Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

Room-is actually a natural bridge,
which has been left as a remnant between
two surface collapses along a 20- to
30-ft-wide underground tube. Only 30ft
after going underground the trail turns a
half circle to the right and climbs a short
incline into a distributary tube that
branches from beneath the natural
bridge. At the top of this incline, 7 ft
higher than the tube junction and 20 ft to
the north, the trail emerges onto the
smooth-surfaced lava pool that gave the
cave its name - Boulevard (fig. 48).
Farther northeast within the cave, the
roof lowers until it is only a 3-ft-high
crawl way. Here the floor of the Boulevard
drops off in a 2-ft-high lava cascade.
The tube continues with a normal
downstream gradient for another 250 ft
with two tight crawls. This part of the
tube was partially drained soon after
solidification of the top of the Boulevard
pool, but the lava level did not drop
enough to reveal the former floor of the
tube. Nearly 150 ft beyond the cascade
marking the north edge ofthe Boulevard,
the tube divides into two distributaries.
The one on the left (west) is filled nearly
to the roof, but the distributary on the
right makes a shallow S-curve and continues
northeast for another 75 ft. Near
its end the tube enters a cupola-like dome
with a ceiling 9 ft above the floor.
However, the tiny outlet of this room is
impossible to traverse because there is
less than a foot of clearance between the
ceiling and floor.
Throughout its length Boulevard
Cave displays well-formed lavacicles
and dripstone walls, but its main attraction
is its short stretch of smooth floor
(the Boulevard). In the cave's entrance
chamber is evidence of how the lava
pond that produced the Boulevard was
partly drained after solidification of its
upper surface, and of the relation of
Boulevard Cave to the other caves of this
group.
Boulevard Entrance Chamber and
East Branch of Balcony Cave
As previously noted, the upstream
end of Boulevard Cave is in a broad
chamber-The Flushing Bird Roomformed
at the junction of two distributary
tubes (map 13, pl. 4). The floors of the
two tubes are at different levels, and the
smooth floor of the Boulevard is 7 ft
higher than that of the eastern tube. Yet,
at one time molten lava must have been
at the same level in both tubes. A
Figure 48. The smooth floor of this tube provided inspiration for J.D. Howard to
name it Boulevard Cave (see fig. 4 and map 13, pl. 4).
high-lava mark present in both tubes
shows that the same level as the Boulevard
is preserved in several places within
the eastern tube's entrance chamber.
Some obstruction downstream that had
ponded the lava evidently gave way and
the lava surface within the entrance
chamber was quickly lowered. The Boulevard
floor had already congealed, but
material just beneath this floor must have
been so hot and plastic that it oozed
upstream as a backflow that formed a
sloping apron 7 ft high and 20 ft long,
which now connects the Boulevard with
the floor of the East Branch. Stretched
bubbles and open tensional cracks remain
frozen into this apron surface. Still
later, a final surge of hot molten lava
entered the entrance chamber from upstream
and left a second high-lava mark
in the form of a thin yellow lava scum
plastered across this sloping apron 2 ft
above the present floor. A curious feature
of the entrance chamber, rarely seen
in lava tubes of this area, is a small patch
of waterworn pebbles and sand deposited
in the lowest part of the chamber. Evidently,
flash floods or rapid snowmelt
cascaded into the entrance chamber from
the shallow drainage just to the west and
deposited loads of sandy gravel in this
tiny basin beneath the natural bridge,
then filtered out through a maze of coarse
collapse blocks. The loose blocks that
provided this filter mark the upper end of
a collapse trench 80ft long, which lies on
the site of the downstream continuation
of the East Branch.
The East Branch tube can be entered
through a stoopway at the north end of
this collapse trench. Despite the difficulty
oflow ceilings and a remarkably rough
and wavy floor, one can traverse the tube
for another 140 ft underground. Evidence
indicates that this part of the East
Branch tube was completely filled with
molten lava at the time of the Boulevard
ponding except for one high-ceilinged
area 30 ft downstream from the low
entrance. Here, a fallen block left a hole
in the roof large enough to stand in and
peer around a flat-bottomed chamber
above the roof of the present cave.
Although this chamber is only 1-3 ft
high, its dimensions and roof features
clearly indicate that it was a high place in
64 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California