The Proceedings of the Eighth International Conference on Creationism (2018)

are too shallow for the sand to fail in such a way (dips that the structures are contained within are often 20° or less). However, if the slump occurred in a subaqueous setting, it is possible the failure could have occurred on a shallow slope and been driven by a down-slope current. B. Cross-beds and cross-bed dips It has been incorrectly stated by some that the Coconino has steep cross-beds which are close to the angle of repose (e.g., Hill et al. 2016, pp. 58, 70, 202). In fact, a whole series of cross-bed dips were published by Reiche (1938) whose mean dip was very close to our mean dip of about 20°. Another misunderstanding that many have is that the angle of repose (about 33°) is less under water than it is in air. This is false. The angle of repose underwater is about the same as that in air (Allen 1970a; Carrigy 1970; Hunter 1985). In aqueous settings a variety of factors (velocity, water depth, amount and type of entrained sediment) control whether cross-beds or plane beds are formed. In general, the faster the current the lower the angle becomes on cross-bed dips until plane beds are formed in the upper flow regime. A possible explanation for cross-bed dips less than the angle of repose, is that the upper (steeper) parts of the cross-beds have been eroded away by the next migrating set of cross-beds (e.g., Poole 1962, p. 148). We do not know how steep the upper parts of the cross-beds were (or how tall the bedforms were) because we do not have the upper parts of the dune to measure, so this may be a possibility. However, in extremely thick (> 15 m) cross-bed sets that were measured by Maithel (personal communication, 2018) in the Ash Fork area, dips remained fairly constant at about 23° from the top of the set until a meter or two near the bottom of the set where the cross-beds rapidly flattened out (see Fig. 28). This pattern occurs throughout much of the Coconino which may suggest the tops of the dunes were never much steeper than the bottoms. Sand waves have not been studied extensively because they occur in underwater settings with strong current flows. However, it is interesting that measured cross-bed dips of sand waves are in the range for the dips that we see within the Coconino and other cross-bedded sandstones. The lee slopes of sand waves in marine and estuarine settings typically display angles of less than 20° but have been reported to reach more than 30° (e.g., Aliotta and Perillo 1987, p. 11; Cornish 1901, p. 170; Dalrymple 1984; Elliott and Gardiner 1981, p. 58; Langhorne 1982, p. 580; Ludwick 1970; Salsman et al. 1966, p. 13; Werner 2000, p. 87). In some instances, smaller sand waves were found to be steeper than larger ones (e.g., Dalrymple 1984). However, the opposite trend was reported for the sand waves off the coast of western Australia (Jones et al. 2009). Lee slope angles in sand waves depend on a variety of factors, including tidal current velocity, tidal current asymmetry, bed load versus suspended load transport, grain composition, grain size and textural characteristics, and these relationships warrant further investigation. C. Laminae Hunter’s types of dune stratification (1977) have been widely cited, but we have had difficulty identifying clear examples in our study as has Maithel et al. (2013, 2014, 2015, 2016, 2017). Hunter Whitmore and Garner ◀ The Coconino Sandstone ▶ 2018 ICC 597 Figure 20. Tabular and wedge cross-bed sets in the Coconino as seen near the bottom of the formation along Hermit Trail in Grand Canyon. Notice how the cross-bed sets have a relatively constant thickness along dip until near the bottom of the set. Sometimes near the bottom they thin and form a “wedge.” The set between the two major bounding surfaces is about 8 m thick. JHW photo 8142-2013. Figure 21. Abounding surface (the horizontal rock surface) within the Co- conino has been exposed to reveal the many thin foreset laminae that can be traced for many meters along strike (toward the girls). The beds are dip- ping to the left. Some gradually pinch out over the length of the outcrop. This is much different than cross-sections of avalanche tongues that can be found in modern dunes (see Fig. 22). This exposure is near Ash Fork, Ar- izona. One m hiking stike near middle of photo. JHW photo 9391-2013.