Urban geology: Monumental geology
Stephen K Donovan (The Netherlands)
My writings on urban geology are normally centred in the area around my home in Noord Holland, but sometimes I am lucky enough to travel. A personal wish that I have had since I was a teenager was to see and, if possible, board a dreadnought battleship. This whim was finally satisfied in March 2014, when I visited the last surviving dreadnought from World War I, the USN Texas, preserved at the San Jacinto Battleground State Historic Site, near Houston (Fig. 1A). What I had not realised was the battleship is interred adjacent to the site of the Battle of San Jacinto, where a rag-tag army of insurgents, following defeat at the Alamo and Goliad, decisively defeated the Mexican army in under 20 minutes in April 1836, thereby winning independence from Mexico for Texas.
The Battle of San Jacinto is commemorated by a towering monument (Fig. 1B), which is the tallest memorial stone column, about 175m, and some 4.5m taller than the much better known Washington Monument in Washington DC. The San Jacinto Monument is visible over a wide area of this flat coastal plane and is distinctive enough at a distance, but up close it is highly distracting to the geologist (Rhodes, 2011, pp. 218-219).
Cordova Shell Stone
The San Jacinto Museum of History, located at the base of the monument, has an explanatory display that helpfully describes the facing stone of the steel-reinforced concrete structure as Cordova Cream Shellstone. This was a new name to me and my favourite reference for American building stones did not mention it (Williams, 2009). However, it appears that the name Cordova Cream Shellstone confuses the names of two limestones.
Adams (2008) noted two different dimension stones (that is, natural stone or rock that has been selected and fabricated to specific sizes or shapes), namely Cordova Cream and Cordova Shell Stone, which are quarried in the Austin area of west-central Texas by the firm, Texas Quarries. Kyle (2011, fig. 5) published a colour photograph illustrating the different appearances of Cordoba Shell and Cordoba Cream stones, presumably an alternative (or erroneous?) spelling of Cordova. These building stones are extracted from the Lower Cretaceous (Comanchean = Middle Albian, about 100 to 105Ma old; Mancini & Scott, 2006, fig. 1) Walnut and/or Edwards formations (Ellison & Jones, 1984; Moore & Bebout, 1989, fig. 6; Adams, 2008), which crop out north of Austin and were deposited on an extensive, shallow-marine platform (Fisher & Rodda, 1969).
What is immediately striking about the Cordova Shell Stone is its numerous fossil molluscs and their mouldic preservation. Moulds of calcareous fossils in a calcareous rock always pose an interesting question: how were the shells dissolved without dissolving the rock? The gastropod shells would most likely have been aragonitic (Tucker, 1991, table 4.1) and we may speculate that so, too, were the bivalves preserved as moulds. Aragonitic shells are not uncommonly found to have been dissolved away in calcitic limestone. Furthermore, the Edwards Formation is locally dolomitised and it may have been so in the San Jacinto Monument, influencing the dissolution of aragonitic shells.
Cordova Shell Stone is widely used as a dimension stone, yet its properties seem to argue against it being well suited to this task. The presence of numerous moulds of molluscs makes the rocks porous and prone to accumulating small pockets of rainwater which, if the rain is to any degree acidic, will accelerate deterioration by dissolution. The monument was built in 1936, so we know that the external surface has been exposed to the elements for about 80 years. Comparison of the exterior surface of the monument (Fig. 2A) with those in the museum, and therefore indoors (Fig. 2B-D), shows the latter to be obviously better preserved. But, again, any dolomitisation may act to retard dissolution in acidic waters.
Identifiable fossils are many, but of limited diversity. There are many indeterminate mouldic fragments (Fig. 2), which might be identified with time and adequate casting in a suitable liquid rubber compound such as latex (unfortunately, I was unprepared, as a piece of Plasticine will give a rough and ready cast in a situation such as this). What appear to be calcareous algal structures, overgrowing shells (Fig. 2B), are present, but not common. The fauna is largely limited to one species of bivalve and one of gastropod (Fig. 2A, C and D). The gastropods are turritellids, possibly close to Turritella seriatimgranulata Roemer, which is known from the Upper Aptian and throughout the Albian, and occurs in Texas (Adkins, 1928, p. 183; Squires & Saul, 2006). Modern Turritella are gregarious, ciliary suspension feeders.
The bivalves are an Exogyra oyster (best seen in Fig. 2A and D). Exogyra and other oysters are so common in the Cretaceous of Texas that they formed the basis of a biostratigraphic scheme in the early twentieth century (Bose, 1919, p. 3). There are many species of this genus named from the state (Bose, 1919; Adkins, 1928) and, without casts to guide any comparisons with illustrated and described taxa, I prefer to be conservative and identify these specimens as simply Exogyra sp.
I thank the Stephen F Austin State University for financing my visit to Texas; and my friends Alan and Karen Carruthers for their hospitality, and especially for taking me to the San Jacinto Battleground State Historic Site.
Note that many of the articles listed below are available free online.
Adams, J. 2008. A long-time producer of Texas limestone. Stone World, July 1: Accessed April 2014.
Adkins, W.S. 1928. Handbook of Texas Cretaceous fossils. University of Texas Bulletin, #2838: 385 pp.
Bose, E. 1919. On a new Exogyra from the Del Rio Clay and some observations on the evolution of Exogyra in the Texas Cretaceous. University of Texas Bulletin, 1902: 22 pp.
Ellison, S.P. & Jones, J.J. 1984. Walking the Forty Acres: Building Stones – Precambrian to Pleistocene. University of Texas Libraries, Austin. Accessed April 2014.
Fisher, W.L. & Rodda, P.U. 1969. Edwards Formation (Lower Cretaceous), Texas: dolomitization in a carbonate platform system. AAPG Bulletin, 53: 55-72.
Kyle, J.R. 2011. Geology of Texas industrial minerals. Austin Geological Society Bulletin, 7: 29-43.
Mancini, E.A. & Scott, R.W. 2006. Sequence stratigraphy of Comanchean Cretaceous outcrop strata of northeast and south-central Texas: implications for enhanced petroleum exploration. Gulf Coast Association of Geological Societies Transactions, 56: 539-550.
Moore, C.H. & Bebout, D.G. 1989. Carbonate Rock Sequences from the Cretaceous of Texas. Field Trip Guidebook T376. 28th International Geological Congress, Washington, D.C. American Geophysical Union, Washington, D.C., viii+46 pp.
Rhodes, A. 2011. Texas. Moon Handbooks, Berkeley, California, 466 pp.
Squires, R.L. & Saul, L.R. 2006. Additions and refinements to Aptian to Santonian (Cretaceous) Turritella (Mollusca: Gastropoda) from the Pacific Slope of North America. The Veliger, 48: 46-60.
Tucker, M.E. 1991. The diagenesis of fossils. In Donovan, S.K. (ed.), The Processes of Fossilization: 84-104. Belhaven Press, London.
Williams, D.B. 2009. Stories in Stone: Travels in Urban Geology. Walker, New York, x+260 pp.