Northern Entrance
The rocks of Natural Tunnel are of
two main types. These are carbonate dolostone (limestone with
magnesium) and siliciclastic rocks (clay with quartz). These are
sedimentary rock and belong to the Knox group. They were lain down
in the early Paleozoic Era during the Ordovician and Cambrian
periods. They contain fossils of algae from 500 million years
ago.
Natural Tunnel lies along the Glenita
Fault which is a structural weakness between the gently folded Rye
Cove syncline (a down fold) to the east and the more tightly folded
Purchase Ridge syncline to the southwest. The Glentia Fault may be
seen here at the north entrance to the tunnel a few feet above the
opening.
Geologists with the Virginia Division
of Mineral Resources believe a large sink developed here at the
north entrance to the tunnel. Stock Creek (you see flowing through
the tunnel) flowed into the sink eroding and dissolving the rock.
The water flowed south and rose as a spring near the south opening.
The Glentia Fault allowed cracks for this flow of water
underground. The water contained abrasive grains of quartz, sand,
and gravel which gradually carved the spectacular
amphitheater.
To claim a find
for this EarthCache you must post a picture (optional) of yourself
with the north opening of the tunnel in the background and answer
the following:
1. Estimate the
height of the tunnel entrance from the streambed to the
roof.
2.
Estimate the width of the entrance.
3. Estimate the
height of the amphitheater from the streambed to the
rim.
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