Roxbury Conglomerate

The Roxbury Conglomerate, also informally known as Roxbury puddingstone, is a name for a rock formation that forms the bedrock underlying most of Roxbury, Massachusetts, now part of the city of Boston. The bedrock formation extends well beyond the limits of Roxbury, underlying part or all of Quincy, Canton, Milton, Dorchester, Dedham, Jamaica Plain, Brighton, Brookline, Newton, Needham, and Dover. It is named for exposures in Roxbury, Boston area. It is the Rock of the Commonwealth in Massachusetts.

The Roxbury Conglomerate comprises the lower part of the Boston Bay Group, which is a 5,000 meter thick (3 miles) sequence of sedimentary rocks that fill the Neoproterozoic Boston Basin in eastern Massachusetts. The upper part of the Boston Bay Group consists of the Cambridge Argillite, which overlies the Roxbury Conglomerate. The Roxbury Conglomerate traditionally has been subdivided into three subdivisions; (1.) basal Brookline Member (conglomerate and sandstone), (2.) medial Dorchester Member (mostly sandstone with minor conglomerate) and (3.) upper Squantum Member (largely diamictite). However, these three subdivisions of the Roxbury Conglomerate complexly interfinger with each other and lack the simple layer cake distribution that past studies have described. The Brookline Member of the Roxbury Conglomerate is the classic ‘puddingstone’ that is typically discussed and illustrated in popular web pages, articles, and other publications. It is about 150–1,300 m (490–4,300 ft) thick and consists of massive clast supported pebble and cobble conglomerate beds interbedded with beds of argillite and sandstone. The conglomerates consist of grey feldspathic sand and well rounded pebbles and cobbles of quartzite, granite, felsite, and quartz monzonite. The 'puddingstone' of the Brookline Member is complexly interbedded with layers of laminated and graded argillite and sandstone and massive diamictite. The Dorchester Member consists of purplish, greenish and grey siltstone, sandstone, and medium to fine grained argillite. As traditionally defined, it is 180–500 m (590–1,600 ft) thick and dominated by medium to fine grained argillite. It contains lesser amounts of sandstone and conglomerate than the Brookline Member. The sandstone beds within this member commonly exhibit full or partial Bouma sequences. The beds within this member commonly exhibit evidence of penecontemporaneous deformation due to downslope slumping. The gravel size portion of the Squantum Member diamictites consists of range from sub rounded to angular clasts, 5–60 cm (2–24 in) in diameter, to well rounded clasts 3–8 cm (1.1–3 in) in diameter. They are composed of multicoloured, locally derived felsic and mafic volcanic rocks, granodiorite, quartzite and massive, graded and laminated sandstone and siltstone. The sand and gravel sized fraction of the diamictites consist of volcanic, granitic and metasedimentary lithic fragments that have the same composition as the sediments of the Brookline and Dorchester members. Glacially striated pebbles, chattermarked quartz grains and dropstones have been reported from these diamictites. However, none of these reports have been substantiated by later research. For example, previously identified dropstones have been re interpreted as having been emplaced by lateral sediment gravity or current processes. The Roxbury Conglomerate has been significantly altered by metamorphism. Metamorphism has altered its sedimentary rocks to subgreenschist facies and created a slaty, well developed, spaced cleavage that oriented approximately perpendicular to bedding within it. Typically, tectonism has flattened, stretched, indented, and fractured the pebbles and associated matrix of the Roxbury Conglomerate to the point that it often has the appearance of flow structure.

The strata of the Boston Bay Group lie within a faulted bounded basin. They are bounded by normal faults to the north and west. Within the Boston Basin, Roxbury Conglomerate lies unconformably upon Dedham Granite, Westwood Granite, and much older Middlesex Fells Volcanic Complex. To the south, the Roxbury Conglomerate unconformably overlies Mattapan Volcanic Complex, which, in turn, overlies the Dedham Granite. The Dedham Granite, Westwood Granite, and older rocks are overlain by the calc alkaline Brighton Volcanic rocks, which consist of the altered basalt and andesite flows, pyroclastic rocks, breccia, tuff, and intrusive rocks. The upper part of the Brighton Volcanic rocks sporadically interfinger and interbedded with the Brookline and Dorchester members throughout the basin in the southern portion of the basin.

The Roxbury Conglomerate is typically unfossiliferous. The only fossils which have been reported from it are 0.5–3.5 cm (0.2–1.4 in) in diameter raised ring structures found in outcrops at Hewitt's Cove and Slate Island and two dislocated stromatolite hemispheroids found in laminated mudstones at Squantum Head.

In the 19th and early 20th centuries it was frequently used to construct walls and house foundations in the Boston area; some of the stone was quarried in Brighton and Newton, but the most extensive workings were those in Roxbury. The American poet Oliver Wendell Holmes wrote a poem called "The Dorchester Giant" in 1830, and referred to this special kind of stone, "Roxbury puddingstone", also quarried in Dorchester, which was used to build churches in the Boston area, most notably the Central Congregational Church (later called the Church of the Covenant) in Boston's Back Bay neighborhood. Roxbury puddingstone is the official rock of Massachusetts. Puddingstone Park is a neighborhood park built as part of the redevelopment of a former puddingstone quarry in the Mission Hill neighborhood of Boston.