
Natural
Bridge, Mount Eccles
The cave is located towards the end of a small lava channel that runs from
a subsidiary crater to the Southeast of Mount
Eccles.
Note that although you can explore much of the cave by natural
light from the entrances, you should bring torches (with decent batteries!)
if you want to go right through as it gets a bit gloomy in the middle and
it is easy to slip on the damp rock. A bright floodlight will show
the full range of colours, which is not normally visible
The origin of the Cave.
Natural
Bridge is a "lava tube" formed by the roofing of a lava channel.
The diagram shows a long section and cross-sections. The sections
show the angular "gothic" roof shape which indicates that this roof developed
by the inward growth of levee banks on each side. This process is
described in another page. As well
as growing out and over the channel to form a roof the layered banks slumped
downward while still semi-liquid. We can see the resulting contorted
layers exposed within the cave where parts of the lining have broken
away.
|
|
Entrance to Natural Bridge Cave,
Note "gothic" roof shape. |
Contorted roof inside the cave indicates slumping of
the levee layers. |
Features of the cave.
If you have a bright light you will see colour variations on the walls
- the green is mosses and lichen growing in the low light from the entrances,
and the various rich shades of cream, brown and orange which are the natural
colours of the basalt rock and minerals weathering from it.
The small hole on the surface connects to the main cave below.
Do not climb into this. The guard rail is there for a good reason
- the apparent floor of the small upper chamber often has a cover of loose
leaves which can hide a dangerous hole and 10m fall into the main cave
below!
Surface features
To the north of the bridge you can see the lava channel which was roofed
to form the cave. The narrow section next to the cave may also have
been roofed, but it has since collapsed. To the south the channel
widens and looses its character rapidly. The lava river must have
spread out at that point to form a broad "delta".
There is an interesting walk from the car park, back up the lava channel
to its source in a small crater, and you can continue from there right
back to the main crater and lake at Mount Eccles.
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Last modified on 2 Sept 1998
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