Geology of Boston (Pre-Ice Age, 4.6 billion years ago to ~2 million years ago)
In the Beginning
Avalon and the Boston Basin
Cambridge Argillite and Roxbury Conglomerate
Avalon On Its Own
Pangaea: Together and Apart Again

In the Beginning

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The geologic history of Boston spans 4.6 billion years and thousands of miles.  For the first 4 billion or so (give or take a year or two), it is similar, if not identical to the rest of New England.  So, here is a link to an excellent web site that will explain and describe much of that early history:

A great deal happened over the course of that 4 billion years, of course, but of special note to Boston is the story of the Grenville Mountains.  These mountains, stretching from Canada to Mexico through what is now western Massachusetts, were pushed up when early North America collided with Baltica, another early continent,  forcing the crust to fold into mountains.  As soon as they formed, they began to wear down due to weathering and erosion, with the sediments flowing to both sides of the mountain range forming a wide flat coastline in the ocean where most of  New England would be in the future.  During this time enormous amounts of sediment (gravel, sand and mud) was deposited in the ocean off the coastlines and in deep sea basins. The remains of the Grenville Mountains are now in Western Massachusetts. What becomes Boston at a later date will eventually collide to this ancient formation.

Avalon and the Boston Basin 

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Around 570 million years ago, there was a subduction zone along the edge of the ancient continent of Gondwana. Note that this continent was in the far Southern hemisphere, whereas early North America (where the Grenville Mountains formed) was near the Equator. As in all subduction zones, there was a great deal of volcanic activity, and this led to the formation of the volcanic Avalon mountain chain.

Rifting occured along the edge of Gondwana, and within the Avalon area major faults and rift valleys formed. Included in this area is what we now know as the Boston Rift Basin.

Around 550 million years ago, Avalon and other microcontinents separated from Gondwana and drift North-West. At this point, Avalon was an "island arc", much like Japan, Indonesia or the Phillipines is today.

Map of the Earth 514 million years ago, as Avalon was splitting from Gondwana. Avalon is the island arc containing
New England and Nova Scotia on the map. (source: paleomap project

Cambridge Argillite and Roxbury Conglomerate

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The bedrock of the Greater Boston area consists of a variety of rock formations.  Under most of downtown Boston is the Cambridge Formation, called Cambridge Argillite (also called Cambridge Slate or Cambridge Mudstone, depending on which geologist you are talking with).  The argillite is a slightly metamorphosed, reasonably weak, layered sedimentary rock.  Slightly uphill from that,  around the city, especially south and west is the Roxbury Conglomerate. 

Remember that mountains are always being weathered and eroded as soon as they begin to grow. As the rift valley opened in Avalon, it soon became filled with sediments which later formed the Roxbury Conglomerate, the Cambridge Sandstone, and the Cambridge Argillite. These rock formations were created between 570 and 550 million years ago, when Avalon was near the South Pole.

A mountain river carrying down large smoothed rocks - this is the same process that carried down the stones that eventually became incorporated into the Roxbury Conglomerate. (Photo taken in Olympic National Park, Washington)

A close-up of Roxbury Conglomerate.

Mudflats along a tidal marsh. Mud, silt, and sand are being gently deposited at the bottom of this tidal area. Eventually this could lead to the formation of solid rocks such as mudstone or sandstone. The Cambridge Argillite initially formed in a similar way - it could have been a tidal marsh like this, or a lake or ocean floor.

Layered Cambridge Argillite and Sandstone.

Avalon on Its Own

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Avalon was only one of many volcanic island arcs and other pieces of continents that were slowly coalescing over the next several hundred million years.

Avalon collided with early North America about 380 years ago.  While happening slowly, it was by no means gentle. A tremendous amount of metamorphic activity occured as each successive piece was added to North America. The action of microcontinents smashing into North America was like a jam-up on a conveyor belt. For more information, visit

To see a very cool animation of an island arc colliding with a continent, visit this site.

Map of the world around 390 million years ago. (source: paleomap project

Metamorphic rock from Lexington. This rock probably went through this intense metamorphism around the time that Avalon smashed into North America ~380 million years ago. Basalt starts out as a fine-grained black or greenish rock. Observe the many quartz crystals and banding that occurred as a result of intense pressure and heat during that collision.

Pangaea:  Together and Apart Again

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Eventually, all the continents came together, forming the giant supercontinent of Pangaea about 250 million years ago. Avalon, where Boston is, was smooshed between Baltica and Africa on one side, and North America on the other side. This formed the Appalachian Mountains, the remains of which are still in existence. The part of the Appalachians that lie in Massachusetts lie in the western part of the state - the Berkshires. 

But nothing on the Earth’s crust lasts forever, and Pangaea started to break apart around 200 million years ago. As it began to break, many rift valleys formed. Most of this rifting ended within a relatively short period of time, leaving only the mid-Atlantic rift zone to continue to this day. During this period of intense rifting, many valleys formed that still exist today. These include the Connecticut and Hudson River valleys. Also as a result of this rifting, many small lava flows seeped through cracks to the surface. You can see some of these as intrusions of igneous rock all over Eastern Massachusetts.

After the initial breakup of Pangaea, the eastern North American plate boundary ended up well offshore of North America, at the mid-Atlantic Ridge out in the middle of the ocean. Slowly, the violent volcanic activity of earlier years quieted.  The continents began their movement towards their present locations.

From here on out, Boston’s geologic story became one of weathering, erosion, deposition.  Rivers ran from the volcanic rock formations surrounding the Boston Basin, leaving deep channels in the softer sedimentary rocks, especially the Cambridge Argillite.  

Map of the world around 237 million years ago, at the time of Pangaea. (source: paleomap project