Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

water well, the cave changes markedly.
The ceiling height drops as the amount of
rubble on the cave floor increases. The
walls narrow until the passage is only
18-20 ft wide, but the west wall swings
out another 10 ft into a rounded alcove
near the top of the incline. This upwardfunneling
tube extends upstream for 170
ft almost to the end of the accessible part
of the cave. Nowhere on this moderately
steep slope is there an exposure of the
original floor of the cave; it appears to be
everywhere buried under collapse rubble.
The length and uniform gradient of
the slope, however, are a strong indication
that this part of the tube was a lava
cascade.
The southernmost end of the accessible
part of Bearpaw Cave is a small
passageway that opens amid the collapse
blocks beneath the northwest comer of a
big collapse trench to the south. This
passageway is only a random hole in the
collapse rubble; it does not indicate the
course of the former upstream continuation
of the original lava tube.
Collapse Trenches
To many monument visitors the deep
collapse trenches, the varied topography
to the south and west of the Merrill Ice
Cave parking lot, and the sharp changes
in biologic life zones may be more
interesting than the caves. This parking
lot is a junction between three zones of
vegetation. To the south the deep collapse
trenches are obscured upstream
beneath dense thickets of mountain mahogany,
which occupy the rough schollendomed
surface between Bearpaw
Butte and Hippo Butte (fig. 4). Immediately
to the west Bearpaw Butte rises
high enough to support an evergreen
forest. North and east of the parking lot
are rough plains cloaked with bitter
cherry, rabbit brush, sagebrush, and
small annuals. Additionally, various
plant communities thrive in the microclimates
of the caves and collapse trenches,
including lichens and mosses that
grow on the shady side of the verticalwalled
collapse trenches and in the
moist, cool air of the cave entrances.
A limited supply of water remains
throughout part of the summer in small
shaded rock niches - similar to the
"tanks" in the Southwest, as they are
called by naturalists. A poorly accessible
but permanent supply of water is found
in the abandoned well of Bearpaw Cave,
and water also comes from partial melting
of ice pools in Merrill Ice Cave.
Availability of water is a critical factor in
supporting a larger than average population
of animals, especially rodents.
Birds are more abundant here than in
most parts of the monument.
The two northern collapse trenches
(map 10, pl. 4) are geologically quite
ordinary. Note their vertical scarps,
which expose flow units of lava (for a
description of flow units and their relation
to lava tubes, see the "Post Office
Cave" section). Below these scarps the
floors are elongate saucers of collapse
rubble. A simple calculation of the volume
of the trenches below ground level
shows that roof collapse of both levels of
Merrill Ice Cave is inadequate to provide
the space occupied by the trenches.
Either or both of two explanations could
account for this discrepancy. ( 1) Beneath
the ice level of Merrill Ice Cave there is
one or more lava tubes whose collapse
provided the extra space to hold the
rubble. (2) Molten lava flowing through
the collapsing tube or tubes was of
sufficient volume to remove most of the
blocks that fell into it. Very likely, both
processes contributed. Good examples
of collapse blocks smoothed over or
rafted away by lava are present in Valentine
(map 8, pl. 3), Catacombs (map 3,
pl. 1), and Tickner and Berthas Cupboard
Caves (both on map 9, pl. 3).
Examples of collapse trenches whose
volume conforms with the size of the
tube, and some that do not, are found
along the course of the same lava-tube
system downstream from Merrill Ice
Cave-especially in the 5-mi stretch
downstream from Skull Cave (see map
20, pl. 6, and the "Skull Cave" section).
Hydraul ic Rampart
At the northeast end of the collapse
trench at the head of Bearpaw Cave is
evidence of uplift and subsidence of the
roof of Bearpaw Cave at a time when it
was filled with molten lava. The evi-
dence here also sheds some light on the
probable origin of the inferred lava cascade
in the upper part of Bearpaw Cave.
The downstream end of this southern
collapse trench is bordered by two unusual
deposits not found around the
edges of the other two downstream collapses.
One is a partial girdle of loose
lava blocks derived from the flow that
formed the vertical cliffs along the edge
of the collapse. These blocks, however,
are piled up in a ridge on the top of the
flow, like a ring of dirt thrown out from
an excavation. The other feature, at the
northwest comer of the trench, is a short
lobe of pahoehoe lava that spilled over
the wall of the trench and extended 50 ft
on the surface before its source of lava
subsided into the trench (see map 10, pl.
4).
Ridges of loose blocks that rise above
the rim of certain breakdowns are interesting
structures, which we have named
"hydraulic ramparts" (fig. 46). Hydraulic
ramparts may form when an obstruction
backs up lava within a tube. The
ponded lava rises upstream inside the
tube until its hydraulic pressure becomes
great enough to either break through the
obstruction or else force a hole through
a weak spot in the roof of the tube,
allowing the lava to spill out onto the
surface as a lava flow.
We infer that hydraulic pressure
above a blockage in Bearpaw tube lifted
the roof of the tube over the downstream
part of the present collapse trench by
breaking the roof along two lines, one
across the course of the tube and the other
parallel to its west wall. This pie-shaped
slab was tilted up, similar to lifting a trap
door, as molten lava rose beneath it. As
this occurred, its north edge collapsed
and formed the hydraulic rampart shown
on the map. Simultaneously, molten lava
squeezed up the break along the west side
and spilled onto the surface as a pahoehoe
lobe (see map 10, pl. 4). Prior to this,
however, the impeded lava enlarged the
outlet underground by prying up the roof
north of the intersection of the two
breakage lines and created a new passage
along the route of the inferred lava
cascade in the upper part of Bearpaw
Cave; this process caused the lava level
to fall in the upstream part of the tube.
56 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California