ian mckenzie wrote:They are asymmetrical in cross section, having a steep wall on the upstream side and a gentler slope on the downstream side.
Marjorie Sweeting (AKA the "mother of karst"), on page 144 of her 1973 book
Karst Landforms, says
A section taken through the crests [of a scallop] transverse to the flow direction always shows that the downstream slope is steeper than the upstream.
Chapter 11 of Art Palmer's
Geological Guide to Mammoth Cave National Park contains very explicit photos and diagrams (similar to my crude ASCII diagram above), and he states:
The direction of the water flow that formed the scallops is indicated by their steep sides, which face in the downstream direction.
Lauritzen and Lundberg's
Solutional and Erosional Morphology, chapter 6.1 in
Speleogenesis, says:
The longitudinal profile of scallops and flutes always has the steepest side facing downstream.
OK, I even dug out Rane Curl's "classic" paper from the 1974 NSS bulletin,
Deducing Flow Velocity in Cave Conduits from Scallops... Rane introduces things by talking about "turbulent jet flow" and then referring to a diagram, which shows the steep side on the downstream side.
Convinced yet?