Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

that passed through it. The reason becomes
clear only after we examine that
part of the tube 220 ft downstream from
the entrance, where it makes an abrupt
turn to the northeast. At this locality the
weight of the lava in the Sunshine lava
lobe forced a breakdown (or else the lava
discovered a roof collapse) into an older
open tube, formed in a deeper flow. A
large but unknown volume of lava tumbled
into this lower distributary and
continued to flow until ultimately all but
200 ft of the upstream part of this lower
tube was filled to its roof, and thus access
was terminated to the northeast.
Collapse blocks from the surface
clutter the Sunshine entrance, but otherwise
the first 200 ft of the cave shows
typical lava-tube features. The roof is
decorated with lavacicles that bend and
flow into dripstone as the roof rounds
onto the walls. Near the entrance a
prominent narrow bench is present on
both walls 12 ft above the floor. A less
conspicuous high-lava mark is 7ft above
the floor. A 5-ft cascade is located 50 ft
from the entrance, and a second 3-ft
cascade is another 120 ft farther downstream.
Between these two cascades is a
55-ft stretch of floor where the partly
solidified pahoehoe broke up into a block
jam. Final draining below the cascades
left a cave floor of frothy pahoehoe.
At 210 ft downstream from the entrance,
the Sunshine tube turns abruptly
to the right (northeast) and just beyond
the turn many of its physical features
change in appearance (map 5, pl. 2). In
this area is where flowing lava discovered
or else forced a breakthrough into an
underlying open lava tube. The exact
place where the breakthrough occurred is
difficult to pinpoint, because once a spill
into a lower tube is effected, further
collapse enlarges and spreads the area of
breakdown. Such spreading of the collapse
clearly did occur in the Sunshine
Cave, but it is likely that the first collapse
may still be preserved in the round pit
crossed by the catwalk 20 ft beyond the
abrupt bend to the right, 230 ft from the
entrance. Inspect the edge of this pit and
note that lava poured into it from all
sides. Unfortunately, collapse filling obscures
the junction of the lower part of
the pit with the underlying tube. Clearly
this hole served to transmit lava to the
lower level, but it may not have been the
site of the first collapse.
Additional collapses in the Sunshine
tube occurred where the floor of the
upper tube disappears piecemeal into the
lower tube 90 ft farther downstream, at
the point where the trail descends into the
lower level via a short stairway. Most of
the collapse debris was carried away by
the flowing lava, but some may remain
buried beneath the lava on the floor.
Indeed, the stairway does not reach the
original floor of the lower level, which
lies beneath ponded lava. Such ponding
may have been caused by a small dam of
collapse debris.
Another 15 ft downstream the congealed
Sunshine lava flood subdivided
around a pillar and cascaded for a vertical
distance of nearly 8 ft before reuniting on
the downstream side of the pillar. Was
there a jam of collapsed blocks against
the upstream end of this pillar that
created the 15-ft-long pond? It seems
likely, for several rafted blocks are frozen
into the lava surface just upstream
from the pillar, and downstream from the
cascades many rafted blocks were being
carried away when the lava solidified.
One large block is shown on the map and
is found on the eastern margin of the lava
stream where it rounds the next bend. A
post-lava collapse that broke through
above ground nearly closes the tube in an
area 50 ft downstream from the catwalk.
At the Mouse Hole, 50 ft farther and a
total of 465 ft downstream from the
entrance, the lava pooled until its floor
was only 6 in. from the roof of the tube.
Battered Sherman Cave and Sunshine Arch
As lava drained out of the channel,
the roof of the short upstream extension
of Sunshine Cave partially collapsed, a
process that formed Battered Sherman
Cave and Sunshine Arch. The entrance
to Battered Sherman Cave is between
collapsed blocks at the downstream end
of the cave. Post-lava-flow breakdown
has obliterated virtually all ofthe original
linings of the cave; indeed nowhere does
the ceiling height exceed 4ft. A chaos of
blocks has to be traversed along its 38-ft
length to reach the collapse area at its
upstream end. A battered Sherman live
animal trap was found in the cave when
it was explored in 1988, hence its name.
Sunshine Arch is located between
Battered Sherman Cave and the entrance
collapse of Sunshine Cave proper. It is a
very small remnant of the roofed tube
upstream from Sunshine Cave. The 7-ft
diameter and 5-ft long arch has undergone
extensive unraveling and nothing
remains of its original interior surfaces.
A similar arch is shown in figure 30.
Another spillover lobe immediately
east of the Sunshine Channel also formed
a lava gutter, became roofed over, and,
after a few right-angle turns, tumbled
into the upper tube beneath Natural
Bridge in an 18-ft cascade. One cannot
walk through this connection today because
access is denied by a roof collapse
at the point where the gutter went underground.
Evidently at the time when lava was
feeding into these overspills and small
distributary tubes the site of most of the
present deep collapse trench must have
been occupied by a large lava lake,
perhaps mostly crusted over but including
parts where molten lava was open to
the sky. Evidence of the lava lake can be
seen near the head of the trench. Two
spillover lobes are shown on the map just
to the east of the shallow basin that forms
a "bay" at the southern end of the
collapse trench. Before the formation of
these small lobes, a much larger flow of
lava evidently escaped from this bay in
the side of the trench. This flow contained
lava tubes within its core. Most of
these tubes filled with lava, but the upper
parts of some still remain open as in the
case of Juniper Pole Cave, a network of
subdividing near-surface distributary
tubes. Downstream, all tubes are filled
with congealed lava.
One of the more interesting features
formed by overspills from the large lava
lake is a small flat-topped ridge known as
The Wall. It borders the south side of the
wide bulge in the middle of the trench.
Two small collapse basins on the south
side of The Wall indicate that another
flow of lava, comparable to the one that
contained the Juniper Pole Cave, spilled
over the rim of the trench at this point.
This lava flow must have also developed
Natural Bridge Area 35