Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

occupies the lava pool just upstream
from the Unga Dunga entrance where the
channel starts its northward trend. In the
area just downstream from the junction
of Hercules Leg Cave with Upper Juniper
Cave (nearly 350 ft southwest of the
Juniper Cave entrance), the similar rapid
congealing of molten lava within a lava
flow thinner than at Catacombs preserved
an early stage in the development
of the anastomosing tube pattern so
typical of the upper part of Catacombs
and some parts of the Labyrinth Cave
system.
Features of Hercules Leg Cave between
the entrance and the right-angle
bends are much like the pillared area
west of the entrance. High-lava marks
and narrow perched lava benches indicate
former highstands of the lava. Sloping
floors covered with frothy pahoehoe
alternate with gently sloping pools of
smoother pahoehoe, much like the alternating
rapids and slack areas in a mountain
brook. Some roof failure occurred
when lava was flowing in the tube, as
indicated by the small piles of collapse
breccia that were overridden with lava
and by blocks rafted in frothy pahoehoe.
Middle and Lower Parts of
Hercules Leg Cave
From the area at the Unga Dunga
entrance, Hercules Leg Cave runs in
gently sweeping curves northward for
750ft and then makes a right-angle turn
to the east for 320 ft. It then angles
northeast 90 ft to its junction with Upper
Juniper Cave, a tributary tube (map 6, pl.
2). In this part of its course the cave is
mostly a single large tube with few
complications. The right turn to the east
is a rounded curve complicated only by
a split of the tube around a large (80 by
45 ft) pillar.
The middle and lower sections of
Hercules Leg have the most intact roof;
only minor amounts of collapse breccia
litter its floor except at the two ends
where large collapses occurred. The central
parts of this area, including the two
tubes which detour around the big pillar,
are excellent examples of unscarred lavacicle
ceilings, dripstone walls (fig.
31), and frothy pahoehoe floor adorned
with rafted blocks.
Near the large pillar where the tube
bends east is a former skylight roofed
over with lava. A roof collapse has
shattered and blocked the upper end of
the tube that went around the east side of
this large pillar. Three additional pillars
are present downstream, including one
260 ft farther with a hanging tube that
widens into a balcony 7-10ft above the
floor of the main passage. Many alcoves
in this part of Hercules Leg show evidence
of slow withdrawal of lava as the
tube drained.
There are more rafted blocks in the
pahoehoe floor of Hercules Leg Cave
than in other tubes of the Cave Loop
Road area. This probably reflects its
closeness to the surface; many blocks fell
into the lava stream from its thin roof
during volcanism. Collapse to the surface
may also have occurred at this time
but was later roofed over, as at the
skylight. Remnants of collapse breccia
penetrated and smoothed over by lava are
not abundant, however. A large floor
jam of blocks occurs at the junction of
Hercules Leg tube with Upper Juniper
Tube.
Juniper Cave
Relation to Hercules Leg Cave
Juniper Cave is the downstream continuation
of Hercules Leg Cave. After it
picks up a short tributary from the south
(Upper Juniper Cave), the combined
system makes a sharp bend to the north,
continues for 200 ft , and then makes a
90° turn east past the Juniper entrance
collapse. The area where two right-angle
bends closely follow one another is a
jumble of large pillars between which the
main tube subdivides and reunites in a
network of separate strands. Large areas
of roof collapse (fig. 32), including an
extensive one at the Petroglyph entrance,
add to the confusion. In this maze of
tubes and pillars, lava cascades, pools,
floor jams, irregularly shaped alcoves,
and piles of collapse rubble are jumbled
together. Another tributary enters Juniper
Cave from the southwest, 200 ft
downstream from where the main tube
turns sharply east.
The tributary from the south (Upper
Juniper Cave) joins Hercules Leg tube at
its junction with Juniper Cave 300 ft
downstream from its source and was fed
from a surface lava flow. At the Swallet
entrance located at the head of this
tributary, a short lava gutter leads from
a wide basin on the surface directly into
the tube. This basin was once filled by a
lava flow. After the flow had drained
through this tributary tube for some time,
the pooled basin lava fill at the surface
broke through an obstruction on its
northeast rim and continued downslope
as a surface flow. This partial draining of
the lava basin fed the upper Juniper tube.
The final trickle oflava down the Juniper
tributary tube made a low central ridge of
frothy pahoehoe, which stands 1-2 ft
above the older lava floor; shallow gutter-like
areas formed on both sides of the
ridge against each wall (see cross section
on map 6, pl. 2 near upper end of tube).
The question of whether the lava
flow that fed Upper Juniper Cave penetrated
a breakdown in an older empty
tube or was simply part of the outpouring
of lava associated with the Hercules Leg
tube has not been resolved. An outpouring
seems more likely because the draining
of both tubes appears to have occurred
at the same time, as shown by
evidence at the floor jam at their junction.
Floor Jam at the Junction of
Hercules Leg Tube and
Upper Juniper Tube
The largest floor jam of lava blocks
in the Cave Loop Road area is in the Bat
Whistle Room at the junction of Hercules
Leg tube and Upper Juniper Tube. The
jam's top in each of these two tubes is
elevated a few feet above the confluence
of the tube floors , but the floor jam is
largest and best developed in the lower
part of Hercules Leg. All of the Hercules
Leg passage is not occupied, however. A
40-ft ledge of collapse breccia seals off
an alcove on the north side of the mouth
of Hercules Leg; this ledge formed a wall
that funneled the floor jam blocks into a
constriction between a large pillar and
38 Selected Caves and lava-Tube Systems, lava Beds National Monument, California