Boulder Geology
Q&A With Professor Mark R. Boardman, Professor of Geology, Miami University, Ohio
Q - Professor Boardman, I came across your photos of the large rocks located just south of the Glass Window Bridge. I took a number of photos of those rocks and the surrounding area including some panoramic images. 
I was told by a gentleman who has lived on Eleuthera for quite some time those big rocks were displaced from the ocean as a result of a tsunami. Can you tell me whether you agree with that? And if they weren't put there from a tsunami, what then?
How Did The Boulders Get There
A - One idea about the origin of the boulders is that they were displaced from some lower and windward location by huge waves (a tsunami). This is a provocative and titillating idea, but this tsunami origin for these rocks is not supported by the data - i.e. the idea is not true.
The boulders are remnants. The laminated, layered sediment of the boulders are in place. What looks like a boulder is the eroded remnant of a more continuous laminated unit.
All of the big boulders have lamination and layers that is concordant with other layers. There are no upside down laminae. There are no laminae on their sides, etc. Could all of the boulders have happened to be torn up by a tsunami and all of them set down right-side up? Maybe. What would be the likelihood of tossing a coin and having it always be heads up and never tails up?
Many of the boulders have clear indications that the lower portions are more eroded (by dissolution) than the upper portions. Extrapolating this idea leads us to believe that most of the rest of the unit was totally eroded (dissolved away) at a particular layer. In only a few areas has the rock not been totally eroded - i.e. the remnants that are viewed as boulders.
It seems that most of the boulders are in swales - not on the highest portions of the undulating surface of Eleuthera. Background: The laminated units are nearly all caused by wind deposition of sand grains. The undulating surfaces that are seen are the result of formation of sand dunes. Sand dune formation alternated with non-dune times - often these non-dune-forming times resulted in the formation of soils at the tops of the dunes - these soil formation times are recorded as harder, redder units today. Each subsequent dune-forming event would most likely add laminated sand in the swale areas of the previous dune building time period.
The observation that the boulders are usually in the swales supports the idea that the uppermost laminated unit of the rocks is the unit that is being eroded most severely. The "boulders" are the remnants of this last (most recent) dune-forming period.
Why don't we see blocks / boulders that have old soil horizons in them? Soil horizons that are upside down? Why don't we see “boulders” of units that are clearly of different lithologies (e.g. corals)?
So - We are not saying that tsunamis are an impossible way for these "boulders" to have been em placed.
We are saying that there is a more reasonable way for them to have formed. Our scenario is not nearly as sexy or titillating or fearful as a tsunami, but it better fits the observational data.
I took some great photos and contributed to the discussions and ideas presented above. However, the principal creator of these ideas is Dr. John Mylroie (Mississippi State University) and by Dr. James Carew (University of Charleston). These two professors are the lead thinkers in our group. Dr. Bruce Panuska (Mississippi State University) added the most critical data that supported the ideas generated primarily by Drs. Mylroie and Carew. Attached is an early draft of a field guide we created for the 11th Symposium on the Geology of the Bahamas. You can get a final version of this document from the Gerace Research Center, San Salvador, Bahamas.
© 2008 Mark R. Boardman
Director, Institute of Environmental Sciences
Professor of Geology
Miami University, Oxford, OH 45056