|
Caves and Natural Bridges
| These natural bridges of
Vanport Limestone formed when a cave in the limestone was exposed by erosion of Hells
Hollow by glacial lake overflow that passed through Hells Hollow for a while during the
advance of the glacier. It is similar to the development of a "hanging
valley," but the stream that was left hanging was underground. These are
essentially "hanging caves." The bridges are now isolated from the
rest of the Vanport Limestone containing the cave, because these pieces of limestone have
moved under the influence of gravity away from the outcrop, similar to rock cities, towards Hells Hollow, or because the intervening
area has been gradually dissolved by water.
|
Click for larger image. |
|
The first natural bridge encountered along
the trail rarely has water flowing beneath it. |
Click for larger image. |
|
The second natural bridge is used by
the trail in crossing the gully, as seen in this photo. It also is usually dry.
The blue blaze on the tree marks the Slippery Rock
Gorge Trail. |
Click for larger image. |
|
The third natural bridge, also used by the
trail, often has water flowing from the cave on the left side, beneath the bridge, and
over a waterfall on the right side. |
Click for larger image. |
|
Note the smooth sides of this small cave
by the third natural bridge, and the scalloped sides, especially on the left. The
cave formed by water flowing along a fracture in the rock dissolving the limestone.
Note the fracture running along the roof of the cave. The Flume,
along the Hells Hollow Trail, was probably formed by the collapse of a similar cave.
This cave is only about 1 or 1½ feet high. There
are other small but dry caves that can be seen in the Vanport Limestone outcrops along
the trail. |
|
