rock steps
CATARACT: A large waterfall
FAN: water spreads horizonally as it desends whileremaining in contact with bedrock
Horsetail: The desending water maintains some contact with bedrock
PLUNGE: the water desends vertically, losing contact with the bedrock surface
PUNCHBOWL: water desends in a constricted form, then spreads out in a wider pool
SEGMENTED: separate (distinctly) flows of water form as it desends
TIERED: the water drops in a series of distinct steps or falls
MULTI-STEP: a series of waterfalls one after another of roughly the same size with each its own sunken plunge pool
Some Of The Types, Which One Am I?
This magnificant waterfall and pool took millions of years to form so enjoy and take care of it.
Note In order to receive credit for the Earthcache you must do the following:
1. Email us the answers to the following:
A. Estimate the height of the waterfall (need to be within +/- 10 feet)
B. Estimate the width of the waterfall (need to be within +/- 10 feet)
C. Using the above classification, what type of waterfall is the Blue Hole?
2. Upload a picture of your GPSr pointing to the front of the waterfall. While it would be nice, you do not have to have to show your face(s) in the photo.
We hope you have enjoyed this little bit of geology as much as we did. We are certainly not geologists………..far from it! We have found the subject becoming more and more interesting so from a strictly amateur point of view, we simply want to share these wonderful local geological phenomena with you. We are learning as we go along.
This Earthcache was approved by the Geological Society of America
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