Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

central section spreads into still-uncollapsed
tubes. The upper part of the Silver
Connector level decreases in height and
continues eastward, where it is nearly
filled with lava. It eventually becomes a
crawlway that continues for 30 ft to
where it is plugged with congealed lava.
It is open another 10 ft farther downstream
as the western end of the downstream
entrance level.
Part of the middle section of the
Silver Connector level is equivalent to
the Upper Cataract Tube, but collapse of
both floor and roof prevents tracing this
connection.
The upstream (western) end of the
Silver Connector level is even more
complex: west of the Silver Connector
breakdown this level extends into parts
of at least five tubes. The highest three
levels are reached from Silver Cave (map
14, pl. 5), whose roof lies only 10-12 ft
below the ground surface. The lowest
passage extending north from the base of
the Silver Connector is 85 ft below the
surface but still 25 ft above the underlying
Cocoa Pipeline.
Two breakdowns in the floor of the
Silver Connector level give access to the
Cocoa Pipeline: the eastern (downstream),
Central Connector is an easy
and safe passage into the pipeline; the
other, 540 ft upstream, is the deep,
steep-sided, and very unstable Cocoa
Connector.
The Silver Connector level seems
geologically uninteresting at first; its
floor is nothing but a hummocky jumble
of large fallen blocks that are difficult to
walk over. The walls are more instructive,
for patches of dripstone show the
perched remains of a collapsed tube here
and there, and irregular shelf-like extensions
from the walls can be identified as
edges of the floor of a broken-off tube.
Only rarely can one see any primary
features in the high and inaccessible
roof. In fact, playing a strong light over
this roof, 15-60 ft overhead, is a disconcerting
experience when one notices
the many precariously perched blocks
that appear ready to fall. However, no
other cave in Lava Beds National Monument
provides such insights into the
structure and mechanics of operation of
lava tubes as does Post Office. Just as the
shambles after an earthquake reveal the
structural details of broken buildings, an
inspection of the collapsed walls of the
Silver Connector level provides details
of tube construction and of the mechanics
of plastic flow in lava tubes that could
never be inferred from well-preserved
lava tubes coated with lava plaster. A
few select features of this remarkable
lava-tube ruin that contribute to the
understanding of lava tubes are described
herein. Areas of special interest or beauty
are also described. Not all features
shown on the map are described herein.
The floor of the Silver Connector
level, 200 ft upstream from the breakdown
that connects it with the Upper
Cataract Tube, is surmounted by an
unusually high and steep conical pile of
collapse debris. The pile rises 45ft above
the thick blanket of tumbled blocks that
forms the floor of the cave. Nearby are
irregular benches jutting from the wall at
various points, remnants of at least three
tubes that coalesced to create a cave
30-45 ft high. Farther upstream (northwest)
65 ft from the base of this huge
collapse pile, the floor of the Silver
Connector level drops away into the
Central Connector, a breakdown 60 ft
long and 6-15 ft wide. This connector
gives easy access to the eastern part of
the Cocoa Pipeline lava tube, 25 ft
below. Before visiting this very different
level, however, continue upstream in the
Silver Connector level by carefully skirting
the Central Connector along the top
of its southeast wall. For some distance
upstream from the Central Connector,
the ceiling of the Silver Connector level
is nearly 35 ft high and coated with
off-white caliche (calcium carbonate
crusts). If illuminated while wet, caliche
has a highly reflective, silvery glow.
Water dripping from this roof has deposited
upon collapse blocks on the floor
fine examples of the fragile arborescent
growths of caliche popularly called
"cave coral." Most growths are white,
but where stained by hydrous iron and
manganese oxides, organic soil compounds,
or fungi, they can be deep red,
lustrous black, yellow, pink, silky gray,
and light blue.
The next major interruption, 540 ft
farther upstream from the Central Connector,
is the Cocoa Connector, a deep,
unstable 22- by 15-ft hole. The descent
through the Cocoa Connector to the floor
of the Cocoa Pipeline, 40 ft below, is
over loose and easily rolled debris. A
descent here is neither recommended nor
necessary because there are two safer
entrances into the Cocoa Pipeline. Upstream
toward the Cocoa Connector, the
blanket of collapse blocks on the floor
thickens, and many huge blocks as much
as 40ft long are embedded in the debris.
Some of these large blocks show surface
features, which testify that they are large
pieces of the separations between two
superposed levels. Collapse over considerable
areas must have occurred in places,
as such large blocks surely are not due
to the slow unraveling upward of roof
slabs loosened along minor joints and
cracks. The truncated edges of some
floor benches still hanging from the walls
also imply large-scale rockfalls. The
piles of collapse blocks are a jigsaw
puzzle of surface features and the broken
walls of lava tubes. It is common to find
large fallen blocks with several coatings
of lava plaster sliced in cross-section on
one or more broken edges.
Downstream 65 ft from the Cocoa
Connector, the Silver Connector level
makes a 45° turn to the left (as viewed
looking upstream) and heads almost due
south. The Cocoa Pipeline below turns
the same corner with an even sharper
bend. The effect of a sharp bend in the
original, pre-lava topographic valley in
controlling the superposition of the two
lava tubes seems clear, because the curve
became gentler as the pre-flow depression
was filled in.
The dangerous Cocoa Connector can
be skirted safely on its northwest side.
Once around it one climbs a steep 15- to
20-ft pile of collapse rubble. In another
190 ft is the large vertical hole through
several levels called the Silver Connector.
Silver Connector
The breakdown 190 ft upstream from
the Cocoa Connector (map 15, pl. 5) has
for several years been called the Silver
Connector (fig. 51) because its top is in
Silver Cave. It extends vertically for 60
Post Office Cave 77