Dark Hole Wind Cave: 43.919167 121.142778

Wind Cave is the longest (1170 m slope length), largest (18 + m, maximum height), and most difficult lavatube to traverse of the Arnold System. The "Dark Hole" refers to a skylite a short distance into the cave. Most of the cave's upper two-thirds has great piles of basalt boulders, some a meter and more in diameter, along the floor. Many rubble piles are higher than 18 m and have angles of repose exceeding 22. In profile, the series of rubble piles and corresponding ceiling domes, formed by basalt-layer spallation, gives the tube an undulating cross-section. The true lava tube profile, determined along uncollapsed ceiling sections and floor sections not covered by rubble, matches the surface topography fairly well, although some slopes along the floor toward the end of the tube are reverse to the surface topographic slope.

Despite massive spallation, many ceiling and wall sections preserve original lava tube lining and glaze, although in many places the lining is broken by polygonal fractures; minerals produced by weathering are prominent. Multiple flow lines, representing drainage stages of liquid lava, are preserved in some walls and can be traced for tens of meters. Some flow lines developed prominent gutters, or ledges along the walls.

In all but the last few hundred meters, Wind Cave is characteristically skull, or keyhole, shaped in cross section, giving the impression of an upper level and lower level. Considering the probable formational mechanisms for large lava tubes, this configuration is reasonable and is perhaps to be expected. During initial stages of formation, two mobile conduits may be stacked vertically, separated by a shear plane. In the case of Wind Cave, the intervening shear zone may have been so weakly developed that it was removed, or incorporated in the liquid lava, in the final stages of lava tube drain-age. Thus, the size and shape of both upper and lower levels would be essentially maintained. Cross sections in straight lengths of the lava tube are symmetrical; in meander bends, cross sections are often asymmetrical, developing cut banks and slip banks.