Excerpt from The Geology and Early History of the Boston Area of Massachusetts: A Bicentennial Approach

The softest Boston rocks are argillite (related to slate and shale but harder) and volcanic ash. The argillite was originally de posited as clay in either a lake or marine embayment; the volcanic ash was blown out Of the many volcanoes that were active in the area during the time the clay or mud was being deposited.

Gravel was interlayered with the clay and cemented into a hard rock called conglomerate, locally called puddingstone. This rock crops out widely in Roxbury, Dorchester, and Brookline. Also interlayered with these sediments were volcanic flows, ashes, cin ders, and the great variety of deposits formed by volcanoes. These deposits - now hard rock - are best seen in nearby Mattapan, Hyde Park, Milton, Lynn, and Saugus.

Hundreds of millions of years have elapsed Since these deposits were laid down, and the ash and mud not only has been com pressed, cemented, and solidified into rock, it has also been deeply buried by later deposits and drawn deeper into the Earth's interior, where it was squeezed, folded, broken, and turned up on end. Today, these badly distorted rocks are at the surface again only because of the erosion Of the many thousands of feet of rock that once overlay them.

What was the ancient landscape like at the time the Boston rocks were being laid down? In all probability, the region was a broad lowland surrounded by hills or mountains of granite. A large body of water, either a large lake or perhaps an arm of the sea, occupied part of the lowland. Small volcanoes and at least one large conethat rose a mile or more in height dotted the plain and surround ing uplands. The ash from these volcanoes blanketed the plain and was carried by the rivers to the lake or sea where it mixed with silt and clay that was being carried by rivers from the upland. The rivers descending onto the plain from the surrounding high lands also carried gravel, which was deposited in flood plains and which makes up much of the Roxbury puddingstone.

This landscape of remote Paleozoic time can be matched with landscapes existing today. One can see similarities in the Puget Sound lowland and the nearby Cascades in the State of Washing ton; the Sacramento Valley, with Marysville Buttes and Mounts Shasta and Lassen in the distance; the Imperial Valley, at the head of the Gulf of California depression, where volcanic centers existed until very recent time; the PO River Plain in northern Italy, where a volcanic center existed not long ago near Rimini; and many of the great rift valleys of Africa that contain deep lakes and nearby active volcanoes.

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