Roadside Geology of Boston: The Medford Dike and the Breakup of Pangaea

A roadcut is an excavation into a hill or mountain created by civil engineers, usually through blasting, for the purposes of building a road. Fortunately, for urban dwellers, such as myself, living in a region where structure is not well exposed, a great deal of geology is visible through roadcuts. This is especially true in the densely-populated, paved-over, heavily-vegetated, highly-eroded and glacially-scoured world of Greater Boston.

Roadcut exposing the dark gray Medford Dike,
as it intrudes its way out from the bowels of Interstate 93 South

In Annals of the Former World, John McPhee, a Pulitzer Prize winner, wrote “The roadcut is a portal, a fragment of a regional story, a proscenium arch that leads imagination into the earth and through the surrounding terrane. In the rock itself are the essential clues to the scenes in which the rock began to form. Unfortunately, highway departments tend to obscure such scenes. They scatter seed wherever they think it will grow. They “hair over everything”---as geologists around the country will typically complain."

Personally, when I see a roadcut, it’s like seeing the Holy Grail. It’s a chance to view the landscape of the ancient past. It's an opportunity to "read the rocks." Roadcuts aren't to be missed. Take a ride with a geologist, and you'll see what I mean. Nothing else matters, with my car lurching and weaving, and my wife admonishing me with “Keep your eyes on the road!” I’ve only got one good shot at seeing that roadcut, as we go careening by on the highway. Better make it good!

McPhee goes on to say, “Without roadcuts, all you could do is drill a hole, or find natural streamcuts, which are few and far between.” Fortunately, living in New England, roadcuts are relatively abundant. McPhee continues, “A roadcut is to a geologist, as a stethoscope is to a doctor. An X-ray to a dentist (he struck home on that note). The Rosetta Stone to an Egyptologist. A twenty dollar bill to a hungry man.” Anyway, you get the point.

Recently, I drove past a “famous” local roadcut. It contained the Medford Dike, a few miles north of Boston. The dike qualifies as a famous rock formation in New England, certainly amongst geologists. The only other geologically well-known structure that comes to mind (and I’m sure there are others) is the “Old Man of the Mountains” in New Hampshire, a granite rock-face (literally) that was decapitated by the forces of erosion and mass wasting in 2003. Except that the Old Man is (or was) known more for historical and touristic aspects than those strictly geological. 

Losing-face doesn’t prevent the State of New Hampshire from missing the old guy. He’s still on their license plates.

Anyway, back to the Medford Dike. I must admit that I knew the roadcut was coming. I was fully prepared and driving alone. The dike is best seen from the southbound lane of Interstate 93 at mile marker 23.6. The speed limit is 65 mph, but everyone does 80. Enough time for one good shot! Heading south, I had my passenger window rolled down, camera in hand, shutter speed set to 1/1000, and my left hand on the wheel. No one to admonish me on this one!

The 375 foot-wide Medford Dike is a volcanic, “feeder” comprised of biotite gabbro and dated at about 190 million years of age. Its magma intrudes the Lynn Volcanic rocks and is one of 1,000 to 2,000 basaltic dikes that intruded rocks of the Avalon terrane in the vicinity of eastern Massachusetts. This was the time when the newly-formed Atlantic Ocean began to spread at the consequence of the supercontinent of Pangaea, which was rifting apart.

Geologic map of the Pine Hill area in Medford, Massachusetts.

The Medford Dike (Jd) can be seen trending northeasterly. Note the swarms of other intrusive dikes in the vicinity and the transection of the dike with I-93.

(From Wilson, 1901)

About 230 million years ago in the Middle to Late Triassic, Pangaea began to initially fragment at the southeastern portion of North America. Pangaea began to split apart along deep-seated faults. The main zone developed into the North Atlantic Ocean basin between Africa and North America, but numerous major rifts also formed along the edge of the newly-forming circum-Atlantic continents. Along the east coast of North America (then south-facing), a complex rift-system developed with many northerly-trending basins, some onshore, and some now submerged beneath the ocean or buried on land.



Seen here in the earliest Jurassic, Pangaea has initiated its break up, beginning with the North Atlantic.



Around 201 Ma, the time of the Medford Dike, magmatic injections in the form of a giant dike swarm began somewhat synchronously with the progression of rifting to the north-northeast. In so doing, the Triassic-Jurassic rift system in eastern North America was formed. The sedimentary fill and lava flows of the rift basins, fed by such dikes as the Medford, are collectively known as the Newark Supergroup and belong to the multi-continental rift system of the Central Atlantic Margin (CAM) system.

This illustration shows the formation of dikes and sills, intrusive rocks containing coarse-grained, mafic rocks such as gabbro. The extrusive flows at the surface fed by the feeder-system have long since eroded away.

(Modified from the University of Toledo’s Website)

The dikes and basins of the rift system are indicative of stretching of the continental crust at the time that the Atlantic Ocean was beginning to form and signaled the dispersal of the continents that we know today throughout the globe. Eventually, North America and South America separated from Africa, creating the Atlantic Ocean by beginning to rift apart initially in the North Atlantic basin.



The breakup of Pangaea has initiated in this 195 million year perspective. The arrows indicate
the drifting that is occurring between the continents of North America and Africa 
that are about to form. The region of Greater Boston is located within the ellipse.
As the young Atlantic Ocean began to form, numerous rift basins developed 
parallel to the rift. This was a time of subsidence and sedimentation within the rift basins,
and flood basalts. The extrusive flows were fed to the surface by a massive, 
intrusive magmatic system with "feeders" such as the Medford Dike.
(Modified from Ron Blakey, Colorado Plateau Geosystems, Inc.
and through the generosity of Wayne Ranney)

Before rifting of Pangaea took place, Boston was located far inland, within the epicenter of the huge Pangaean supercontinent. The only reason that Boston is on the Atlantic seaboard today is because rifting initiated right near it. 

CAM rift basins (black) of the circum-North Atlantic shown on an early pre-drift reconstruction.

So, when you drive by the Medford Dike, let it be a reminder of the time when Pangaea was about to break up, and give birth to the Atlantic Ocean, dispersing the contemporary continents throughout the globe. The Medford Dike is also easily visible (and more safely) by hiking the Pine Hill area in the town of Medford. There, you can observe the dike as part of the bedrock rather than as a roadcut. If you choose to photograph it from I-93 South, like me, remember to “Keep your eyes on the road!”