Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

lava tubes, and the upper parts of these
tubes collapsed to form the two small
basins. The Wall itself developed from
inch-thick sheets of lava continually deposited
and cooled from slight spillovers
of the oscillating lava lake level. This
thick and sturdy rock wall, 4-15ft thick
and 5-8 ft high along its south side,
consists of dozens of thin lava sheets
stacked on top of one another. Some
layers are only small rod-like trickles
instead of sheets, as seen on the outside
of The Wall. The interior side of The
Wall is a caved surface formed when the
lava lake dropped to a lower level and
drained out. It shows broken edges of
sheets, smoothed over in places by a
coating of lava plaster stuck against them
as the molten lava lowered.
What caused the lava lake in the first
place? And how did it disappear and its
site become covered with talus? Judging
by what we can see of the continuation of
the major lava tubes downstream in such
places as Ovis, Crystal, Sentinel, and
Post Office Caves, it seems many fillings
and drainings of the lava lake occurred as
different eruptions rose to a climax and
then waned. Lava also backed up into the
tubes when roof collapse partly or completely
blocked its flow (see the maps of
Crystal (map 18, pl. 6), Skull (map 12,
pl. 4), and Post Office (map 15 , pl. 5)
Caves). The lava tubes that formed within
the site of this particular stretch of
trench undoubtedly have repeatedly lost
parts of their walls and roofs as molten
lava coursed through them, for in no
other way can we account for the size of
the present trench except by the rafting of
both old tumbled and newly congealed
thin roof blocks and wall linings down
the tube. After collapse and rafting of
this rock, intermittent blockage of the
tubes downstream created the rise of a
lava lake until it overflowed. Such a
sequence of events was probably repeated
many times during the buildup of the
lava plain that slopes away on both sides
of this major trench. This plain also
includes the northeast slope honeycombed
with the near-surface lava tubes
upon which Cave Loop Road is built.
Within the area shown on the Natural
Bridge map (map 5, pl. 2), two more
breakdowns upstream from the deep
collapse trench give additional clues to
the position of the line of major lava
tubes underground. One is a deep trench,
80 ft long and about 50 ft wide. The
upstream side is breached by a shallow
draw 50ft long. From the south edge of
this draw a broad spillover lobe of lava
escaped to the south but narrowed to a
point and congealed within 100 ft.
Another 125ft upstream is a deep pit
called Duffys Old Still Well that is 85 ft
long and 25 ft wide at the surface but with
vertical to overhanging walls in its deeper
parts. The collapse debris on its floor
is only a few feet thick, and an open lava
tube extends both upstream and down-
Figure 30. Arch-shaped natural bridge of lava is similar to that at Sunshine Arch. This arch was formed by collapse of lava
tube carrying basalt of Giant Crater on south flank of Medicine Lake volcano (see fig. 1).
36 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California