Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

Merrill Ice Cave, Ice level
The ice level of Merrill Ice Cave is a
360-ft-long remnant of a medium-sized
lava tube that is 15-20 ft wide and
originally was 12 ft high. It is closed by
roof collapse at both ends and its floor is
completely obscured by collapse rubble
and ice ponds. The roof and walls have
been so greatly enlarged by slow unraveling
of collapse blocks that few original
features remain. Low benches are partially
visible from beneath the collapse
debris in the downstream part.
The most striking geologic feature in
this level is a steep-walled depression in
the floor 40 ft upstream from the foot of
the ladder. An ice pond 25 ft in diameter
occupies the central part of this depression.
Steep slopes of ice-encased collapse
rubble rise on both ends of the ice
pond, and the upstream slope actually
forms a frozen cascade. It seems that the
pond marks the site where the floor of the
ice level collapsed into a third, and
probably larger, lava tube below. Moreover,
it seems probable that this lower
tube is currently filled with ice.
An account of how ice develops
within caves and descriptions of more
impressive ice deposits are given in the
"Crystal Cave" and "Ice Deposits" sections.
Merrill Ice Cave, however, is a
much safer cave than either Crystal Cave
or Skull Cave's ice level. Moreover, the
well-planned trail makes the extensive
ice accumulations in Merrill Ice Cave
easy to visit.
Three ice pools (map 10, pl. 4)
occupy one-fourth of the floor space.
One at the foot of the ladder and another
near the upstream end of the ice level
tube that extends 70 ft and includes many
"islands" are situated in low spots on the
floor of the lava tube. The third pool is
at a lower elevation and, as mentioned,
fills a round hole that apparently connects
the ice level with a lower ice-filled
tube.
Most ice in the shallow upstream
pool is fairly transparent, and shadowy
outlines of the blocks that project above
the pool's surface can be traced to depths
of a few feet below the surface. Much of
the cave's ice, however, is murky with
dust, small bubbles, cracks, and organic
growth.
The beautiful draperies of transparent
icicles and expanses of permanent
hoarfrost in Crystal Cave are totally
absent in Merrill Ice Cave. Thin rimes of
hoarfrost form in parts of Merrill Ice
Cave during the winter months, and
short, thin icicles develop for a brief
period during the spring thaw; but these
features come and go with the changing
seasons. The formation of a water puddle
in the late summer and autumn over the
top of all three ice ponds is evidence of
a greater seasonal exchange of warm air
for cold air in the two levels of the cave.
Why the difference between the two
caves? Two explanations seem probable.
(1) Crystal Cave is much larger and
deeper than the ice level in Merrill Ice
Cave. A far greater volume of air must be
exchanged before there is a marked
temperature change in Crystal Cave. (2)
Although both caves have only one
obvious entrance, one must consider air
flow from a second area into Merrill Ice
Cave through the ice level. Note on the
map that the south end of the ice level is
closed by the steep slope of collapse
rubble from the northernmost collapse
trench. Openings between these large
blocks are surely big enough to allow
considerable interchange of the cave air
with the outside air. The sun shines
strongly upon this part of the collapse
trench at midday, affording an ideal case
where cold air of the cave flows out into
the collapse depression, is warmed, and
rises-a continual mechanism pumping
cold air from the cave between day and
night during the summer months.
Bearpaw Cave
Figure 45. Stairway leading down to entrance of Merrill Ice Cave (see fig. 4 and map
10, pl. 4).
Bearpaw Cave is a large lava tube
that has lost nearly all of its primary
features by slow unraveling of its roof
and walls (map 10, pl. 4). The entire
cave floor is mantled with collapse rubble.
A long-abandoned well, dug into
this rubble at a point 55 ft upstream from
the entrance contains water; thus the
pahoehoe floor of the cave was probably
reached by the well. In the same area,
remnants of a lava bench, almost obliterated
by the cover of rubble, are present
in places along each wall. On the west
wall, above the bench, is a large curving
alcove on which drips tone still remains.
The alcove slopes out at its base and
merges with the bench below.
Upstream beyond the large cavern
that contains the alcove, bench, and
Merrill Ice Cave, Bearpaw Cave, and Nearby Collapse Trenches 55