Geological Survey Bulletin 1673 Selected Caves and Lava Tube Systems in and near Lava Beds National Monument, California

Sentinel Cave

Sentinel Cave consists of the ruins of four large lava tubes superposed, with the fourth and lowest level slightly offset from the upper three. They have tumbled into a chaos of large angular blocks by partial collapse of their roof and floors after volcanism ceased. In places segments of a tube's floor and walls survive as benches or balconies perched above piles of rubble. In other places tube floors have survived the general collapse, but are seldom seen because they are deeply covered by rubble from above. Many floor segments gradually gave way under this load and broke through into the underlying tube. Roof areas covered by lavacicles or other primary features are uncommon; indeed, a light played over the ceiling in many places reveals only spaces where roof blocks have tumbled out. Many loosened blocks remain precariously perched, indicating that collapse of the roof is still underway. Collapse to the surface has occurred at all four entrances. In fact, the near-total collapse of the uppermost tube to the surface, a process which left a train of breakdown depressions, reveals the course of this main lava feeder channel connecting Natural Bridge to Ovis, Indian Well, and Sentinel Caves. This is the tube system that also spawned the Labyrinth, Catacombs, and Hercules Leg-Juniper Cave systems. Sentinel Cave's roof is the largest remnant to survive along the course of this highest feeder tube of the Cave Loop Road area.

Features Near Upper Entrance

At the upper entrance to Sentinel Cave are two large collapse pits with a natural bridge between them (map 7, pl. 2). The ceiling of the upper tube has been thinned almost to the surface by roof collapse, and collapse breccia covers the floor.

Located 50 ft from the entrance and 10 ft above the present floor is a fragment of the tube wall and a lava bench. They were protected from collapse because they were in a 20-ft-diameter alcove on the southeast wall of the tube. As the floor of the alcove cooled and solidified, the underlying lava began to drain back into the main tube, a process which formed a 2-in.-wide tension crack in the upper crust. Another 50 ft downstream the collapse blocks on the cave floor end abruptly, and a remnant of each wall of the tube is perched on either side. One remnant runs beneath the stairs on the west side; the other is separated from the present east wall of the cave by collapse blocks that represent an 8-ft retreat of the original southeast wall.

Both remnants have a lava bench, or balcony, 3 ft above the pahoehoe, which probably marks the former floor of the tube. Evidently, throughout most of this area the floor of the upper tube collapsed into the middle tube below. A little farther downstream, at the abrupt bend of the tube to the north, is an almost impassable chaos of deep pits partly filled with huge blocks. High ceilings, caused by unraveling of the roof almost to the surface, characterize this area. These features indicate collapse of all three upper-tube levels, an event confirmed by the lowest tube being closed by debris in this area.

The trail avoids this chaotic section of the cave by following a small branch of the upper level that swings around a pillar to the west and reenters the main tube 90 ft downstream. Here a natural catwalk crosses the deep collapse pits in the main cave. This catwalk is anchored at either end to remnants of the pahoehoe bench that formed the floor of the upper tube. An upper balcony, 3 ft higher than the catwalk, still adjoins the lower-floor bench at either end of the catwalk. Pahoehoe lava in a tributary to this upper balcony is exposed at intervals within the small tube forming the trail bypass. This balcony continues on the north wall of the bypass tube for 20 ft west of the catwalk. A remnant of the floor of the upper tube extends as a narrow natural bridge about halfway across part of the deep chasm 15 ft south of the catwalk. Together with fallen roof blocks at the west side, the remnant forms a second precarious crossing over the collapsed tube. The numerous benches, partial bridges, and balconies where these two passages join indicate a complex series of events.

North of the catwalk the chasm of collapsed blocks ends abruptly. The trail from the east end of the catwalk continues along the narrow bench, 3 ft below the balcony bench, and over a lava fall until it reaches the intact floor of the upper tube. Downstream this tube floor, littered with fallen roof blocks, can be traced for 95 ft to where it is buried under collapse rubble.

Two interesting features appear along this stretch of intact floor. One is a collapse pit resembling a well only a few feet in diameter, which gives access to the middle level below. The other is a small tube-in-tube less than 2 ft deep and 3 ft wide; its cross section is best seen on the north wall of the collapse pit, just beneath the surface. South of the pit most of the tube-in-tube has collapsed, either from its own weight or by bombardment of falling roof blocks (map 7, pl. 2). In this section of the cave, the main passage measures 10 ft wide and as much as 25 ft high.

Nearly 25 ft north of the collapse pit the trail begins to rise steeply on a huge mound of collapse rubble. No primary features of the lava tube are seen in this section except on the surfaces of fallen blocks. From the top of this collapse mound the trail rounds a sharp corner, passes a tiny branching tube from the north, and descends a stairway into a deep collapse. Here the entire floor of the upper level has fallen into the lower tubes. This collapsed area extends northeast for 210 ft, where the trail again ascends to a partially rubble filled upper tube.

At the east end of the collapsed area where the stairs descend from the upper level, the middle level extends south beneath an overhang. Through the floor of the middle level, beneath this overhang, a narrow vertical chimney-like pit opens below steep slopes of rubble. The pit gives access to the lowest levels of Sentinel Cave and is covered by a gate.

Middle Level of Sentinel Cave

The entire length of the middle level of Sentinel Cave is covered by rubble from the collapse of the upper level. Small remnants of lava benches and balconies provide glimpses of tube walls in and above the middle level (map 7, pl. 2). These remnants have not been correlated with one another, nor with the two benches described in the upper level. Halfway along this east-trending, 20-ft-wide passage is a double skylight, the Tube and Pillar entrance, named for the small natural bridge and pillar within a near-surface tube. The ceiling height of the main passage gradually decreases from 12 ft to 9 ft at the lower entrance.

Tubes Intersecting Sentinel Cave

A side tube that crosses the collapsed middle level of Sentinel Cave approximately 400 ft upstream from the lower entrance is the continuation of the tiny branching tube just upstream from the stairway. At its upstream entrance, this small tube is a 3-ft-diameter crawlway with a smooth pahoehoe floor. The tube enlarges slightly before swinging east and rejoining the main tube 16 ft above the floor of collapse rubble. The eastern continuation of this side tube hangs 5 ft lower on the east wall of the main passage because of a lip formed by draining of the lava back into Sentinel Cave immediately after collapse joined the tubes. From the sill above this lip the empty tube can be followed downstream 115 ft, where it again intersects the main Sentinel tube by means of a 7-ft lava fall. Midway along its course this side tube is partly choked by a mass of collapse breccia that was overrun and smoothed out by the withdrawing lava. From this point, the tube cannot be followed downstream into the west wall of Sentinel Cave.

Another tube (which may be an earlier westward continuation of the upper level) enters Sentinel Cave from the west 6 ft above the main passage floor at the corner where the trail along the middle tube rises over a rubble scarp 8 ft high and then turns east along upper Sentinel Cave's collapsed continuation. This 20-ft-wide upper-level tube quickly disintegrates into a maze of collapse rubble and irregular low rooms. The tube continues back to the south with most of its pahoehoe floor buried. This tube can also be reached by the Wabbit entrance (map 7, pl. 2).

Lower Entrance and Downstream Extension

Exit from Sentinel Cave is through the lower entrance, a large collapse pit 66 ft vertically below the upper entrance. To the east, an entrance hole, the Annex entrance, leads down into a 15-ft-diameter tube with a relatively clean pahoehoe floor. The character of this northern part of Sentinel Cave is very different from the main part previously described. Here the walls and floor are largely intact; the walls are moist and nearly black, and the temperature of the cave is much colder. Multiple benches, partially peeled wall linings, and a ropy pahoehoe floor are present along this section of the tube. Some collapse rubble and a long peeled lining along the north wall are found just upstream from a low tube-in-tube that meanders down the center of the cave for over 100 ft. The main tube then turns to the north and becomes triangular in cross section (fig. 33) for several tens of feet. Along this segment, the elevation of the floor is 85 ft below the upper entrance. Farther along the passage, the ceiling drops to 5 ft near a large pile of collapse rubble; just beyond this pile is the start of another collapsed tube-in-tube, which can be followed for 50 ft. This main passage contains two sets of benches, one located just below ceiling height and the other directly above the collapsed tube. The remaining several hundred feet of cave consists of a 10-ft-wide passage whose ceiling gradually lowers. The tube is sealed by pahoehoe lava at a point some 106 ft vertically below the upper entrance. Spalled wall linings cover the floor of the last section of the cave and make it difficult to traverse. The downstream continuation of this tube contains Indian Well Cave.

Figure 33. Two explorers negotiate a narrow passage of Sentinel Cave (see fig. 14 and map 7, pl. 2). Portions of left wall linings have sloughed off.