Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

underway. Collapse to the surface has
occurred at all four entrances. In fact, the
near-total collapse of the uppermost tube
to the surface, a process which left a train
of breakdown depressions, reveals the
course of this main lava feeder channel
connecting Natural Bridge to Ovis, Indian
Well, and Sentinel Caves. This is
the tube system that also spawned the
Labyrinth, Catacombs, and Hercules
Leg-Juniper Cave systems. Sentinel
Cave's roof is the largest remnant to
survive along the course of this highest
feeder tube of the Cave Loop Road area.
Features Near Upper Entrance
At the upper entrance to Sentinel
Cave are two large collapse pits with a
natural bridge between them (map 7, pl.
2). The ceiling of the upper tube has been
thinned almost to the surface by roof
collapse, and collapse breccia covers the
floor.
Located 50 ft from the entrance and
10 ft above the present floor is a fragment
of the tube wall and a lava bench. They
were protected from collapse because
they were in a 20-ft-diameter alcove on
the southeast wall of the tube. As the
floor of the alcove cooled and solidified,
the underlying lava began to drain back
into the main tube, a process which
formed a 2-in.-wide tension crack in the
upper crust. Another 50 ft downstream
the collapse blocks on the cave floor end
abruptly, and a remnant of each wall of
the tube is perched on either side. One
remnant runs beneath the stairs on the
west side; the other is separated from the
present east wall of the cave by collapse
blocks that represent an 8-ft retreat of the
original southeast wall.
Both remnants have a lava bench, or
balcony, 3 ft above the pahoehoe, which
probably marks the former floor of the
tube. Evidently, throughout most of this
area the floor of the upper tube collapsed
into the middle tube below. A little
farther downstream, at the abrupt bend of
the tube to the north, is an almost
impassable chaos of deep pits partly
filled with huge blocks. High ceilings,
caused by unraveling of the roof almost
to the surface, characterize this area.
These features indicate collapse of all
three upper-tube levels, an event confirmed
by the lowest tube being closed by
debris in this area.
The trail avoids this chaotic section
of the cave by following a small branch
of the upper level that swings around a
pillar to the west and reenters the main
tube 90 ft downstream. Here a natural
catwalk crosses the deep collapse pits in
the main cave. This catwalk is anchored
at either end to remnants of the pahoehoe
bench that formed the floor of the upper
tube. An upper balcony, 3 ft higher than
the catwalk, still adjoins the lower-floor
bench at either end of the catwalk.
Pahoehoe lava in a tributary to this upper
balcony is exposed at intervals within the
small tube forming the trail bypass. This
balcony continues on the north wall of
the bypass tube for 20 ft west of the
catwalk. A remnant of the floor of the
upper tube extends as a narrow natural
bridge about halfway across part of the
deep chasm 15 ft south of the catwalk.
Together with fallen roof blocks at the
west side, the remnant forms a second
precarious crossing over the collapsed
tube. The numerous benches, partial
bridges, and balconies where these two
passages join indicate a complex series
of events.
North of the catwalk the chasm of
collapsed blocks ends abruptly. The trail
from the east end of the catwalk continues
along the narrow bench, 3 ft below
the balcony bench, and over a lava fall
until it reaches the intact floor of the
upper tube. Downstream this tube floor,
littered with fallen roof blocks, can be
traced for 95 ft to where it is buried under
collapse rubble.
Two interesting features appear
along this stretch of intact floor. One is
a collapse pit resembling a well only a
few feet in diameter, which gives access
to the middle level below. The other is a
small tube-in-tube less than 2ft deep and
3 ft wide; its cross section is best seen on
the north wall of the collapse pit, just
beneath the surface. South ofthe pit most
of the tube-in-tube has collapsed, either
from its own weight or by bombardment
of falling roof blocks (map 7, pl. 2). In
this section of the cave, the main passage
measures 10ft wide and as much as 25
ft high.
Nearly 25 ft north of the collapse pit
the trail begins to rise steeply on a huge
mound of collapse rubble. No primary
features of the lava tube are seen in this
section except on the surfaces of fallen
blocks. From the top of this collapse
mound the trail rounds a sharp comer,
passes a tiny branching tube from the
north, and descends a stairway into a
deep collapse. Here the entire floor of the
upper level has fallen into the lower
tubes. This collapsed area extends northeast
for 210 ft, where the trail again
ascends to a partially rubble filled upper
tube.
At the east end of the collapsed area
where the stairs descend from the upper
level, the middle level extends south
beneath an overhang. Through the floor
of the middle level, beneath this overhang,
a narrow vertical chimney-like pit
opens below steep slopes of rubble. The
pit gives access to the lowest levels of
Sentinel Cave and is covered by a gate.
Middle Level of Sentinel Cave
The entire length of the middle level
of Sentinel Cave is covered by rubble
from the collapse of the upper level.
Small remnants of lava benches and
balconies provide glimpses of tube walls
in and above the middle level (map 7, pl.
2). These remnants have not been correlated
with one another, nor with the
two benches described in the upper level.
Halfway along this east-trending, 20-
ft-wide passage is a double skylight, the
Tube and Pillar entrance, named for the
small natural bridge and pillar within a
near-surface tube. The ceiling height of
the main passage gradually decreases
from 12 ft to 9 ft at the lower entrance.
Tubes Intersecting Sentinel Cave
A side tube that crosses the collapsed
middle level of Sentinel Cave approximately
400 ft upstream from the lower
entrance is the continuation of the tiny
branching tube just upstream from the
stairway. At its upstream entrance, this
small tube is a 3-ft-diameter crawlway
with a smooth pahoehoe floor. The tube
enlarges slightly before swinging east
and rejoining the main tube 16 ft above
40 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California