Roxbury Puddingstone, the Massachusetts state rock

Why this time of year

It is bedrock and old building stone, so in the Boston area it is there to see in every month, in rock ledges and cliffs and in some old stone structures.

How to notice it

1. In Roxbury, Dorchester, Brookline, and nearby parts of the Boston Basin, look at bold rock ledges and cliffs. In that area the rock was also used more than other kinds for retaining walls, bridge abutments, and stone arches.
2. Look closely at the rock face: puddingstone is full of rounded pebbles, some as big as cobbles, held firmly in the rock around them, which gives it a rough look.
3. Compare the pebbles with each other. They are pieces of older rocks, including volcanic rock, granite, and quartzite, so they do not all match.
4. A 1976 U.S. Geological Survey guide says most of the churches in Boston's Back Bay were built of this stone, so the old church walls there are worth a look.

Sources

• malegislature.govMassachusetts General Laws, Part I, Title I, Chapter 2, Section 22 (primary, statute; read via curl 2026-11-10). Section title 'Rock or rock emblem of commonwealth'. Text: 'The Roxbury Puddingstone (Roxbury Conglomerate), shall be the rock or rock emblem of the commonwealth.' Supports the title, the deck's 'official rock of Massachusetts by state law', and the two names. The page does not show the year of enactment.
• pubs.usgs.govUSGS Bulletin 1476, Clifford A. Kaye, The Geology and Early History of the Boston Area of Massachusetts, A Bicentennial Approach, 1976 (primary, federal; read via curl + pdftotext). 'Gravel was interlayered with the clay and cemented into a hard rock called "conglomerate," locally called "puddingstone."' (gloss, deck). 'This rock crops out widely in Roxbury, Dorchester, and Brookline.' (step 1). 'Hundreds of millions of years have elapsed since these deposits were laid down' (deck 'old'). The harbor 'is part of a large topographic depression, commonly referred to as the Boston Basin.' (deck). 'Harder volcanic rocks and conglomerates stand out as hills and knobs'. 'It is a mottled brown and, because of the embedded round pebbles, has a rough appearance.' (step 2). 'Most of the churches in the Back Bay were built of this stone.' (why_now, step 4). Origin as worded here: rivers 'also carried gravel, which was deposited in flood plains' (a scanned line is missing after this) and the setting is called 'remote Paleozoic time'; both are superseded on age by Thompson 2014, so neither is on the card.
• pubs.usgs.govUSGS Bulletin 839, LaForge, Geology of the Boston Area, Massachusetts, 1932 (primary, federal; read via curl + pdftotext). 'The main area of the formation occupies nearly all of Roxbury and large parts of Dorchester, West Roxbury, Brookline, Brighton, and Newton.' 'As a rule the rock is resistant and forms bold hills with many ledges and cliffs.' (step 1). The Brookline member's base contains 'cobbles and small boulders' which 'for the most part rather well rounded' (deck 'rounded pebbles and cobbles', step 2). 'the pebbles are generally so firmly embedded in the matrix that blocks can be trimmed' (step 2 'held firmly'). Used 'for such structures as retaining walls, bridge abutments, and stone arches.' (why_now, step 1). Also: 'Except at one locality in Watertown it is not exposed on the surface or in excavations north of the Charles River.'
• ajsonline.orgThompson, Ramezani, and Crowley, American Journal of Science v. 314, June 2014, p. 1009-1040 (primary, peer-reviewed; Wellesley/MIT/Boise State; read via curl + pdftotext). Clasts: 'Volcanic rocks including volcaniclastic sediments of Lynn-Mattapan origin dominate the clast assemblage' and 'granite and quartzite are typically present among the largest clasts' (step 3). Coarse clasts 'locally reaching 20 to 30 cm'. Age (notes only): youngest detrital zircon gives 'the best maximum age estimate' of 595.14 +/- 0.90 Ma, and intrusions of 584.19 and 585.37 Ma 'represent minimum ages of associated conglomerate'. Setting: 'Roxbury Conglomerate accumulated in fault-bounded intra-arc basins near the end of this magmatic cycle.' It also records that the older Carboniferous reading rested on tree-trunk fossils 'later discredited'.
• boston.govCity of Boston Parks and Recreation, Urban Wilds: Roxbury, last updated 3/5/26 (primary, city). Rockledge Urban Wild 'is characterized by a wooded puddingstone outcrop that connects a lower and upper clearing.' Buena Vista (Warren Gardens) Urban Wild is 'Comprised of a 50-foot rock cliff'. Shows a public walker can see puddingstone outcrops in Roxbury today, and supports 'woodland-edge' as a habitat. Place names are for notes only.

Checked against these sources on November 10, 2026.

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