Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

the floor of the trench, where it spread in
a tiny delta over nearby collapse blocks.
The second lobe, which rounded the west
side of the obstruction on the rim, was
even more sluggish. It congealed in place
as it began the drop over the rim.
A third lobe of molten lava spilled
into the northwest end of the entrance
trench. This lobe had more energy and
supplied a larger volume of lava. The
lava eventually congealed after half filling
the large Craig tube 80 ft upstream.
The lava stream forms half of the floor of
the large room called Craig Temple. If
the eruption feeding the basalt of Valentine
Cave had continued slightly longer,
both Craig Cave and Craig Temple
would have been filled and completely
buried.
Craig Temple
The part of the Craig tube that lies
upstream from the entrance collapse is
240 ft long, but only 125 ft of its
downstream segment forms the highceilinged
room that Howard called
"Craig Temple" (map 16, pl. 5). The
remaining 115 ft is a broad low-ceilinged
area, parts of which can be reached only
by crawling.
Craig Temple was formed by the
underground merger of two flows of
lava. The younger one, the basalt of
Valentine Cave, advanced upstream in
the Craig tube over the apron of collapse
rubble. Its surface is very rough and
uneven and reveals how the flow subdivided
into trickles and tongues among
the collapsed blocks as it cascaded down
the steep apron of rubble onto the floor
of the Craig tube. The other flow was a
late tongue of basalt of Mammoth Crater,
the same basalt that built the Craig tube.
It spread downstream from within upper
Craig tube. This compound flow is the
lava fill that blocks the tube 240 ft
upstream from the entrance and ends
downstream with a sharp junction
against the younger upstream-flowing
lobe of basalt of Valentine Cave. This
junction is 160 ft downstream from
where the tube is blocked by lava rising
to its roof and is 80ft upstream from the
west edge of the entrance. This junction
is the lowest area in Craig Temple; the
arched roof above is more than 50 ft wide
and 10 to 18 ft high.
The late Mammoth Crater flow within
Craig Temple is not a single flow that
filled the 25-ft-high Craig tube at one
time. Instead it is composed of dozens of
small flow units, most only 1-5ft thick.
They moved slowly and piled on top of
one another as the flow advanced. The
congealed flow fronts of some of the
thicker flow units form a series of lobelike
steps, 3-5 ft high, that form the
dais-like structure of the temple. Each
successive bench near the upstream end
of the cave becomes thinner and more
discontinuous, and the stack of benches
finally reaches the roof, blocking further
upstream access. The lava seems to have
been fed very slowly into the tube and to
have lost its energy by the time it arrived
at this section.
Fern Cave
The entrance to Fern Cave (map 17,
pl. 5) is 1.5 mi southeast of Hospital
Rock near the northeast comer of the
monument. Ferns flourish within the
circle of light from a small (8 by 10 ft)
entrance hole in the cave's roof. J.D.
Howard named the cave and mentioned
the abundance of toads as well as ferns
beneath the entrance in his notes. The
hole in the roof provides the only access
for humans; however, a locked grate has
been installed across the entrance hole to
protect the ferns and some well-preserved
Indian pictographs from vandalism.
Visits to the cave can be arranged at
the Visitor Center. Below the grate, a
steel staircase in the entrance hole leads
to the top of a fern-covered mound of
loose blocks and soil (fig. 58). The top
of this mound is flat and only about 9-10
ft in diameter, but it spreads outward to
both walls of the cave. Other than this
mound of rubble, and another large pile
from a roof collapse upstream, most of
Fern Cave is relatively free of collapse
blocks or other debris.
The cave can be traversed for 1 ,300
ft. At each end, further access is completely
blocked by lava. At the downstream
end the roof lowers until along the
last 30-50 ft there is only a crawlspace
between ceiling and floor. The upstream
end, by contrast, is a near vertical semicircular
wall about 12 ft high. Into this
underground amphitheater, lava that
now forms the cave floor boiled upward
from some deeper source. This molten
lava almost certainly rose through a
connector from an overfilled lava tube
below.
Fern lava tube is large; in places the
passage is more than 60ft wide, and most
of the tube is 30-40 ft wide. Ceiling
Figure 58. Fern-covered mound at entrance gives Fern Cave (see fig. 4 and map 17,
pl. 5) its name. Entry is through 12-ft-diameter roof collapse.
Fern Cave 83