Lava cascade in Thunderbolt Distributary of Labyrinth Cave system
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tubes created a cave floor that is hummocky<br />
and difficult to traverse. Many<br />
blocks are exceptionally large-a few<br />
are as long as 40 ft. Very few "<strong>in</strong>-place"<br />
remnants <strong>of</strong> the walls <strong>of</strong> the orig<strong>in</strong>al lava<br />
tubes have survived. The remarkable<br />
feature about this level is that it did not<br />
completely collapse to the surface and<br />
form an open collapse trench like the one<br />
at the downstream entrance. One possible<br />
explanation <strong>of</strong> the strength <strong>of</strong> this part<br />
<strong>of</strong> the ro<strong>of</strong> is that the highest accessible<br />
lava tube at both the upstream and<br />
downstream ends <strong>of</strong> the Post Office <strong>Cave</strong><br />
is filled with congealed lava. This fill<strong>in</strong>g<br />
may have formed a strong massive strut<br />
that has prevented <strong>in</strong>terior collapse from<br />
work<strong>in</strong>g upward to the surface.<br />
Remnants <strong>of</strong> certa<strong>in</strong> passageways<br />
that collapsed together to form the high<br />
central part <strong>of</strong> the Silver Connector level<br />
have been given names to facilitate description<br />
(see map 15, pl. 5). Where a<br />
passageway consists <strong>of</strong> an open<strong>in</strong>g<br />
formed by the <strong>in</strong>tegration <strong>of</strong> two or more<br />
tubes by collapse, we call it a "level. " If<br />
only one ma<strong>in</strong> lava tube is dom<strong>in</strong>ant, we<br />
call that one a "tube." Only the larger<br />
tubes and levels have been named. Small<br />
vertical breakdowns between tubes or<br />
levels are unnamed, but we use "connector"<br />
for the larger semicyl<strong>in</strong>drical<br />
breakdowns that cross vertically through<br />
the position <strong>of</strong> two or more tubes and yet<br />
do not reach the surface.<br />
Two connectors described here<strong>in</strong> at<br />
some length are the Silver Connector<br />
(fig. 51 and map 15, pl. 5) and the<br />
Cataract Connector. The Cataract Connector,<br />
near the east end <strong>of</strong> Post Office<br />
<strong>Cave</strong>, is an excellent example <strong>of</strong> a<br />
connector that formed by collapse between<br />
the lava tubes while one or more<br />
were occupied by molten lava. One <strong>of</strong> its<br />
walls has an 18-ft-high lava cataract that<br />
was frozen <strong>in</strong> place while molten lava<br />
was cascad<strong>in</strong>g <strong>in</strong>to the hole. Other parts<br />
<strong>of</strong> this connector's walls are plastered<br />
with lava dripstone that leaked through<br />
holes <strong>in</strong> the fragile floor <strong>of</strong> a level above<br />
them.<br />
The Silver Connector gives its name<br />
to the Silver Connector level. The Cataract<br />
Connector gives its name to two<br />
lava tubes, the Upper Cataract Tube and<br />
the Lower Cataract Tube. Each tube is<br />
compound and <strong>in</strong> places splits <strong>in</strong>to two or<br />
more parts, but this splitt<strong>in</strong>g is due to the<br />
build<strong>in</strong>g <strong>of</strong> <strong>in</strong>ternal balconies and not to<br />
breakdown between separate tubes.<br />
The Upper Cataract Tube lies just<br />
beneath the eastern part <strong>of</strong> the Silver<br />
Connector level and at its west end<br />
merges with that level through a ro<strong>of</strong><br />
collapse. In the much deeper parts <strong>of</strong> the<br />
Silver Connector level farther west, the<br />
Upper Cataract Tube is undoubtedly one<br />
<strong>of</strong> the components <strong>of</strong> the enlarged Silver<br />
Connector level (see longitud<strong>in</strong>al section<br />
on map 15, pl. 5).<br />
At the upstream end <strong>of</strong> the Post<br />
Office <strong>Cave</strong> the relations <strong>of</strong> passageways<br />
are similar. Above the Cocoa Pipel<strong>in</strong>e<br />
the four largest tubes or levels have been<br />
given names. Above the Cocoa Pipel<strong>in</strong>e<br />
tube is the Cocoa entrance level, which<br />
<strong>in</strong>volved two tubes, one above the other.<br />
Still higher is the upper entrance level; it<br />
subdivides eastward <strong>in</strong>to upper and lower<br />
tubes. And still higher are the Silver<br />
<strong>Cave</strong> levels (maps 14 and 15, pl. 5).<br />
The Silver Connector extends vertically<br />
through at least four and perhaps<br />
seven <strong>of</strong> these levels; it stops below them<br />
but above the Cocoa Pipel<strong>in</strong>e. With<strong>in</strong><br />
200 ft downstream from the Silver Connector,<br />
most <strong>of</strong> these levels have merged<br />
through collapse to form the highceil<strong>in</strong>ged<br />
part <strong>of</strong> the Silver Connector<br />
level. The Cocoa Connector, and the<br />
Central Connector downstream, are<br />
breakdown holes that reach the Cocoa<br />
Pipel<strong>in</strong>e.<br />
Because <strong>of</strong> the difficulty <strong>of</strong> show<strong>in</strong>g<br />
superposed levels on a map, some levels<br />
are <strong>of</strong>fset for the sake <strong>of</strong> clarity and l<strong>in</strong>e<br />
variations are used to help dist<strong>in</strong>guish<br />
certa<strong>in</strong> levels. Collapse rubble completely<br />
covers the floors <strong>of</strong> the Silver Connector<br />
level and Cocoa entrance levels<br />
but is almost absent <strong>in</strong> the ro<strong>of</strong>ed-over<br />
parts <strong>of</strong> the Cocoa Pipel<strong>in</strong>e and a few <strong>of</strong><br />
the other tubes.<br />
With this prelim<strong>in</strong>ary explanation <strong>of</strong><br />
the features, an upstream traverse<br />
through Post Office <strong>Cave</strong> is described. It<br />
starts at the collapse trench that gives<br />
access to the ma<strong>in</strong> entrance and ends at<br />
the upstream end <strong>of</strong> the Cocoa Pipel<strong>in</strong>e<br />
with a climb through overly<strong>in</strong>g tubes and<br />
levels to the upstream exit.<br />
Collapse Trench at<br />
Downstream Entrance<br />
The collapse trench that leads to the<br />
downstream entrance <strong>of</strong> Post Office<br />
<strong>Cave</strong> (map 15, pl. 5) is 220 ft long, 80<br />
ft across at its widest po<strong>in</strong>t, and 50 ft<br />
deep. Its walls <strong>in</strong> most places are nearly<br />
vertical, and large underground caverns<br />
open at either end <strong>of</strong> the trench. The<br />
downstream cavern leads beneath Post<br />
Office Natural Bridge to a downstream<br />
breakdown trench; the upstream cavern<br />
conta<strong>in</strong>s the entrance to Post Office<br />
<strong>Cave</strong>.<br />
In January <strong>of</strong> 1918, J.D. Howard<br />
clambered down the upstream wall <strong>of</strong> the<br />
trench, explored the cavern at its head,<br />
and from the side <strong>of</strong> this cavern crawled<br />
upstream along the downstream entrance<br />
tube (see map 15, pl. 5) to the po<strong>in</strong>t<br />
where his way was blocked by lava that<br />
filled the tube to its ro<strong>of</strong>. Evidently he<br />
did not f<strong>in</strong>d the small and <strong>in</strong>conspicuous<br />
crawlway through collapse debris <strong>in</strong> the<br />
middle <strong>of</strong> the entrance cavern floor that<br />
gives access to the ma<strong>in</strong> parts <strong>of</strong> Post<br />
Office <strong>Cave</strong>, for he disappo<strong>in</strong>tedly<br />
wrote, "it isn't much <strong>of</strong> a cave, as it is<br />
only an open<strong>in</strong>g with a crawler at the<br />
back end." Howard called the cave Post<br />
Office because <strong>of</strong> the many "pigeon<br />
holes" <strong>in</strong> the cliff above the entrance<br />
cavern. These rem<strong>in</strong>ded him <strong>of</strong> post<br />
<strong>of</strong>fice boxes. This clutter <strong>of</strong> small open<strong>in</strong>gs<br />
is one <strong>of</strong> the particularly <strong>in</strong>terest<strong>in</strong>g<br />
features <strong>of</strong> the collapse trench. They<br />
apparently formed <strong>in</strong> either a skylight on<br />
the top <strong>of</strong> a large tube or <strong>in</strong> a surface<br />
channel through which molten lava was<br />
actively mov<strong>in</strong>g downstream. In either<br />
case the surface <strong>of</strong> the molten flood (as<br />
observed <strong>in</strong> Hawaii and at other active<br />
volcanoes by Peterson and Swanson,<br />
197 4) congeals from the walls <strong>of</strong> the<br />
open<strong>in</strong>g, but before it can crust over<br />
completely, m<strong>in</strong>or fluctuations <strong>in</strong> the<br />
height <strong>of</strong> the lava river cause recurrent<br />
partial crusts to form and attach to the<br />
walls <strong>of</strong> the open<strong>in</strong>g at slightly different<br />
72 Selected <strong>Cave</strong>s and lava-Tube Systems, lava Beds National Monument, California