The Horse Lava Tube System (or Horse system) is series of lava tubes within Deschutes County, Oregon, of the United States. The system starts within the Deschutes National Forest on the northern flank of Newberry Volcano and heads north into and near the city of Bend.The system continues north again to Redmond at the Redmond Caves and into the Redmond Canyon, where the last known segment is known to exist; however, the basalt flow that created the system goes beyond to Crooked River Ranch and terminates just short of the Crooked River Canyon. The lava flow that created the Horse system is also referred to as the Badlands Excursion and it filled the ancient channel of the Deschutes River which at that time flowed around the east side of Pilot Butte. The Badlands Excursion is apart of the Basalt of the Badlands which also consists of the Arnold Lava Tube System, the Badlands rootless shield, and the Lava Top butte basalt. All have a geologic age around 80,000 years old.
Horse Lava Tube System is composed of complex anastomosing, parallel, and often disconnected lava tubes, all formed apparently in a single basalt flow. Many segments probably formed within the flow independently of other segments. Some sections were probably only partly drained of fluid lava, or were plugged by subsequent flow units. The relationship of individual segments to the overall system is apparent in map view. In addition to uncollapsed lava tubes, all depressions found by aerial and ground reconnaissance, indicated on topographic maps, and visible on aerial photographs. The depressions are often elongate parallel to the direction of Horse Lava Tube System and from field observations are interpreted as collapsed structures. Some structures are collapsed lava tubes, others probably represent collapsed lava ponds similar to those associated with the Arnold System and ponds not directly associated with tubes, but which resulted from drainage of liquid lava beneath a partly cooled, thin crust.
Individual segments trend nearly parallel to regional topographic slope. The system forms a broad, low ridge approximately two kilometers wide (indicated on topographic maps) similar to the ridge formed by Arnold Lava Tube System, and represents the most mobile part of the lava flow.
Elements of the system are located in basalt, the oldest unit mapped in the area and the same basalt in which the Arnold System formed. Both the source and terminus of Horse Lava Tube System is covered by younger. The system can be traced from Lewis Farm Cave, downslope more than 11 km to Barlow Cave. Because the source and terminus of the system are concealed by superposed younger basalt, it is not possible to determine the total system length. The tube system probably extends beneath the younger basalt toward the source and northeast toward the flow terminus. Individual tube descriptions begin with Lewis Cave at the source end and progress northeast toward the terminus.
Lewis Farm Caves consist of two lava tube segments. The tube is entered through a skylight and a vertical drop of about 7.5 m. It trends north and southeast from the entrance, with the north passage extending 110 m over sand and basalt blocks before terminating in sand blockage. Parts of the west wall have well-developed pahoehoe flow lines. A side passage, 14 m wide and 55 m long, extends northeast and has pahoehoe flow lines along sections of the southern wall.
The passage southeast from the entrance is about 90 m long, sinuous, and has well-preserved original walls and ceiling with tube lining and glazing present. Although most of the floor is covered with sand, original floor (pahoehoe) is exposed in a short section. The tube pinches out with the ceil-ing and sand floor converging.
Two caves on the Johnson Farm, east of Lewis Farm, were not surveyed. Entrances are through crawlways in roof collapse near the farm house and service buildings.
Garbage Caves, east of Arnold Market Road, are located in the reclaimed Bend City garbage dump. Two individual lava tube sections were examined, although there are reportedly several other large caverns in the area. Garbage Cave #1, is nearest the present parking area; entrance to the tube is a crawlway leading from a large collapsed lava tube trench currently being filled with refuse. The tube consists f three sections: a west room, 35 m x 10 m, and east room of nearly equal dimensions, and a south passage extending 17 m from the west room. In all sections floors consist of sands and blocks collapsed from the ceiling and passages are terminated by ceiling collapse. Some ceiling and walls in the east room exhibit lava tube lining and glazing. Solid basalt on the south passage floor may represent an in situ part of the original flow.
Garbage Cave #2 is about 150 m northeast of Cave #1 and consists of an uncollapsed lava tube segment about 51 m long. It leads from the end f a large sinuous depression (collapsed lava tube segment) and parallels the general trend of the Horse Lava Tube System. Sand and collapse blocks compose the floor, and the tube is blocked by collapse. Small glaze patches are preserved on parts of the ceiling toward the passage end.
Stevens Road Cave is a small lava cave entered through a collapse on the south side of Stevens Road. The cave is about 32 m x 27 m and from the shape of the interior apparently is the uncollapsed part of a lava blister. The floor is comprised of sand and basalt blocks; some of the original ceiling is preserved.
Horse Cave is a well-known lava tube east f Bend and the lava tube system name was derived from it. Entrance to the tube, through a collapse depression, leads to parallel and interconnecting passages that are typical for Horse Lava Tube System. Except for painted walls and litter left by vandals, the ceilings and walls are well preserved, exhibiting flow lines, lava tube lining, and glaze. The floor is covered by sand and the ends f the passages are blocked by sand. Depth of the sand apparently fluctuates considerably. Some walls display multiple sand and silt varves to a height one meter above the present floor. The roof is fairly thick compared to the diameter of the lava tube and probably explains the lack of abundant collapsed sections. One collapsed section formed a skylight over the eastern passage.
Wilson Cave is entered at the south end of a collapse depression and extends 75 m over sand floor before it is blocked by sand and spelled basalt. Two prominent troughs representing collapsed segments of the tube extend southwest from the terminus of Wilson Cave. A small crawlway, trending toward Barlow Cave, at the northern end of Wilson Cave collapse was not examined.
Barlow Cave entrance is on a fence line about 90 m south of Butler Market Road. Entrance, through a skylight, leads to a north passage extending about 51 m before blockage by ceiling collapse and sand. At the south end the tube is plugged by wet sand and collapse blocks. An elongate pond about midway be-tween the southern terminus of Barlow Cave and the entrance to Wilson Cave is probably part of the lava tube that collapsed while the roof was semi-molten and plastic. The floor of the depression is relatively impervious and water is retained in the pond throughout much of the year; however, fractures evidently permit some seepage of water into the tube, indicated by wet sand in Barlow Cave.
A prominent pillar and side passage are located about halfway in the southern part of the lava tube. The side passage leads about 15 m from the main tube and terminates in a circular collapse de-pression with a crawlway leading to the surface. The depression was formed by drainage of molten lava beneath the partly cooled surface crust before the crust was sufficiently thick to support its weight. As a result, the plastic crust sagged under its own weight, ruptured, and created the collapse depression. The depression containing the pond, described above, probably formed similarly but the crust did not rupture.