Neponset River Gorge - Boston-Milton, MA - Places of Geologic Significance on Waymarking.com

In Boston and Milton, along the Neponset River Greenway, is a windy gorge where the Neponset River passes through, and a sign on a bridge over the river discusses the geology of this location.

The Neponset River Greenway is a pedestrian walkway that runs along the river from its mouth up currently to Mattapan. From the junction of Adams Street and Eliot Street, first walk on the Adams Street Bridge over the Neponset River. This is not the bridge with the sign, but it is a great way to see the gorge on the west side. The river flows under a large brick building on the left and under the bridge, then, the river turns sharp left, and disappears going right. Now walk back to the intersection and turn left (east) onto the Neponset River Greenway, and walk about 200 ft to a bridge over the Neponset River and the sign on the left.

The sign has the following text:

"The rock of Milton Hill and the remnant outcrop we find here formed approximately 600 million years ago when the North American plate was still connected to the African plate. Deposited as horizontal layers of sand and gravel, the sedimentary sandstones and shales and the metamorphic conglomerates were later tilted to the south. The accompanying photos show details of these rocks that can be seen upon exploring the site.

The current shape of the Neponset River Valley is the result of the last great Ice Age, the Wisconsin Glacier which ended 10,000 years ago. Ice covered New England in a sheet one mile thick and the ocean was 400 feet lower than today. When the glacier melted, the ocean level rose creating the present coastline.

The aerial photograph clearly shows the tortuous path the Neponset River follows on its trip to the sea. The hard conglomerate rock shoulder of Milton Hill ot the east forced the river to turn north until it encountered a less resistant layer of sedimentary shale. The river cut through the shale layer to the east and south until it reached the plain of what is now the river estuary."

The first photograph shows a greyish rock with a round stone in it. The caption explains:

"This conglomerate contains pebbles of older rock that became rounded as they tumbled along a riverbed. The pebbles are similar to lavas and volcanic ashes that make up the bedrock in nearby Mattapan. The conglomerate found here is dominated by reddish andesite pebbles, and occasional large granite cobbles. It is quartzite-free, distinguishing it from the nearby 'Roxbury Puddingstone.'

The next image is a layered grey rock with texture. The text on this is as follows:

"The undulating surface of this sandstone can best be seen on a rainy day when the delicate ripples are dramatized. These features formed as currents in an ancient river disturbed the sandy bottom to create ripples like the ones visible in modern stream channels. The current flowed toward the north over the flat0lying layers before they were thrust upward."

The third image is of the river and the shore. The text next to it is as follows:

"This photo shows the broken shale and sandstone that crumbled as the river cut and eroded the river channel. Larger more resistant metamorphic rocks make up the cliffs seen on either side of the gorge."

If you walk east, you will end up in the flat estuary an area of hummocks of reed grass to the Granite Avenue bridge and beyond. If you walk west, across Adams Street, you will first find the Baker dam blocking the Neposnet River, then further west the path runs to Mattapan. Along the path are sample boulders of the rocks that are pictured on this sign.

Waymark is confirmed to be publicly accessible: yes Requires a high clearance vehicle to visit.: no Requires 4x4 vehicle to visit.: no Public Transport available: yes Website reference: [Web Link] Parking Coordinates: Not Listed Access fee (In local currency): Not Listed

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