Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

long and 300ft in maximum width. The
rim of the breakdown is continuously
surrounded by a rampart of talus and
tilted roof blocks ranging in height from
8 to 20 ft. However, before the rampart
was formed, the heart -shaped area had
sagged in toward the center of the depression,
and so the rampart itself is
surrounded by a "moat" up to 8ft deep.
Most interior breakdown now consists of
a "cork" of somewhat broken and variously
tilted, but still relatively intact,
roof rock. The inner edge of the breakdown
is separated from this central cork
by talus and broken rock, which spread
from the low cliff of the breakdown and
from the shattered edges of the cork (map
20, pl. 6).
The position of the large lava tube
apparently was beneath the west side of
the heart-shaped depression. The
blocked cave upstream projects into the
west side of the depression, whereas at
the widest part of the depression a skylight
drops into another segment of tube.
Also near this part of the depression a
shallow collapse trench of the normal
width for the tube leaves the breakdown
and continues to the east side of the next
large breakdown to the north. This collapse
trench extends north-northwest for
440 ft, changes into a 250-ft-long shallow
linear sag with narrow turndown
sides, and disappears 70 ft short of the
southeast comer of the next breakdown
The next breakdown is shaped like a
rectangle with rounded-off comers. It is
similar in size but a little smaller than its
heart-shaped companion. A mixed
block-and-tilted rampart 8-35 ft high
surrounds the collapse with part of this
rampart standing in a moat 2-8 ft deep.
The interior of this breakdown, however,
does not contain any intact cork. Instead
a hummocky carpet of loose blocks
spreads from all sides.
The four breakdowns that complete
the N. 40° W. trend are fairly shallow
collapse trenches of a size and depth
consistent with the collapse of a tube
averaging 30-50 ft wide. They are distributed
in two pairs of small breakdowns
with 345 ft of relatively uncollapsed lava
tube between. A skylight 45 ft deep
punctures the west edge of a lava-tube
cave at a point 50 ft north of the southern
pair. A small fissure that deviates to the
northeast from the south side of the
northern pair has a small block rampart
rising above it.
At the downstream end of the northem
pair the lava-tube system turns on a
N. 10° E. course and maintains this trend
for a distance of a little more than 1 mi.
It then fades out into a widespread area
of low sag basins north of the Three
Bridges area. The actual walking distance
is more than a mile because this
part of the lava-tube system meanders in
a series of broad open curves.
Many interesting features occur
along this stretch of trench, and most are
variations of features previously described.
The first breakdown is rimmed
by a block rampart 8-20ft high, parts of
which are bordered by a moat as much as
5 ft deep. A skylight is present at the
northwest comer of the collapse, whereas
on the opposite (east) side a schollendome
with almost vertical sides may
indicate a second lava tube that contributed
to the collapse. Both of these inferred
lava-tube caves appear to converge
farther north into a curving
(concave to the west) small breakdown.
At the north end of this breakdown a cave
can be entered that swings to the east,
almost at a right angle to the former
trench. At the northeast end of this cave,
a very large oval breakdown 980 ft long
and 350ft wide trends N. 10° E. Block
ramparts 15-20 ft high are perched above
its north periphery, but they lower to
10-12 ft around its south end. The
interior of this large collapse consists
mostly of hummocky piles of loose
blocks, but a dozen large fragments of
the roof can be shown on the map. The
largest is a thin flat-topped table 430 ft
long (labeled on map 20, pl. 6 as "table")
that occupies the north-central part of the
breakdown.
A smaller oval breakdown overlaps
the northwest comer of the large collapse.
It has no ramparts, and its central
part is a cork of intact roof rock, warped
into a spoon shape at its northwest end.
A cave curves sharply right from the
north end of this breakdown and then 200
ft farther downstream turns sharply left
and connects with a collapse pit about 50
ft in diameter. This lava tube is the first
of the three caves separated by breakdowns
that inspired the name "Three
Bridges."
The smaller breakdown between the
first and second caves is separated by a
sill from a larger 320-ft-long and 75-
ft-wide larger neighbor. A spillover
channel lies over the sill. This breakdown
is rimmed on all sides by a hydraulic
rampart ranging from 5 to 15 ft
high and sits in a narrow moat as much
as 5 ft deep. At the downstream end of
this breakdown a small collapse pit provides
entrance into the second section of
lava tube-a cave 260ft long-which, in
tum, is interrupted at its north end by a
section of collapse trench 330 ft long. At
the downstream end of this trench the
third cave extends 220 ft to where it is
interrupted by a collapse pit. Another
cave entrance is present on the north side
of the collapse pit, but we did not explore
this cave beyond its entrance.
North of the Three Bridges is a broad
area of indefinite shallow sag basins and
a few linear streaks that may be surface
lava channels. The course of the lavatube
system across this area is indefinite,
probably because the main tube split into
a cluster of anastomosing smaller tubes.
If so , some of these reunite downstream
and continue the overall N. 10° E. trend
beyond the north edge of the map area.
The offset in the lava-tube system
just north of the Three Bridges area (map
20, pl. 6) was inferred primarily from
anastomosing tube channels associated
with the shallow sag basins. Two additional
tiny breakdowns lie directly on this
line within the belt of indefinite sag
basins. Other, larger breakdowns are
enclosed within still larger sag basins to
the west. Farther northeast four other
large collapses are clustered within one
large sag basin with indefinite boundaries.
Another large collapse lies still
farther north inside a much larger shallow
sag basin.
As mentioned previously, the lavatube
system continues northward beyond
the map area, then it swings eastward
toward Juniper Butte (fig. 4), and then it
probably continues northward to Fern
Cave. Along its easterly course it spawns
many distributaries to the northwest and
north.
100 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California