Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

dripstone walls, and lavacicle roof. Only
50 ft downstream from the base of the
slide, this tube is demolished by a slide
of massive amounts of red breccia
through its roof. At its northeast end the
Silver Clouds Cavern terminates downstream
where it plunges east into a lower
level, but this lower tube is inaccessible
because of lava accreted to its walls.
The name "Beaconlight" was given
to this cave by J.D. Howard, after the
Modoc Indian practice of illuminating
the interior of caves in the search for
water by building a row of bonfires of
sagebrush inward from the entrance.
Water does stand for a part of the year in
the deepest part of the Silver Clouds
Cavern, and also in a low spot in the
lower tube. The name "Silver Cloud"
derives from irregular patches of offwhite
caliche covering part of the roof of
this room- when wet after seepage from
rain, the caliche reflects light with a
silvery glow.
Beaconlight Cave is the partly collapsed
remnant of a large lava tube. Little
of the original walls and virtually none of
its ceiling have survived the slow unraveling
of blocks loosened by frost action.
Thick collapse rubble covers almost the
entire floor of the cave except for a short
stretch in the lower and smaller tube.
Retreat of the walls by collapse provides
good exposures of thin flow units in the
thick lava flow in which the upper large
tube developed. In the lowest parts of the
two upper rooms this thick flow of lava
rests in a few places upon a red breccia,
which appears to completely encompass
the lower small tube. The red breccia
may be the underground extension of the
tuffs and breccias, which make up the
Schonchin Butte cinder and agglutinate
cone (see map 12, pl. 4 and the "Skull
Cave" section).
The most interesting geological area
in Beaconlight Cave is the northeastern
end of the Silver Clouds Cavern. The
chamber narrows, turns abruptly almost
due east, and plunges steeply into a lower
level through an inclined tube 20 ft in
diameter. This tube cannot be entered
because it is blocked by many accretionary
linings (each the remnant of a tubein-tube)
along the east wall, and because
it is filled with collapse rubble on the
west side. This downslope termination of
a lava tube by accretionary linings is very
similar to that of Arch Cave, Silver
Cave, and to parts of Tickner Cave,
except that it is larger and not as well
exposed. Not all of the collapse rubble in
Beaconlight Cave is of post-lava origin.
At two places along the northwest and
northeast walls of Silver Clouds Cavern,
older collapse breccia was partly covered
with dripstone of congealed lava.
Skull Cave
Skull Cave (map 12, pl. 4) was first
visited by modem explorers in 1898
following E.L. Hopkins' sighting of the
long collapse trench at the head of the
cave from the top of Crescent Butte. On
investigation he found on the lower
level, partly embedded in icc, two human
skeletons and abundant bones of
antelope, bighorn sheep, and mountain
goats. A broken log also lay on the ice.
Apparently two Indians had attempted to
lower the log to serve as a ladder but let
it slip, and they were swept off the
balcony to their deaths.
Skull Cave is located at the end of a
well-marked paved road. It is a remnant
of two large lava tubes superposed upon
one another. Like Kirk Whites and Beaconlight
it is on the line of collapse
trenches and caves, which skirt the south
side of Schonchin Butte. The upper tube
has suffered so much collapse of its roof,
floor, and walls that most details of its
original form are lost. The lower tube,
however, retains many of its primary
features despite large collapses through
its roof and extensive slides through its
walls.
Two balconies can be seen from the
stairs built to access the downstream part
of the lower tube. These might be considered
two additional separate levels;
however, because the upper balcony
ends downstream in a blunt semicircular
wall, and the lower balcony ends 60 ft
upstream by filling to its roof, they are
more appropriately considered local balconies
marking periods during which
draining of the eastern part of the lower
tube halted. This downstream part of the
lower level is nearly cut off from a large
upstream chamber by a huge pile of
collapse blocks (see longitudinal section
on map 12, pl. 4). Passage between the
two chambers of the lower level is
through a narrow crawlspace up and
around the northeast side of this roofcollapse
pile. The upstream chamber has
been nicknamed "Boneless Cavern" by
disappointed spelunkers who had hoped
to find another rich store of animal bones
after they negotiated the narrow crawl.
Neither of the two balconies that are so
conspicuous in the lower level are
present in the Boneless Cavern.
The complexities of Skull Cave make
it a prime example for studying a large
feeder-tube system. If we consider the
two levels and balconies and count the
overlaps where they are superposed,
over 1,000 ft of linear passage is available
for study and exploration. These
passages record some information, but
they also provide some unsolved puzzles
concerning the origin and hydraulics of
the large feeder tubes that transmitted
vast quantities of lava from vents for the
basalt of Mammoth Crater to the lowlands
bordering Tule Lake (a distance of
approximately 15 mi).
Upper Level
The entrance into Skull Cave (fig.
47) is at the east end of an impressive
collapse trench, 460 ft long and 100 ft
wide. The upper level resembles a huge
tunnel, 60-80 ft wide and 30-65 ft high.
At a point 480 ft east of the entrance the
upper tube ends abruptly at a semicircular
wall. Here, lava in the upper tube
found an opening into a lower level. A
small remnant of the lava that poured into
this connector survives as a frozen lava
fall. The lava cascade marking the top of
this fall can be seen from the base of the
first (upper) stairs on the trail to the south
wall of the tunnel, and then it curves east
against the south wall of the tube for 25
ft. The north half of the lava fall remnant
is completely buried beneath the debris
of a later roof collapse.
The only locality where the original
floor of the upper level can be seen is at
the top of the lava fall , in the southwest
corner of the upper level. Upstream from
Skull Cave 59