From Another Shore EarthCache

Timely Tragedy

Great tragedies in human history are responsible for a geologic curiosity in Fulton, Missouri. It is at this location one may find the Church of St. Mary the Virgin, Aldermanbury. Designed by famed architect Sir Christopher Wren in 1677, the church replaced another destroyed in the 1666 Great Fire of London. Brilliant white stone blocks quarried from the Isle of Portland, Dorset on the Wessex Coast of southern England were chosen. Two hundred and sixty-four years later, an incendiary bomb dropped during the Second World War left the building a hollowed out husk. Efforts led by British architect Patrick Horsbrugh to restore the church brought the structure, block by block, across the ocean to Missouri.

What makes the Church of St. Mary the Virgin, Aldermanbury, a curiosity? Consider the geology of Callaway County. All of the local, natural stone hails from the Paleozoic Era (541 to 252 million years ago). Dolomites and limestones formed at that time are distinctive and reflect the ecology of the area. The stone blocks used to build the church come from the Mesozoic Era (specifically, the late Jurassic period 145 to 163 million years ago) making this both the youngest (geologically) and oldest (construction) stone building in Callaway County!

How Ooid

Looking carefully at the exterior walls of the church, one may notice something odd. Each of the stone blocks is made up of very small, round nodules that appear to be glued together. These are ooids! To understand their formation, we have to travel back 152 to 145 million years ago to the Tithonian stage of the late Jurassic period.

At that time, southern England was a shallow, sub-tropical sea with a low graded slope not far from shore at the thirty-eighth parallel. Warm, wet winters were followed by hot, dry summers. These temperatures lessened the sea water's ability to hold dissolved gas. Carbon dioxide was released leaving calcium and bicarbonate in the water that readily bonded.^* With the water thus supersaturated, grains of sediment or shell fragments on the sea floor easily accumulated calcium carbonate as they rolled in the currents (and the calcium-rich mud, or micrite). The resulting nodules (ooids) bonded together. Pressure and time transformed these joined balls of goo into the Portland Freestone member of the Portland Stone Formation.

(* For the curious: Ca²⁺ (aq) + 2HCO₃^- (aq) ⇌ CaCO₃ (s) + H₂O (aq) + CO₂ (g))

Layers of Time

When one thinks about it, the small sliver of geologic time (7 million years) that created Portland Freestone is really not all that small! Ecology can change and create differing types of the same rock. Portland Freestone (oolitic limestone) is no different; there are four classifications that are differentiated by fossil content. In order of depth from top to bottom (i.e. youngest to oldest):

• Roach (1 meter thick) is heavily fossiliferous and consists of empty molds left by former gatropod and bivalve fossils. These remains are thought to have been dissolved away by ground water as they were composed of aragonite (a crystalline form of calcium carbonate, CaCO₃).
• Whitbed (up to 2.5 meters thick) contains some minuscule fossil fragments (mostly shells). This form of Portland stone is finely grained and withstands the elements well.
• Curf (also called "small roach", 1 meter thick) is a combination of shelly limestone and chert. Sometimes being easily weathered, curf is not usually used as a masonry stone.
• Basebed (up to 2 meters thick) is the most blemish-free and smooth version of Portland stone. Identifiable fossils are rare at best as it is a combination of micrite and ooids.

Quarrying Knowledge

Stand in front of the church's south door (posted coordinates). Note the grey stone gives way to rows of white stone forty-two inches from the ground to the left of the door. Locate a white stone block approximately three feet long and one foot tall in the first row from the ground. This block is eroded on the upper left corner and bordered by the door's surrounding masonry and the grey stone.

1a) How many fossils are visible on this stone (none, few or many)?
1b) Identify the type of Portland stone (roach, whitbed or basebed).

Go left two stones in the same row. This white stone block is approximately two feet long and one foot tall.

2a) How many fossils are visible on this stone (none, few or many)?
2b) Identify the type of Portland stone (roach, whitbed or basebed).

For both stones . . .

3) Are there visible ooids on either or both stones?
4) Which stone type is most prevalent on the church (roach, whitbed or basebed)?
5) How are ooids formed?

Your responses can be sent to Unitzoid by clicking on this link.

Building Blocks

Godden, Mark. Portland's Quarries and its Stone. http://www.dorsetgeologistsassociation.com/Portland-Stone/Portland_Stone_Document_-_7_June_12.pdf

Missouri Department of Natural Resources. Generalized Geologic Map of Missouri. http://www.dnr.mo.gov/geology/adm/publications/map-gengeomap.pdf

National Churchill Museum. History of the Church of St. Mary the Virgin, Christopher Wren Church. http://www.nationalchurchillmuseum.org/church-of-st-mary-history.html

Railsback, Bruce. Buildings and building stone: St. Mary's, Aldermanbury, in Fulton, Missouri. http://www.gly.uga.edu/railsback/BS/BS-SMFM.html

West, Ian M. Portland Bill, Isle of Portland. Geology of the Wessex Coast (part of Jurassic Coast, Dorset and East Devon World Heritage Site). http://www.southampton.ac.uk/~imw/Portland-Bill.htm

Wikipedia. Portland Group (geology). http://en.wikipedia.org/wiki/Portland_Group_(geology)

Wikipedia. Portland stone. http://en.wikipedia.org/wiki/Portland_stone

Permission to place this EarthCache was kindly provided by Dr. James Williams, Executive Director of the National Churchill Museum.