Geologic conditions in the subsurface are, of course, uncertain, and are shown hypothetically in the figure. Field observations of exposures in the quarry face and at the ground surface around the deck indicate two rock units are present: 

QSPh - a quartz-sericite phyllite interlayered with lesser amounts of quartz-sericite schist; and, 

SQR - a sericite-quartz rock interlayered with subordinate lenticular bodies of quartz-sericite phyllite. 

The light-colored QSPh unit is best exposed in the quarry face.  Sericite is a general term used for fine-grained, light- colored micaceous minerals. The term phyllite refers to a rock texture where aligned, fine-grained micaceous minerals impart a shiny appearance to the foliation surfaces. A schist is a similar rock, but the individual mica grains are large enough to be visible with the unaided eye. At the surface the phyllite and schist unit is partly to completely decomposed, meaning that the rock is remoldable (i.e., crumbles) with hand pressure. Although pyrophyllite was not positively identified in the sericite schist unit within the quarry, it is likely present as microscopic grains not visible to the unaided eye. 

Underlying the QSPh unit is a sericite-quartz rock (SQR) that contains lenticular bodies of quartz-sericite phyllite and schist.  In general, the sericite-quartz rock is less weathered and more resistant than the quartz-sericite phyllite and schist because of the greater percentage of quartz in the former. The lenticular bodies of the QSPh rock unit may act as planes of weakness within the SQR unit. This light-colored, resistant, quartz-rich rock appears to hold up Occoneechee Mountain.  Both the QSPh and SQR rock units likely originated as felsic volcanic or sedimentary rocks that were subsequently altered by hydrothermal fluids, metamorphosed, folded, sheared and fractured. 

The contact zone between the QSPh and SQR units appears to intersect the ground surface beneath the deck. Localized zones of completely decomposed phyllite are exposed around the foundation piers of the deck within the SQR unit. The discontinuous nature of the contact, and the shear surfaces noted throughout the quarry indicate the QSPh and SQR rock units may be in fault contact. These observations indicate the possibility for weak zones to occur in the contact zone between the QSPh and SQR rock units.

Information on planar discontinuities observed in the rock mass is presented to aid in evaluating alternative stabilization concepts, as well as to provide basic information to prospective contractors. Through-going, planar discontinuities in the quarry judged to be relevant to slope stability in the vicinity of the observation deck are grouped into five categories based on their orientation and origin. Three, smooth-surfaced foliations (F1, F2, and F3) defined by the planar alignment of platy (micaceous) minerals are present. These foliations formed during the metamorphism, folding and shearing of the rock, and are developed best in the QSPh unit exposed beneath the observation deck. Two joint sets (fractures) are also present (J1 and J2) in both the QSPh and SQR units. Both joint sets cut across, and are therefore younger than, foliations F1, F2, and F3. 

F1 corresponds to the primary failure planes in the recent rockslide, and dips about 55° to 70° NW toward (out of) the quarry face. In the vicinity of the deck, F1 also appears to be subparallel with compositional layering in the rock, and the contact between the two rock units. Foliation 2 (F2) is less steep than F1, and dips about 35° to 45° NW toward (out of) the quarry face. The intersection of F1 and F2 forms the saw tooth pattern on the quarry face immediately below the deck (Figure 5A).  Foliation 3 (F3) is a younger shear surface that cuts across and deforms both F1 and F2. F3 dips about 15° to 35° NW toward (out of) the quarry face. 

All three foliations are potential failure planes. Failures along F1 within the QSPh unit, or near the contact with the SQR are most likely because of its steeper dip angle and assumed lower shear strength of the foliations compared to the rock mass. Failures along F2 and F3, however, cannot be ruled out. Although the foliation planes (potential slip surfaces) are better developed within the QSPh unit than the SQR; it is prudent to assume failure-prone discontinuities may be present in the sericite quartz rock until determined otherwise. It is very unlikely that rock dowels used to anchor the observation deck's footings to the weathered rock would prevent movement of the rock slopes along the steeply dipping foliation planes that intersect the quarry face.

Two joint (fracture) sets are present that indirectly influence the stability of the slopes. Because they cut across the foliations F1, F2 and F3 the joints act a planes of separation that break the rock mass into a series of blocks. For this reason the spacing and orientation of the joints will affect the placement of rock bolts shown in stabilization alternative B. Iron oxide staining along the joint surfaces indicates that they allow water to infiltrate the rock mass, a further destabilizing effect. Joint set 1 (J1) dips about 55° to 60° SE into from the quarry face. J1 is well exposed in the main scarp of the recent rockslide. Joint set 2 (J2) dips about 72° to 82° SE, oblique to the quarry face.  The trace of J2 can be observed as the high-angle dark lines in the quarry face immediately below the observation deck. Dilation (opening) can be observed along some segments of J2. Joint sets J1 and J2 do not dip towards the quarry face; therefore, it is not anticipated they will act as slip surfaces.