by Michael J. Oard and Peter Klevberg

Well-rounded coarse gravel provides clues to the depositional process. The coarse gravel of the Mogollon Rim in central and northern Arizona, called Rim Gravel, was examined at two widely separated and representative locations. Further characteristics of the coarse gravel was obtained from the literature. The coarse gravel occupies the highest terrain in the region and is very coarse in east-central Arizona. It is deduced that this coarse gravel was deposited as a sheet and eroded into remnants during the Recessional Stage of the Genesis Flood. We conclude that the Rim Gravel provides evidence that the Flood/post-Flood (D/P) boundary corresponds to the stratigraphic location of rocks termed “late Cenozoic” in the uniformitarian geological column in this part of the western United States. This interpretation is relevant to theories for the formation of many notable geomorphic features, including the Grand Canyon of the Colorado River.


Gravel, cobbles, and boulders contain information on the depositional process. They are lumped together and called coarse gravel. Sometimes paleocurrent indicators are present, such as clast imbrication and cross-beds. The degree of rounding of the coarse gravel determines the amount of action by water. Well-rounded coarse gravel is an indicator of significant transport by water during some time in its history. The larger the clasts in the deposit usually the stronger the current needed to transport the coarse gravel. If the coarse gravel is lithified within a matrix, it is called a conglomerate. There are clast-supported and matrix-supported coarse gravels. In the former, the rocks are touching each other with the matrix filling the voids, while in the latter the rocks are almost entirely surrounded by matrix.

Many (if not most) deposits probably have complex histories, and this is likely the case for the Rim Gravel. For example, material when first eroded may be transported via mass wasting (a debris flow, landslide, slump, etc.). As transport continued, and with the addition of water, the material could have been carried in traction at the bottom of the flowing water. The rounding of coarse gravel would more likely occur during this process rather than from mass wasting. It is also possible that the material was eroded by water, rounded, and then mixed in with fine-grained sediment to become matrix supported at deposition. The fine-grained sediment between clasts could have resulted from the breaking up of subjacent material in situ or the erosion of finer-grained upstream substrate during the transport process. Regardless, rounded coarse gravel is an indicator of the action of water.

Uniformitarian scientists would normally interpret rounded rocks as the result of a river or beach process. When they observe rounded rocks, they have a tendency to interpret them as fluvial (Miall, 1996). Generally, one does not encounter littoral (along beaches) environmental interpretations of rounded coarse gravels. Creationist geologists also expect much rounding of rock during the Deluge. So, the latter need to carefully examine the characteristics of the coarse gravel to be able to distinguish between a Flood-laid coarse gravel and one laid down in the postdiluvian (post-Flood) period….

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