Sunday, July 14, 2013

Cave in Gravel Pit and Iron/Manganese Coated Pebbles

I recently visited a cave within a gravel bed that had been unearthed at a small gravel pit. Caves and gravel pits are not a very consistent association so when I heard about the cave from a local, I figured it would be worth a visit.

Cave within gravel pit wall

View of the inside of the cave

The cave appeared to go back approximately 15 to 20 feet. The pit operator indicated that the cave appeared after they had recently excavated along this wall of the pit. He reported that it appeared water had blown out of the side of the pit over night. The pit is located within an area that was recently mapped as Vashon ice contact kames and kame deltas by Polenz and others (2012). The site is on the east side of the Olympic mountains near Hood Canal. The mapping as well as observations I have made in this area indicates that there was a sediment accumulation area along the margins of the Puget Ice Lobe during the last glaciation. The gravel beds shown are classic delta front gravel deposits which are very common in this area (although the exact delta settings does vary).

The pit operator indicated that the floor of the pit was underlain by hard pan that was like concrete which suggests glacial till. The tilted sand and gravel units along with some topographic features suggests that a zone of perched saturated sand and gravel was located in the side of the pit. When the water pressure blew out into the excavation the saturated sand and gravel flowed with it leaving a temporary cave. The very short duration of water flow suggests it was an isolated zone of perched groundwater versus the draining of a larger aquifer (although rare, that has happened in other gravel mines - not good).

Another noteworthy feature was the abundance of iron/manganese coated pebbles. Perhaps not a rare thing, but not something I had previously encountered in such abundance.

Iron/manganese coated pebbles and cobbles
Past water movement through these gravels must have been rich in metals and the metal precipitated onto the pebble surfaces.  

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