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By Mary C. Bourke, Heather A. Viles (Eds.)
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Figure F12 Percussion fracture ridges (distal) (a) on a basalt clast in Oak Creek Canyon, Arizona. Muted ridge fractures are indicated (b). Image courtesy of M. Bourke. 36 Chapter 3: Fluvial Features Bulb of Percussion Scale: sub-facet Feature description: A single bulb of percussion is almost always evident in a conchoidal fracture facet. On the proximal end of the concave facet of a core, the bulb of percussion describes a prominent dish-shaped surface (Fig. F13). The bulb represents a partial hertzian cone, which is responsible for initiating the conchoidal fracture.
Bourke. , 2000). In bedrock channels they are often well developed on the top of bedrock protrusions and the crest of large boulders. They are very well developed on fine grained rock and are associated with joints, fractures and small bed irregularities. Similar to their aeolian counterpart (see Figs. A7-A11), fluvial flutes are closed at the upstream end. They are thought to migrate in an upstream direction and occasionally have a more rounded rather than elongated shape. Hancock et al (1998) attributed this to a diminished strength of flow separation due to changes in the feature itself or in the surrounding bed.
Boulder has been split from larger boulder (not in image) along angular facet on left. Note also the percussion pits on the surface of the sandstone boulder in the foreground. Image courtesy of M. Bourke. 31 Chapter 3: Fluvial Features Percussion fracture facets (concave and convex) Scale: facet Feature description: During fluvial transport, clasts may collide with sufficient force to propagate percussion fractures (Fig. F2). Clast breakage by percussion, particularly on fine-grained rocks, creates smooth, shell-like convexities and concavities.
A Photographic Atlas of Rock Breakdown Features in Geomorphic Environments by Mary C. Bourke, Heather A. Viles (Eds.)