Week 20: Cambridge Argillite, Ludwig Wittgenstein, The Dana Brothers

Cover Image. “Structure discontinuities, Porter Square pilot tunnel,” from D.E. Thompson et al., Field Evaluation of Advanced Methods of Geotechnical Instrumentation for Transit Tunneling: Final Report (Washington, D.C.: Urban Mass Transportation Administration, 1983).

Waxing crescent moon; 37 days until summer solstice. The full Lectio Terra archive can be found here.

Headnote

It’s worth at least checking your bedrock. You may find a surprising history beneath your feet.

As for me and my apartment, the story goes something like this:

A while back, when the continents were arranged much differently, the bedrock of present-day Boston was adrift on the Iapetus Ocean as part of a microcontinent called Avalonia. More or less a chain of volcanic islands, Avalonia had split off from the supercontinent of Gondwana together with parts of what are now the UK and northern Europe. When all dry land on the planet eventually converged into the mother of all continents, Pangaea, the islands of Avalonia were crushed between the tectonic plates of Baltica (Europe) and later Laurentia (North America). So they rested until Pangea broke apart, and the Atlantic Ocean rifted Avalonia into two parts: New England and parts of Canada to the west, and old England and parts of Europe to the east.

The fault line where Avalonia fused to Laurentia is visible in a few places across Massachusetts, including the so-called “Bloody Bluff” in Lexington. The land to the east of this bluff, where I live, shares rocks and fossils and other subtle features with the hills of Wales and other regions across the wide ocean. So while the flora and fauna surrounding Boston may be part of North America, the region’s bedrock originated from another continent entirely. We live atop strange layers of time and stuff; but as Wittgenstein notes below, sometimes what we are looking for is less the facts of the matter than the knowledge that they exist.

As a sedimentary rock, Cambridge Argillite likely was formed off the coast of Avalonia, when a combination of underwater currents and submarine mudslides laid down thick layers of fine-grained silt in what eventually would become the Boston basin. This mud was later buried, compacted, and lightly metamorphosed, creating a dense grey rock (or sometimes purplish-grey) that looks like shale but breaks into irregular chunks rather than thin sheets. Such characteristics created many headaches when they were digging the subway tunnels under my current apartment. But Cambridge Argillite also provides a generally stable foundation for heavy urban infrastructure, and has conductive properties that make it suitable for shallow geothermal applications, including under my kids’ school. Rock on.

Fig. 1. “Reconstruction of the continental blocks for the Great Ice Age,” from Alfred Wegener, The Origin of Continents and Oceans, third ed. (1922), trans. J.G.A. Skerl (New York: E.P. Dutton, [1924]), p. 111.

Readings

1. “How am I able to obey a rule?”—if this is not a question about causes, then it is about the justification for my following the rule in the way I do.
   
   If I have exhausted the justifications I have reached bedrock, and my spade is turned. Then I am inclined to say: “This is simply what I do.”
   
   (Remember that we sometimes demand definitions for the sake not of their content, but of their form. Our requirement is an architectural one; the definition a kind of ornamental coping that supports nothing.)

Ludwig Wittgenstein, Philosophical Investigations, trans. G.E.M. Anscombe (Oxford, England: Basil Blackwell, 1953), §217 (p. 85).

2. Its colours are grey and red. Of grey, it occurs bluish grey and blackish grey; these are disposed sometimes in stripes or spots; and of red, it occurs brownish purple red. The surface is sometimes rendered beautifully pavonine, irised and chatoyant from the presence of Oxide of Iron.
   
   Its lustre is glimmering.
   
   It is opaque.
   
   It is amorphous.
   
   The streak is pearl grey.
   
   It is moderately hard.
   
   The fracture is fine grained uneven , passing in some specimens, into imperfectly straight foliated, and in others into earthy, and it is always slaty.
   
   It is moderately tough.
   
   It is easily frangible.
   
   The fragments are tabular and splintery, and often perfectly rhomboidal.
   
   Its specifick gravity is about 2.888.

James Freeman Dana and Samuel L. Dana. “Argillaceous Slate,” from Outlines of the Mineralogy and Geology of Boston and its Vicinity, with a Geological Map (Boston: Cummings and Hilliard, 1818), p. 57.

Fig. 2. View of Cambridge Argillite in the Boston Harbor Islands, from Peter J. Thompson et al., “Bedrock Geology of the Boston Harbor“ (January 2014).

Supplementum

• Ellen Wayland-Smith and Marcia Bjornerud, “The Languages and Lessons of Stone: A Conversation with Author and Geologist Marcia Bjornerud,” Orion Magazine (August 13, 2024).

• George Ehrenfried, “This Old Land of Cambridge,” adapted from a lecture presented at the Mount Auburn Branch of the Cambridge Public Library (April 1991).

• Patrick J. Barosh and David Woodhouse, “Geology of the Boston Basin,” Civil Engineering Practice (2011/2012): 100-236.

Fig. 3. Screenshot from The National Geologic Map Database (U.S. Department of the Interior and the U.S. Geological Survey).

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Exported from DFOS · May 21, 2026 2:46 PMWeek 20: Cambridge Argillite, Ludwig Wittgenstein, The Dana Brothers