Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

part talus was reversed. Moreover, after
volcanism had ended, further collapse
toppled the roofs of both the original tube
and its bypass. This produced the odd
shape of the breakdown and also closed
entry to this part of the lava tube. A
downstream section of the tube, however,
can be visited beneath Peninsula
Bridge, which lies at the northeast end of
this oddly shaped breakdown.
Northeast from Peninsula Bridge are
three short collapse trenches separated
by two natural bridges. At the north edge
of the third trench a filled skylight is
present on the surface. Out of this skylight
narrow spillover lobes of lava
emerged and built miniature lava tubes
above ground. These surface tubes
spread to the north and east, and they
cross over an uncollapsed part of the
main lava tube. One of these surface
tubes then splits into four smaller distributary
tubes (map 20, pl. 6).
The next feature downstream is a
collapse trench 400 ft long. The ground
adjacent to this trench is riven by a few
curved fissures labeled "cracks" on map
20, plate 6. Access to a section of the
lava tube can be made from the northwest
edge of the breakdown. The tube continues
beneath a natural bridge for 300 ft
to a point where it is demolished within
a collapse trench that trends northwestalmost
at a right angle to the upstream
trend. At its downstream end, however,
this collapse trench curves right (north)
and this curved trench gradually resumes
the former northeast trend of the lavatube
system. Along the northeast side of
this curving collapse trench, many short
spillover lobes of molten lava emerged,
solidified, and then were decapitated by
collapse into the trench. On the opposite
(southwest) side of this curving collapse
trench, a group of shallow sag basins
lowered the ground surface irregularly
and a short lava spillover emerged from
one of the sag basins. The lava tube
continues northeastward beneath another
lava bridge at the northeast end of this
curving breakdown and is closed by a
small collapse pit 350 ft farther downstream.
The next breakdown to the north is
one of the most interesting collapse
features along this part of the lava-tube
system. It is shaped like half a coke bottle
lengthwise and laid on its side with the
sawed surface upward. It is located just
southwest of Frozen River Cave and of
the point where the tube system passes
beneath Lyons Road (map 20, pl. 6). The
neck of the bottle is a normal-width small
collapse trench. The body of the bottle,
however, is a large, shallow collapse
area more than twice as wide as nearby
accessible parts of the lava tube. The
collapse's central part is strewn with
blocks from the former tube's roof, some
of which are as much as 20ft in length.
Similar large roof blocks, partly included
within a talus pile of smaller roof blocks,
are tilted outward away from the breakdown
and form a continuous hydraulic
rampart perched on the rim of the breakdown.
This encircling rampart rises 3-35
ft above the surface of the surrounding
plain. The tilted blocks and accompanying
talus forming the rampart must have
been shed outward upon the rim when
molten lava, under hydraulic pressure
within the underlying partially blocked
lava tube, pried loose and heaved upward
a large area of the tube's roof, a process
which caused it to shed collapse material
on all sides. Evidently the pressure was
relieved downstream before molten lava
could surface through this heaved-up
roof. As the lava drained from the tube,
the shattered roof blocks were lowered
and jostled together to form the floor of
the present depression whose surface
now lies well below the rampart on its
rims. Originally the rampart was continuous
across the neck of the bottle, but
this part of the rampart has tumbled into
the short collapse trench forming the
neck of the bottle.
The entrance into Frozen River Cave
is at the mouth of the bottle. One hundred
feet inside the cave a steep loose connector
can be negotiated to a lower level
that contains ice during most of the year.
J.D. Howard named Frozen River Cave
from the ice floor in this lower tube and
in an alcove on its east side.
From Frozen River Cave the lavatube
system can be traced downstream on
the east side of Lyons Road on a northeasterly
course for 1,300 ft before it turns
north. Two small collapse trenches reveal
the position of the lava-tube system
along this northeast trend, but entry into
the tube is impossible from these breakdowns.
The northerly trend continues without
entrances to caves for 1, 700 ft to
Captain Jacks Bridge. The approximate
position of the lava-tube system, however,
is apparent from two large breakdown
complexes, which resemble-but
are more complicated than-the coke
bottle-shaped breakdown upstream.
Both lie a short distance east of Lyons
Road. The southernmost is a teardropshaped
area 760ft long and 240ft wide
across its blunt northern part, but it tapers
to only 40 ft wide at its south end. On its
east side this area is bordered by a 5- to
25-ft-high block rampart containing
some short tilted-block sections. This
rampart curls around the north end for a
short distance, but the west margin of the
teardrop is a jagged low cliff with little
or no rampart material on its top.
The interior of this area is a maze of
broken roof blocks. Five large pieces can
be recognized (shown on map 20, pl. 6).
Each consists of a fairly intact, although
somewhat broken, part of the roof. Each
piece appears to have been rafted toward
and slightly raised against its neighbor to
the north. Minor block ramparts tend to
form on the north and northeast edges of
the interior blocks, but in other places a
carpet of loose blocks separates the
different pieces. One large fissure, with
blocks piled above its east edge, mirrors
parts of the eastern wall of the breakdown.
Out of this fissure, and much
more copiously out of the break that
forms the east edge of this complicated
area, small spillovers of molten lava
emerged and flowed to the north and east
beneath or through gaps in the rampart.
Three of these spillovers are large
enough to be shown on the map, whereas
other miniature spillovers barely reached
the surface between the interior rafted
blocks. That two tiny holes on the floor
of the western part of the broad area of
the teardrop were probed with a 13-
ft-long stadia rod without touching bottom
indicates the presence of a drained
lava tube beneath.
98 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California