Saturday, January 22, 2011

My Mother-in-Law Asks A Question

One summer morning, several years ago, my mother-in-law settled herself on the couch in my cottage and began tracing some of those time-worn conversational paths so familiar to people who have known each other for decades, those comfortable conversations that border on the repetitious.  Then, after taking a drink from a mug of freshly brewed coffee, she said, “You know, I’ve never understood it.  If mountains have been eroding all of these years, why isn’t the planet getting bigger with all of that eroded stuff?”

. . . a decidedly unlikely question from most mothers-in-law, but, in fact, not from my mother-in-law.  Still, I don’t think there was any lead-up to it.  Just . . . boom.  There it was.

I remember trying to describe the rock cycle with perhaps a touch of plate tectonics tossed in, but she wasn’t persuaded by my inarticulate exposition.  The idea of creation, destruction, and re-creation of rock gained no foothold, and it was my fault.

The memory of that conversation bubbled to the surface recently when I visited her in a retirement community in Hartford, Connecticut.  For nearly all of her adult life, she’s lived in the Hartford area, a landscape blessed with a dynamic geological history.  The turmoil of millions of years ago was certainly not the inspiration for her question, but that geological history relates wonderfully to what she asked.

The city of Hartford sits in the Hartford Basin (or Central Valley) on a swath of Triassic and Jurassic bedrock running from New Haven in the south, through Hartford, and continuing northward into Massachusetts.  This wedge, narrow at New Haven and expanding to the north, dominates the center of the state.

My understanding of what happened here depends primarily on geologist James W. Skehan’s Roadside Geology of Connecticut and Rhode Island (2008).  Errors of fact and interpretation are all mine.

(An aside:  Skehan is a very interesting figure.  This Harvard educated geologist and professor emeritus of Boston College is also a Jesuit priest who has written widely on the relationship between religion and science.  To give a bit of the flavor, in Modern Science and the Book of Genesis, published in 1986 by the National Science Teachers Association, he writes, “If we were to misrepresent the Bible as a scientific presentation, rather than as a theological document of Judeo-Christian religious history, we would do a great disservice to religion.”)

The Hartford Basin is the product of the unraveling of the supercontinent Pangea which began about 200 million years ago.  Some 50 million years earlier, the process of creating this single supercontinent had ended, a process that had launched mountain building over millions of years.  Now, as Pangea came apart along various fault lines, basins formed, including the North Atlantic Ocean Basin and the Hartford Basin, and as their thin crusts stretched and parted, magma flowed up to the surface.  In the Hartford Basin, ridges of basalt protrude from the basin floor, marking the remnants of the three principal lava flows that occurred here between 195 and 185 million years ago.  The most spectacular of the lot was the flow that created the Holyoke Basalt formation; its ridges dominate.  The rest of the floor is largely Jurassic sedimentary rock, material that eroded from the uplands to the east and west of the Hartford Basin.

In his general description of the interplay between the rock cycle and plate tectonics, Skehan describes succinctly the geologic process that played out here:
The forces inside the earth drive the rock cycle because compression builds mountains to be eroded and extension forms basins where sediments accumulate.  (Roadside Geology of Connecticut and Rhode Island, p. 5)
In the section from the 1985 bedrock geologic map of the state shown below, the wedge that is the Hartford Basin is prominent, a couple of very broad brush strokes of white and pale beige coloring.  Reddish fingers in the Basin are basalt bedrock.  (The map, published by the Connecticut Geological and Natural History Survey, is available from the U.S. Geological Survey’s National Geologic Map Database.)

The formation colored white (if labels were visible on the map above, this one would be “TRnh”), occupying a substantial portion of the southern portion of the wedge beginning on its western edge, is the New Haven Arkose from the Late Triassic.  The Early Jurassic Portland Arkose (pale beige, labeled “Jp”) and East Berlin Formation (darker tan, “Jeb”) make up much of the northern end.  This is all sedimentary rock, much of it sandstone and siltstone. 

(An aside:  Quarries in the Basin’s Portland Arkose formation produced the brown sandstone (which came to be called brownstone) used to construct many of the brownstones in New York City.  David B. Williams devotes a chapter to brownstone in his book Stories in Stone (2009), and he’s posted about it on his blog as well.)

The many substantial orangey-red finger-like extensions in the wedge, running roughly from the south toward the north-northeast, are Holyoke Basalt (“Jho”).  The fewer and narrower darker red strips are Talcott Basalt (“Jta”).  At places, they abut the Holyoke Basalt on the west; elsewhere, in the far west of the Basin, they stretch along its western boundary.  Both of these are from the Early Jurassic when Pangea was dismantling.

So appropriate that my mother-in-law’s retirement community sits on a ridge overlooking a portion of the Hartford Basin.  The ridge is called a mountain but hardly merits that designation.  The road to the top twists and turns, hugging a road cut exposing nearly vertical columns of rock, columns that, on occasion, must shed a chunk or two onto the asphalt.

According to the 1985 bedrock map of Connecticut, this exposed rock is Holyoke Basalt (orangey-red, "Jho").

The black arrow points to the approximate location of the retirement community on the basalt ridge.  Among the other prominent bedrock formations shown above are the Portland Arkose (pale beige, “Jp”), the East Berlin Formation (darker tan, “Jeb”), and the Hampden Basalt (light pink, "Jha"); all are from the Early Jurassic.

On my first visit to the retirement community, I took a walk along the road cut and collected a few pieces of rock.  The specimens are not particularly impressive and, in my geological ignorance, I have assumed they are all basalt, a tentative conclusion somewhat supported (well, not rejected) by a couple of colleagues who, at least, talk a good game of geology.  Here are two of the rocks.

I think much of what I saw in the road cut matches the specimen on the left.  In contrast, that on the right has a distinctive sheen, smooth to the touch, as well as some apparent layering.

Have I broached my mother-in-law’s old question with her after all these years?  No.  I doubt it’s of any interest and I suspect I would flub the explanation again.  Rather, I simply appreciate the whimsy of it all, having that exposure of Jurassic basalt outside her door.


  1. Very nice post.

    I've never worked on the Hartford Basin at all, but a possibility for your layered rock is that it's a contact metamorphosed sediment from just outside the basalt flow (or maybe a sediment xenolith from within the basalt, which would also be metamorphosed).

  2. Thank you. Your suggestions make sense and reinforce my appreciation (but not my understanding) of geological complexity, particularly at the margins where different rocks, minerals, and forces come into play.


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