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    • What Are We Digging?
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What Are We Digging?

As the Central Artery/Tunnel Project built the new underground expressway through downtown Boston, variations in thickness and lateral extent of soils, rock, and other materials along the highway made tunneling especially challenging. A typical tunnel section reached 85 feet deep and dug through four types of soil to rest on solid bedrock.

The tunnel's deepest point was 120 feet below street level, where the highway passes under the Red Line subway tunnel at Atlantic Avenue and Summer Street. The shallowest point was near State Street where the tunnel roof reaches ground level. 

Fill
Most of downtown Boston is built on landfill placed between the late 1700s and the 1960s. Soil mixed with bricks, wood, ash, concrete, gravel, sand, and other materials was dumped in low-lying areas and fronted by piers and bulkheads.

Organics
Beneath the fill is a layer of organics, including silt, sand, and peat. These organics were deposited between 12,000 and 8,500 years ago during glacial melting.

About 12,000 years ago the glacial ice sheets that covered the land began to melt, releasing the land below from a great weight that had pushed some areas below sea level. The resulting "isostatic rebound" saw these areas rise back above sea level over the following 3,500 years. Vegetation sprouted on this exposed land in meadows, ponds, and salt marshes much like those on present day Cape Cod.

Approximately, 8,500 years ago, as the glaciers continued to melt, the sea overtook the land, covering its vegetation with bay mud rich in marine life. The joining of the land vegetation and the sea's mud formed dark brown or black organic deposits peppered with shells.

Clay
A marine clay known as Boston Blue Clay lies below the organic layer. The most widespread soil in the Boston area, Boston Blue Clay is composed of fine particles transported by glacial meltwater and deposited in deep ocean waters between 37,000 and 12,000 years ago.

Glacial Till
For nearly a million years during the last ice age, towering sheets of ice advanced and retreated, grinding away the Boston area's soil down to bedrock. The bottom of this 1,000 foot tall ice sheet deposited boulders, sand, silt gravel, and clay atop the bedrock to form a dense layer called till. 
 
Bedrock
The walls of the underground Central Artery rest on bedrock. The majority of the bedrock around Boston Harbor is a metamorphosed shale called Cambridge Argillite. Deposited 500 to 600 million years ago, volcanic activity, earthquakes, and mudslides laced the shale with layers of ash, igneous rocks, and other deposits.

See Facts and Figures for more dirt information. Also, check out our archaeology program section.