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Medium sand; flow from back; channel 1 m wide; foot rule

Medium sand; flow from back; channel 1 m wide; foot rule. Crests are sinuous and semi-continuous. Note that trough depth varies laterally. That ’ s the key to generation of small-sale trough cross stratification.

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Medium sand; flow from back; channel 1 m wide; foot rule

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  1. Medium sand; flow from back; channel 1 m wide; foot rule Crests are sinuous and semi-continuous. Note that trough depth varies laterally. That’s the key to generation of small-sale trough cross stratification.

  2. Flow from R to L; very fine sand with dark plant material; screws spaced about 20 cm The foreset laminae show up well owing to the dark material. Note that not all of the ripples have the classic triangular profile; some are “streamlined”. The one that’s third from left is (temporarily?) riding up over the next downstream. The second from lest has ridden over the remains of the foresets that were deposited in an unusually deep trough of an earlier ripple.

  3. Moenkopi Fm (Triassic), east of Las Vegas On the bedding surface they look like standard current ripples, but in cross section they don’t. Might these have formed in combined flow, with a subordinate oscillatory component?

  4. Medium sand; flow from L to R An “art” photo, without a scale. Some ripple-crest segments are linguoid (convex downstream), others are lunate (concave downstream). Note the subtle streamwise lineation on ripple stoss sides, which is characteristic of bed-load transport.

  5. Dakota Ss, Four Corners region; Scale bar 10 cm The flow was from left to right. The fully three-dimensional geometry tells us that the ripples were fully developed, or almost so.

  6. Linguoid current ripples, I70 W of Green River, Utah Flow was from top to bottom of the photo. It’s unusual to see such a regular lunate pattern. Note the tiny ridges on stoss sides; they are the eroded edges of previously deposited foreset laminae.

  7. Incipient ripples in medium sand; upper marks on scale bar = 1 cm Flow was from left to right. The ripples were triggered by a small cross-stream artificial ridge (out of photo to left). The three-dimensional irregularity is just getting started on the far left (older ripples than on right). The flow separation from the rightmost ripple is just starting to generate a new ripple.

  8. Planar sand surface exposed at low tide, Plum Island, Massachusetts There was a planar sand surface, produced somehow by an earlier strong tidal current. Then a weaker current has produced local trains of incipient current ripples, triggered at certain points. Note how the ripple train expands laterally as it develops. Some three-dimensionality is just starting in a few of the trains.

  9. Barchan ripples in very coarse silt; channel is 17 cm wide. Most of the silt had sufficient cohesion not to be entrained by the weak flow, but there was enough noncohesive sediment to form grossly starved ripples, in the form of miniature barchans.

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