Lava cascade in Thunderbolt Distributary of Labyrinth Cave system
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clear evidence <strong>of</strong> hav<strong>in</strong>g developed from<br />
an earlier collapse after which molten<br />
lava lay ponded <strong>in</strong> a wide lava lake or<br />
sluggish stream open to the sky dur<strong>in</strong>g at<br />
least some major periods <strong>of</strong> volcanism.<br />
Today the rock rim <strong>of</strong> this trench is a cliff<br />
rang<strong>in</strong>g from a few feet to 45 ft <strong>in</strong> height.<br />
From the base <strong>of</strong> the cliff a talus slope <strong>of</strong><br />
loose blocks descends to the deep central<br />
part <strong>of</strong> the trench, which is at the projected<br />
elevation <strong>of</strong> the two large superposed<br />
lava tubes beneath Natural Bridge.<br />
The deepest hole with<strong>in</strong> this collapse<br />
trench is at the trench's extreme northeast<br />
corner, directly beneath the upstream<br />
entrance to the caves beneath<br />
Natural Bridge. Here, a collapse that<br />
extends through the upper tube and <strong>in</strong>to<br />
the lower tube beneath Natural Bridge<br />
has formed a pit over 50ft deep. This pit<br />
is bordered on the southwest by a steep<br />
ridge <strong>of</strong> tal us that forms a berm across the<br />
head <strong>of</strong> the trench and divides the ma<strong>in</strong><br />
part <strong>of</strong> the trench from the deep hole just<br />
upstream from Natural Bridge. Only<br />
near this cave entrance can any clearcut<br />
relation be seen between the trench and<br />
the passages <strong>of</strong> the lava tubes. It is clear<br />
that both the upper and the lower lava<br />
tubes exposed beneath Natural Bridge<br />
extended upstream <strong>in</strong>to the northwestern<br />
part <strong>of</strong> the area now occupied by the deep<br />
collapse trench. Yet a simple calculation<br />
will show that the volume <strong>of</strong> the collapse<br />
trench is at least three times greater than<br />
the space required to completely fill lava<br />
tubes the size <strong>of</strong> those beneath Natural<br />
Bridge, which are almost certa<strong>in</strong>ly the<br />
upstream cont<strong>in</strong>uations <strong>of</strong> Ovis <strong>Cave</strong> and<br />
the lower tube <strong>in</strong> Crystal <strong>Cave</strong>. No<br />
remnants <strong>of</strong> other tubes that might help<br />
with thi s space problem are known. If<br />
they ever did exist, which seems unlikely,<br />
they are hidden <strong>in</strong> the talus <strong>of</strong> the<br />
collapse trench.<br />
Other <strong>in</strong>terpretations <strong>of</strong> the history <strong>of</strong><br />
this trench can be <strong>in</strong>ferred from a study<br />
<strong>of</strong> the rock rims along its borders. First<br />
we note that the highest elevations<br />
around the trench are on these rims; the<br />
sagebrush-covered ground on both sides<br />
slopes away from the trench, not toward<br />
it. As <strong>in</strong>dicated on the map, molten lava<br />
spilled over the trench rim as small lobes<br />
<strong>in</strong> many places. Some lobes escaped over<br />
the rim through low areas along the<br />
northwest side and then coursed northward<br />
down the slope. A larger lobe,<br />
which conta<strong>in</strong>s Sunsh<strong>in</strong>e <strong>Cave</strong>, spread<br />
beyond the area <strong>of</strong> the map. The Sunsh<strong>in</strong>e<br />
Channel, at the head <strong>of</strong> this cave,<br />
and its connection with the trench, is<br />
shown on an <strong>in</strong>set <strong>of</strong> the Natural Bridge<br />
map (map 5, pl. 2); its passages are<br />
described <strong>in</strong> the section "Battered Sherman<br />
<strong>Cave</strong> and Sunsh<strong>in</strong>e Arch." The<br />
Sunsh<strong>in</strong>e Channel is partly lava gutter<br />
and partly small lava tube; farther downstream<br />
it is still ro<strong>of</strong>ed over to form<br />
Battered Sherman <strong>Cave</strong>; a m<strong>in</strong>iature<br />
natural bridge, Sunsh<strong>in</strong>e Arch; and the<br />
upper part <strong>of</strong> Sunsh<strong>in</strong>e <strong>Cave</strong>. Note that<br />
where the channel leaves the big collapse<br />
trench, backflow features developed.<br />
Thus it was a lower<strong>in</strong>g <strong>of</strong> the lava lake<br />
with<strong>in</strong> the large collapse trench that<br />
stopped the fill<strong>in</strong>g <strong>of</strong> Sunsh<strong>in</strong>e tube and<br />
allowed it to dra<strong>in</strong> and survive as an open<br />
cave.<br />
<strong>Lava</strong>-Tube <strong>Cave</strong>s <strong>in</strong> the<br />
Natural Bridge Area<br />
Ovis <strong>Cave</strong>, Paradise Alleys, and<br />
Hercules Leg <strong>Cave</strong> all have their upstream<br />
entrances with<strong>in</strong> the Natural<br />
Bridge area. Ovis <strong>Cave</strong> and Paradise<br />
Alleys are shown on map 4, plate 2,<br />
Hercules Leg and Juniper <strong>Cave</strong>s are<br />
shown on map 6, plate 2. <strong>Cave</strong>s shown<br />
on map 5 (pl. 2) <strong>of</strong> the Natural Bridge<br />
area <strong>in</strong>clude: (1) a complex <strong>of</strong> two large<br />
lava tubes and two small lava tubes<br />
beneath Natural Bridge; (2) Gail <strong>Cave</strong>, a<br />
small lava tube with two collapse bas<strong>in</strong>s<br />
strung along it, which can be traced for<br />
400 ft northeast after branch<strong>in</strong>g from the<br />
upper tube beneath Natural Bridge; (3)<br />
Juniper Pole <strong>Cave</strong>, south <strong>of</strong> the embayment<br />
at the southeast comer <strong>of</strong> the large<br />
collapse trench upstream from Natural<br />
Bridge; (4) Sunsh<strong>in</strong>e <strong>Cave</strong>, Battered<br />
Sherman <strong>Cave</strong>, and Sunsh<strong>in</strong>e Arch,<br />
northwest <strong>of</strong> Natural Bridge; and (5)<br />
Duffys Old Still Well and Prohibition<br />
<strong>Cave</strong>, upstream from the segment <strong>of</strong><br />
deep collapse trench.<br />
<strong>Cave</strong>s Beneath Natural Bridge<br />
The caves beneath Natural Bridge are<br />
remnants <strong>of</strong> four different lava tubes,<br />
each <strong>of</strong> a different size and at a different<br />
elevation; all these caves can be entered<br />
from the large passage beneath Natural<br />
Bridge.<br />
At the downstream (northeast) side<br />
<strong>of</strong> Natural Bridge is a large arched cavern<br />
12 ft high and 40 ft wide. It quickly<br />
narrows to half this width, because a<br />
slide <strong>of</strong> loose blocks spilled onto the<br />
floor from a collapse <strong>of</strong> part <strong>of</strong> the<br />
southeast ro<strong>of</strong> and wall. Farther upstream<br />
80 ft, however, the tube suddenly<br />
widens on both walls, subdivides around<br />
a 75-ft-long pillar, and then reunites at<br />
the pillar's upper end. In another 20 ft the<br />
s<strong>in</strong>gle tube, now 20 ft high and 45 ft<br />
wide, was demolished by the northeast<br />
end <strong>of</strong> the deep collapse trench, which<br />
forms the southwest side <strong>of</strong> Natural<br />
Bridge. This tube, labeled "upper tube"<br />
on map 5, plate 2, is almost certa<strong>in</strong>ly the<br />
upstream cont<strong>in</strong>uation <strong>of</strong> the large tube<br />
that forms Ovis <strong>Cave</strong>. Another remnant<br />
<strong>of</strong> it forms the open<strong>in</strong>g beneath Ovis<br />
Bridge.<br />
Beneath the upper tube is another<br />
large tube-the Humm<strong>in</strong>gbird Flyway.<br />
It does not split around the pillar like the<br />
upper tube but cont<strong>in</strong>ues upstream beneath<br />
the area adjacent to the pillar (see<br />
the cross section <strong>of</strong> Natural Bridge on<br />
map 5, pl. 2). Only about 60 ft <strong>of</strong> this<br />
lower tube is accessible; the rest <strong>of</strong> it<br />
(southeast <strong>of</strong> the pillar) was demolished<br />
by collapse <strong>of</strong> the upper tube's floor.<br />
Along part <strong>of</strong> this collapse, however, the<br />
southeast wall <strong>of</strong> the lower tube can be<br />
traced, curv<strong>in</strong>g up <strong>in</strong> places beneath an<br />
overhang<strong>in</strong>g ledge formed by the broken<br />
floor <strong>of</strong> the upper tube. In this stretch the<br />
Flyway is more than half filled with<br />
collapse debris.<br />
Two crawlways and a pit permit<br />
entry <strong>in</strong>to the relatively undamaged part<br />
<strong>of</strong> the lower Humm<strong>in</strong>gbird Flyway tube.<br />
One crawlway is located <strong>in</strong> the upper<br />
tube 15 ft southwest from the downstream<br />
edge <strong>of</strong> Natural Bridge, at the<br />
northeast marg<strong>in</strong> <strong>of</strong> the slide that spills<br />
out from the southeast wall. The second<br />
crawlway is through a maze <strong>of</strong> huge<br />
collapse blocks where the southeast wall<br />
<strong>of</strong> the upper tube sw<strong>in</strong>gs out around the<br />
pillar. It is near the northeast corner <strong>of</strong><br />
the collapse that dropped part <strong>of</strong> the floor<br />
<strong>of</strong> the upper tube <strong>in</strong>to the lower tube. The<br />
Natural Bridge Area 33