Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

heights. Withdrawal may occur when a
crust is so thin, hot, and pliable that it
cannot support itself; it breaks and may
curve downward into a roll. A rise in lava
level may break it and curl it upward. In
other places the broken-off edges of such
crusts are plastered over by lava splashing
upon them, or by rise of the lava
surface. The holes are by no means as
geometric and regular as post office
boxes. Some are rectangular, most are
irregular, and a few are almost cylindrical.
Some also contain curls of lava
crust, like rolled magazines thrust into a
small post office box. The wall above the
Post Office entrance cavern is an excellent
example of the filling to be expected
on the edges of a crusted-over skylight.
The collapse trench continues downstream
after interruption by the Post
Office Natural Bridge and another natural
bridge. Similar skylight-type features
show in places on the roof and in parts of
the ends of these natural bridges as well,
but these features do not extend through
the entire length of the bridges.
Flow Units
Another feature of interest in the
collapse trench is the division of the lava
in the sides of this trench into many flow
units (Nichols, 1936). Flow units are
created when a surface lobe of erupting
lava advances fast enough to continuously
override the lava surges that recurrently
burst out from its front and sides
as the lava advances. Flow units are easy
to recognize when pahoehoe rubble has
been formed and welded together at the
top and bottom of each successive surge.
Often, however, the contact between
many flow units consists only of lava
froth with bubbles of steam and air that
was smeared into patches and partings by
either an overriding or underriding flow
unit (Waters, 1960). The order in which
flow units congeal is not necessarily
from the bottom of the flow upward.
Basalt flows may produce a thick crust
on their top while the interior of the flow
is still molten and moving forward. It is
in this kind of layered lava that many
large lava tubes develop (Ollier and
Brown, 1965; Hatheway and Herring,
1970; Greeley and Hyde, 1972; Wood,
1976). Most flow units in the walls of the
Post Office collapse trench are 2-12 ft
thick, but some appear to be thicker. On
closer inspection one finds that most
thicker units are compound and subdivided
by platy partings that show evidence
of shearing and recongealing
along surfaces of flow. The north wall of
the collapse trench shows a continuous
75-ft-thick section of flow units; the
thickest unit is as much as 20ft thick, but
it is a compound unit divided into several
subunits along discontinuous zones of
flattened vesicles or along platy joints
that were formed by the collapsing and
shearing of vesicles.
Lava tubes, according to one prevailing
theory, are supposed to form within
the thickest and most rapidly moving
parts of lava flows. Such flows generally
subdivide into flow units of the various
kinds well displayed on the walls of this
trench. Halfway up the south wall, the
flow units dip gently inward as they
approach the axis of the Post Office tube
system-a relation commonly seen adjacent
to the walls of most large lava
tubes. By contrast, high on the north wall
of this trench the flow units dip outward,
roughly parallel to the ground surface.
These upper flow units may indicate that
the valley which contained the vertical
stack of Post Office tubes had filled and
that younger flows subsequently overtopped
the valley walls to the north; then
levees of flow units and thin flows spread
outward from the channel. When this
levee-building situation is reached, the
topmost feeder tube within a pile generally
begins to subdivide into numerous
small distributaries. A characteristic example,
previously described, is the complicated
distributary pattern of Catacombs
Cave.
Entrance Cavern and
Downstream Entrance Level
Howard's statement, "an opening
with a crawler at the back end" is an
appropriate summary. The big cavern at
the entrance has ceiling heights of as
much as 40 ft. Its southeast wall has
collapsed extensively and forms a slope
of loose blocks that extends to the middle
of the cavern floor. The northwest wall
is more intact with patches of dripstone
and small rubble piles. Upstream the
cavern walls narrow markedly, and at a
point 160 ft upstream the ceiling lowers
to 10-12 ft. Still farther upstream the
downstream entrance level is free from
collapse but is filled nearly to its roof
with lava. Continuous lavacicles and
dripstone cover its walls and ceiling. The
last 30 ft is a "crawler" only 1-2 ft high,
which is impassable farther upstream.
Mapping of the remainder of Post Office
Cave showed, however, that this tube is
a continuation of the final tapering downstream
end of the upper part of the Silver
Connector level. The inaccessible part
between them is only 10 ft long, but it
prevented Howard from discovering the
main parts of Post Office Cave.
A breakdown in the ceiling of the
cavern 200 ft from the cavern entrance
opens a hole that leads to a small upper
tube. A ladder 20ft or longer is needed
to reach the hole. Complete collapse of
its floor contributed to the 40-ft ceiling
height of the entrance cavern.
Upper Cataract Tube
At a point 180 ft upstream from the
mouth of the entrance cavern, a crooked
2- to 3-ft crawl way leads 20ft downward
through collapse blocks. The passageway
drops through the ceiling into a
7-ft-high lava tube called the Upper
Cataract Tube (see longitudinal section
on map 15, pl. 5). This tube continues
upstream 375ft to a point where its roof
opens into the overlying, extensive Silver
Connector level.
The walls of the downstream part of
the Upper Cataract Tube arch upward
steeply for 50 ft and meet overhead at an
acute angle. The remainder of the tube
upstream has a normal, smoothly rounded
ceiling. Many features indicate that
the Upper Cataract Tube has had a long,
complex history of repeated draining of
and refilling by molten lava. Both angled
and rounded ceilings show several sue-
Post Office Cave 73