December 19, 2006 | David F. Coppedge

Frozen Storms in Sandstone

Impress your friends at the water cooler with this phrase: “hummocky cross-stratification.”  Let’s call this mouthful HCS and talk about it.  It has a story to tell.
    HCS is a kind of geological formation characterized by alternating three-dimensional hummocks (convex up) and swales (convex down).  Discussed in the geological literature since the 1970s, it is generally thought to represent the work of cyclonic storms along continental coastlines at a depth where violent waves sculpt the shallow ocean floor (up to 10m).  Most geological layers are deposited flat.  Level strata might subsequently be deformed or tilted, but HCS is deposited in its hummocky, swaley form.  Additional layers deposited on top produce the cross-bedded signature characteristic of HCS.
    Two Canadian geologists decided to test the storm formation theory.  They performed flume experiments with sand under controlled conditions and published their results in the journal Geology.1  “There is general consensus that both hummocky and swaley cross-stratification form during storms,” they began.  Basically, they confirmed this hypothesis:

Based on these findings, we suggest that hummocky cross-stratification optimally forms above (but near) storm wave base where aggradation rates during storms are high enough to preserve hummocks but unidirectional current speeds are sufficiently low to generate low-angle, isotropic cross-stratification.  Swaley cross-stratification is also hypothesized to be deposited by an aggrading hummocky bed between fair-weather and storm wave base, but in shallower water where aggradation rates are low enough to cause preferential preservation of swales.

The main things their experiments identified were the relative contributions of oscillatory and unidirectional flow rates on the resulting bed forms.  They also wanted to find out if the waves simply scour and drape the sand, or if the bed itself is dynamic.  Using a 15m flume with capabilities for oscillatory and directional flow, they concluded that some or most HCS is generated by “actively aggrading and migrating hummocky bed forms under long period (8-10 s), high oscillatory velocity … and oscillatory-dominant combined flow” higher than 50 cm/s for the oscillations and less than 10 cm/s for the directional flow.  The directional flow contributes sand to the area of deposition.

1Simon Dumas and R.W.C. Arnott, “Origin of hummocky and swaley cross-stratification— The controlling influence of unidirectional current strength and aggradation rate,” Geology, Volume 34, Issue 12 (December 2006), pp. 1073�1076, DOI: 10.1130/G22930A.1.

OK, you learned something about a peculiar geological formation, which the authors describe as “enigmatic sedimentary structures.”  So what?  Well, what is interesting is where these are found.  The Grand Canyon has lots of HCS.  You can see a prominent outcropping along the Supai Trail, for instance, up from the springs.  A creation geologist was pointing these out on a hike one day, and said, “What’s unusual about these examples is their size.  These are very large hummocks – much larger than anything that is being formed today.”  It must be remembered that these sedimentary layers exposed in the Grand Canyon extend throughout several Western states and some of them throughout a good portion of North America.  Considering not only their size, but their widespread distribution throughout the canyon, he considered these as uncontrovertible evidence of a flood catastrophe like the world has never seen.  Perhaps so; nobody was there to witness these rocks being laid down, but models can help us evaluate the plausibility of such inferences.  Think about it.  Whatever happened at this point on the Supai Trail, it was certainly not a good day for a beach picnic.

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