Friday, April 12, 2013

Things You Find in the Field: Dikes and Drill Holes

After hiking up the north ridge, I wandered over to a flat area that looked like an old drill pad, and found an iron-stained felsic dike at the end of the pad.
Looking south down the dike toward the canyon.
You can follow the dike to a linear outcrop on the middle ridge, and then across the canyon to a rocky patch just left of the darker, more prominent quartzite bed.
Same view, with the dike marked in turquoise.
The casing for the drill hole was still in the ground, verifying that the flat area was indeed a former drill site. I dropped a rock into the hole; it took about 2.5 seconds to hit bottom, indicating a depth of about 30 m (about 100 feet). The estimating method doesn't allow for rocks getting slowed down by hitting the sides of a narrow drill hole, so depths should be considered a maximum. Also, drill holes can be caved short of their original T.D., and a dropped rock might hit the water table prior to reaching the bottom. A splash or funny hollow sound can usually be heard when this happens; nothing can be heard from the rock after it hits water. The sound of the rock hitting bottom is hard to discern in deep drill holes.

It's been quite a while since I tested drill holes of known depth (this can help calibrate how much a rock might slow down by hitting the walls); drill holes now are plugged with mud and cement as the rods are being pulled out, so very few recent holes are left open the way this old hole was.

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