Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

heights are between 12 and 20 ft in the
upstream part of the cave. Downstream
from the entrance, the distance between
floor and roof gradually decreases, but in
most places one can stand upright until
about 150 ft from the downstream end.
Fern Cave is large enough to be a
downstream continuation of the Bearpaw-Skull
lava tube system (fig. 4). It is
likely that this major artery for the
dispersal of molten lava turned east near
the north end of the Schonchin Butte
flow, then north near Juniper Butte, and
connected with Fern Cave. However, we
were unable to trace such a direct connection
through the field of lava that lies
south of Fern Cave.
The events of the last volcanism are
recorded on the floor of Fern Cave as two
recent lava flows; each can be traced the
full length of the cave. The older flow is
of smooth pahoehoe, which originally
stood at a higher level long enough to
start solidifying along its walls, but then
drained down and pooled at a level 3- 5
ft lower than its former stand. This
process left benches 2-4 ft high with
sagging edges in the downstream part of
the tube where a firm crust had formed.
The benches narrow and grade into a
sloping apron, which connects a highlava
mark on the wall with the pahoehoe
floor in the upstream third of the cave. A
final flow of spiny pahoehoe occupies
the central part of the cave's floor
throughout its length, but it failed to
overrun the aprons and downstream
benches of the older flow along the
walls. Thus, almost continuous gutters
border the edges of this lobe in the
upstream two-thirds of the cave. In most
places the gutter is 2- 3 ft deep; its inner
wall is formed by the steep edge of the
spiny pahoehoe lobe (fig. 59), and its
outer wall is formed by the sloping apron
or bench of the smoother older flow. The
gutter is narrow in most of the upper tube
but widens, and so large patches of the
surface of the older flow can be seen in
the downstream part of the tube. The
spiny last lobe had lost most of its energy
by the time it reached this downstream
area; it was too viscous to spread clear to
the benches, let alone cover and overwhelm
them, except at the downstream
end where it blocks the tube.
Downstream Through Fern Cave
In the amphitheater at the upstream
end of Fern Cave is a low mound of lava
that rose into the cave from an unknown
source below (map 17, pl. 5). During the
final stages of this upwelling, a few
blocks fell from the roof and stuck in the
pasty half-molten lava on the surface of
this mound. Other, mostly larger collapse
blocks within the same area tumbled
onto the surface after the lava had
congealed.
The relatively smooth mound of upwelled
lava becomes spiny pahoehoe
downstream. As previously mentioned,
the final lava lobe does not cover the
entire floor of the cave: its edges form the
inside walls of the lava gutters that
developed along each wall of the cave.
Figure 59. Lava gutter formed at edge of slow-moving lava flow that last occupied
Fern Cave (see fig. 4 and map 17, pl. 5).
84 Selected Caves and lava-Tube Systems, lava Beds National Monument, California