Also known as puddingstone, the Roxbury consist of varicolored pebbles, cobbles, and boulders suspended in a fine grained matrix. In the field, the rock ranges in color between light blue-gray and dark gray. Water often stains the iron in many of the blocks to a red-orange color. This rock might be more properly called ‘church stone’; over 35 churches were built in the Boston area with Roxbury Conglomerate in the 19th century. These include Christ Church in Brookline, First Church on Elm in Jamaica Plain, and St. James Episcopal Church in Roxbury.
Close up of the puddingstone
The Roxbury is to geologists what the dropped ‘R’ is to linguists, a sign that you are in Boston, for the puddingstone only occurs in and around the Hub. And like this linguistic trait, no one knows exactly where these rocks originated. It shares some affinities with rocks in west Africa. But it also shows traits of South American rocks. About the only thing that geologists agree on is that the Roxbury was deposited between 570 and 600 million years ago.
Like most sedimentary rocks, the Roxbury Conglomerate is the product of the breakdown of other rocks. Recent analysis indicates that it may have originated in a landscape that looked like modern-day Japan, with a range of volcanoes separated from the sea by a flat plain. Like all mountains, they were involved in a battle between uplift and erosion. And like all mountains, erosion won. Streams washed these eroded bits and pieces into a chaotic mix of sediments at the base of the mountains. In some places, gray to green lava flows abut the puddingstone, indicating that some of the volcanoes were active with molten rock flowing off their slopes.
The Roxbury sits in the middle of a suite of rocks that includes granite found in Dedham and muddy slates that occur in Braintree. These rocks formed between 650 and 505 million years ago on, under, and in oceans bordering the drifting land mass known as Avalon. In its slow movement toward North America, Avalon may also have picked up a hitchhiker or two, which further complicates its history. Ongoing research, especially finding precise dates for all these rocks, continues to clarify details of the Avalonian picture.
Others though do not even agree with these theories. In The Dorchester Giant, Oliver Wendell Holmes wrote:
They flung it over the Roxbury hills,
They flung it over the plain,
And all over Milton and Dorchester, too
Great lumps of pudding the giants threw:
They tumbled on thick as rain.
Giant and mammoth have passed away,
For ages have floated by;
The suet is hard as a marrowbone,
And every plum is turned to a stone,
But there the puddings lie.
No matter what you want to call the stone, it is a nice looking rock and apparently a better stone to work with than the Leesburg Conglomerate.