The fills of Wind Cave are quite varied. Detrital fill varies in size from clays to breakdown blocks tens of feet across. Chemical sediments appear frequently as a cementing agent for the detrital material. The types of sediments may be classified as follows:
- Crystallized fills
- Cemented fill breccias
- Filled sink structures
- Loose fill breccias
- Clay fills
The crystallized fills are apparently related to the layers of silty cemented sediment that appear interlayered in the crystallization sequence of the boxwork, but it is not clear which sedimentation epoch they correlate to.
Cemented fill breccias
In the room, "The Bachelor's Quarters", the cave walls are composed of a cemented breccia. The breccia is composed of angular limestone blocks up to 5 inches across with some sandstone and chert fragments. The matrix in red silt cemented with a fine-grained calcite. The breccia occurs in the J and K Stratigraphic Horizon. (See section on stratigraphic relationships.) The cave passage cuts the breccia and extends into solid bedrock above and below. The possibility exists that this breccia is related to the Pennsylvanian age karst surface known to exist at the Pahasapa-Minnelusa contact. Weathering of the walls contributes irregular blocks of breccia, weathered out angular fragments, and silt to the detritus on the floor. A similar area occurs in parts of the Attic.
Loose fill breccias
Commonly displayed in much of the upper levels of the cave but particularly well developed in Plummer's Pit are thick sections of loose fill breccia. They commonly consist of angular limestone fragments, sharp angular fragments of gray chert up to 5 inches across, and some partly rounded red to purple sandstone pebbles. The matrix is a fine grained deep red silt. The material is not cemented and will easily fall apart and yet is consistent enough to support vertical walls 30 to 50 feet high in the pit. On casual inspection the loose fill breccia appears to be very similar to the cemented fill breccia. However, a detailed microscopic study of the matrix will be necessary to prove this.
Filled Sink Structures
Filled sink structures are masses of sandstone intersected by the cave within the limestone. One example exists at the northwestern end of the upper level passage above the Pearly Gates By-Pass. A second and more spectacular filled sink occurs on the Fairy Palace Loop. Both of these occur at the ends of passageways and only one side is accessible. The filled sink on the Fairy Palace Loop consists of an irregular shaped body of red-brown medium grained sandstone completely filling the passage. The body lies unconformably against the limestone. The sandstone shows bedding of various shades of brown and varies in thickness from fractions of an inch to more than a foot. While the limestone is nearly horizontal here, the sandstone dips 42°. Some cross-bedding is present. A sketch is shown in Figure 9.