Day 1, Stop 1

Semiprimitive Campsite

The meter-high outcrop at the semiprimitive campsite contains two different bedding planes, both oriented N22W/11NE. The top bed consists of a very coarse-grained, highly lithified metamorphosed sandstone with conglomeratic clasts towards the top, approximately 80 cm thick. The bottom bed is a medium-to-coarse-grained sandstone, and is less indurated than the top bed.

The top bedding plane contains elongated, foliated pebbles, composed primarily of quartz. Foliation is a preferred orientation of crystal planes in mineral grains. These pebbles are oriented perpendicularly to the direction of maximum shortening (N12E/31SE) of an overturned fold. The foliation of the minerals dips more than the bedding plane, in the same direction, indicating that the outcrop is part of the upper limb of a fold.

The orientation of the fold is consistent with the regional Paleozoic stress directions, demonstrated by the elongation of the pebbles in the outcrop.


The white grains shown here are the foliated inclusions on the top of the outcrop.

Evidence of a steep fault on the north side of the outcrop was left by chattermarks. Chattermarks are minerals deposited between fault blocks as they move. Faults leave other indicators on fault surfaces, such as striae (scratched lines) and fault gouge (ground-up powdered rock formed when two rocks slide past each other).

Plumose structures, ridgelike tracing in a plumelike pattern on the surface of a master joint, also appeared on the north face of the outcrop. These may have formed when the rock jointed before the fault slid.


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