The Roxbury Conglomerate, also informally known as Roxbury puddingstone, is a name for a

rock formation A rock formation is an isolated, scenic, or spectacular surface rock outcrop. Rock formations are usually the result of weathering and erosion sculpting the existing rock. The term ''rock formation'' can also refer to specific sediment ...

that forms the bedrock underlying most of

Roxbury, Massachusetts Roxbury () is a Neighborhoods in Boston, neighborhood within the City of Boston, Massachusetts. Roxbury is a Municipal annexation in the United States, dissolved municipality and one of 23 official neighborhoods of Boston used by the city for n ...

, now part of the city of

Boston Boston (), officially the City of Boston, is the state capital and most populous city of the Commonwealth of Massachusetts, as well as the cultural and financial center of the New England region of the United States. It is the 24th- mo ...

. The bedrock formation extends well beyond the limits of Roxbury, underlying part or all of Quincy, Canton,

Milton Milton may refer to: Names * Milton (surname), a surname (and list of people with that surname) ** John Milton (1608–1674), English poet * Milton (given name) ** Milton Friedman (1912–2006), Nobel laureate in Economics, author of '' Free t ...

, Dorchester, Dedham,

Jamaica Plain Jamaica Plain is a neighborhood of in the City of Boston, Massachusetts, United States. Settled by Puritans seeking farmland to the south, it was originally part of the former Town of Roxbury, now also a part of the City of Boston. The commun ...

Brighton Brighton () is a seaside resort and one of the two main areas of the City of Brighton and Hove in the county of East Sussex, England. It is located south of London. Archaeological evidence of settlement in the area dates back to the Bronze A ...

Brookline Brookline may refer to: Places in the United States * Brookline, Massachusetts, a town near Boston * Brookline, Missouri * Brookline, New Hampshire * Brookline (Pittsburgh), a neighborhood in Pittsburgh, Pennsylvania * Brookline, Vermont See ...

Newton Newton most commonly refers to: * Isaac Newton (1642–1726/1727), English scientist * Newton (unit), SI unit of force named after Isaac Newton Newton may also refer to: Arts and entertainment * ''Newton'' (film), a 2017 Indian film * Newton ( ...

, Needham, and

Dover Dover () is a town and major ferry port in Kent, South East England. It faces France across the Strait of Dover, the narrowest part of the English Channel at from Cap Gris Nez in France. It lies south-east of Canterbury and east of Maidstone ...

. It is named for exposures in Roxbury, Boston area.Zen, E., R. Goldsmith, Richard N. M. Ratcliffe, P. Robinson, R. S. Stanley, N. L. Hatch Jr., A. F. Shride, E. G. A. Weed, and D. R. Jones (1983
"Bedrock geologic map of Massachusetts".
Special Map, 3 sheets, scale 1:125,000. U.S. Geological Survey, Reston, Virginia.Goldsmith, R. (1991

In N. L. Hatch, ed., pp. E1–E62, Chapter E, "The bedrock geology of Massachusetts". Professional Paper. no. 1366-E. U.S. Geological Survey, Reston, Virginia.

Description

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 The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...

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 Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

), (2.) medial Dorchester Member (mostly sandstone with minor conglomerate) and (3.) upper Squantum Member (largely

diamictite Diamictite (; from Ancient Greek ''δια'' (dia-): ''through'' and ''µεικτός'' (meiktós): ''mixed'') is a type of lithified sedimentary rock that consists of nonsorted to poorly sorted terrigenous sediment containing particles that ra ...

). 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.Emerson, B. K. (1917) "Geology of Massachusetts and Rhode Island. United States". ''Bulletin no. 597'', U.S. Geological Survey, Reston, Virginia.Dott, R. H. (1961) ''Squantum ‘Tillite’, Massachusetts, evidence of glaciation or subaqueous mass movement?'' Geological Society of America Bulletin. 72(9)1289–1306.Billings, M. P. (1976) '"Geology of the Boston Basin". In: J. W. Skehan and D. P. Murray, eds., pp. 5–30, ''Studies in New England Geology''. Memoir no. 46. Geological Society of America, Boudler, Colorado.Socci, A. D., and G. W. Smith (1990) "Stratigraphic implications of facies within the Boston Basin". In: A. D. Socci, J. W. Skehan, and G. W. Smith, eds, pp. 55–74, ''Geology of the Composite Avalon Terrane of Southern New England''. Special Paper no. 245. Geological Society of America, Boudler, Colorado.Carto, S. L., and N. Eyles (2011) "Chapter 43 The Squantum Member of the Boston Basin, Massachusetts, USA". In: E. Arnaud, G.P. Halverson, and G. Shields-Zhou, eds. pp. 475–480, ''The Geological Record of Neoproterozoic Glaciations''. Memoirs no. 36. Geological Society, London, England. 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 A pebble is a clast of rock with a particle size of based on the Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than granules ( in diameter) and smaller than cobbles ( in diameter). A rock made predominant ...

and

cobble Cobble may refer to: * Cobble (geology), a designation of particle size for sediment or clastic rock * Cobblestone, partially rounded rocks used for road paving * Hammerstone, a prehistoric stone tool * Tyringham Cobble, a nature reserve in Tyr ...

conglomerate beds interbedded with beds of

argillite :''"Argillite" may also refer to Argillite, Kentucky.'' Argillite () is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts ...

and sandstone. The conglomerates consist of grey feldspathic sand and well-rounded pebbles and cobbles of

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

, felsite, and

quartz monzonite Quartz monzonite is an intrusive, felsic, igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. It is typically a light colored phaneritic (coarse-grained) to porphyritic granitic rock. The plagioclase ...

. The 'puddingstone' of the Brookline Member is complexly interbedded with layers of laminated and graded argillite and sandstone and massive diamictite.Bailey, R. H. (1987) "Stratigraphy of the Boston Bay Group, Boston Area, Massachusetts". In: D. C. Roy, ed., pp. 209–212, Northeastern Section of the Geological Society of America. ''Centennial Field Guide no. 5''. Geological Society of America, Boudler, Colorado.Smith, G. W., and A. D. Socci (1990) "Late Precambrian sedimentary geology of the Boston Basin". In: A. D. Socci, J. W. Skehan, and G. W. Smith, eds., pp. 75–84, ''Geology of the Composite Avalon Terrane of Southern New England''. SpecialPapers no. 245. Geological Society of America, Boudler, Colorado. The Dorchester Member consists of purplish, greenish and grey

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

, 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 well-known Squantum Member is also known as the Squantum diamictite, Squantum 'Tillite', Squantum Tillite, or the Squantum Tilloid. It consists largely of diamictites that are a heterogeneous and poorly sorted admixture of rare boulders up to 1.2 m in diameter, pebbles, cobbles, and sand in a silty-clay matrix. These diamictites occur as beds, which range in thickness from 18 to 215 m (60 to 705 ft) and are typically interbedded with purplish, greenish and grey siltstone, sandstone, and medium-to fine-grained argillite. Typically, the diamictites are mostly massive and lenticular in form; some are crude to moderately well sorted; and some exhibit normal grading. Most of the diamictite outcrops exhibit chaotic bedding in the form of contorted and folded patches of sand, local clusters of gravel, and coherent slump blocks of

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

. In outcrops,

lapilli Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range f ...

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

beds have also been identified in close association with the diamictite of the Squantum Member. At Squantum Head, the diamictite is interbedded with laminated mudstone beds that are 2 to 10 cm (0.8 to 4 in) thick. Some of laminated mudstone beds contain a few outsized pebbles or cobbles that have depressed the underlying laminae and giving them the appearance of

dropstone Dropstones are isolated fragments of rock found within finer-grained water-deposited sedimentary rocks or pyroclastic beds. They range in size from small pebbles to boulders. The critical distinguishing feature is that there is evidence that the ...

s.Thompson, M. D. (1993) "Late Proterozoic stratigraphy and structure in the Avalonian magmatic arc southwest of Boston, Massachusetts". ''American Journal of Science''. 293(8):725–743.Rehmer, J. A. (1981) "The Squantum tilloid member of the Roxbury Conglomerate of Boston, Massachusetts". In: M. J. Hambrey and W. B. Harland, eds., pp. 756–759, ''Earth’s Pre-Pleistocene Glacial Record''. Cambridge University Press, London, England. 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

s 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.Bailey, R. H., and B. H. Bland (2001) "Late Proterozoic stratigraphy and structure in the Avalonian magmatic arc southwest of Boston, Massachusetts Recent developments in the study of the Boston Bay Group". In: D. P. West Jr., and R. H. Bailey, eds. pp. U1–U23, ''Guidebook for Geological Field Trips in New England'', 2001 Annual Meeting of the Geological Society of America, Boston, Massachusetts.Wolfe, W. (1976) "Geology of Squaw Head, Squantum, Massachusetts". In: B. Cameron, ed., pp. 107–116, ''Geology of Southeastern New England'': Princeton, New Jersey, New England Intercollegiate Geologic Conference, 68th Annual Meeting, Guidebook, Boston, Massachusetts. The Roxbury Conglomerate has been significantly altered by metamorphism. Metamorphism has altered its sedimentary rocks to sub

greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite ...

facies and created a

slaty Slate gray is a gray color with a slight azure tinge that is a representation of the average color of the material slate. As a tertiary color, slate is an equal mix of purple and green pigments. Slaty, referring to this color, is often used ...

, 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.Mansfield, G. R., 1906, "The Origin and Structure of the Roxbury Conglomerate". ''Bulletin of the Museum of Comparative Zoology at Harvard College''. no. XLIX, p. 180

Nature of contacts

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. Overlying the Squantum Member, the Cambridge Argillite consists of up to 5 km (3 miles) of laminated, dark to olive grey, graded,

turbiditic A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...

siltstone and sandstone beds. Graded beds, starved ripples, scour marks, load casts and micro-faults are numerous. Soft sediment deformation structures, such as mega slump folds many meters in amplitude, and pinch and swell bedding, are also common. Discrete ash beds measuring a few centimeters to tens of centimeters in thickness have also been documented in this unit.

Depositional environment

A number of processes have been proposed for the deposition of the Roxbury Conglomerate. Initially, it was proposed that the Boston Basin was a nonmarine basin, in which rivers and mountain glaciers transported and deposited the sediments, which comprise the Roxbury Conglomerate. Further and more detailed examination of the sedimentology of rocks comprising it has shown that the Boston Basin was a deep marine basin in which the sediments comprising the Roxbury Conglomerate accumulated as deep sea fans as the result of non-glacial subaqueous mass flow and

turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...

deposition. This is consistent with the thick turbidites, which accumulated within a

submarine fan Abyssal fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater geological structures associated with large-scale sediment deposition and formed by turbidity currents. They can be thought of as an underwater ve ...

or slope environment, of the overlying Cambridge Argillite. The volcanic and coarse character of the Boston Bay Group points to deposition associated with volcanic activity. Past identification of a glaciogenic origin for the Roxbury Conglomerate was based entirely on the identification of ‘dropstones’ and striated pebbles, which have not been substantiated by later and more detailed research.Passchier, S., and E. Erukanure (2010) ''Palaeoenvironments and weathering regime of the Neoproterozoic Squantum ‘Tillite’, Boston Basin: no evidence of a snowball Earth.'' Sedimentology. 57(6):1526–1544.

Fossils

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 Stromatolites () or stromatoliths () are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). The ...

hemispheroids found in laminated mudstones at Squantum Head.

Age

Based on radiometric dates from volcanic and plutonic rocks underlying it, igneous gravel it contains, from volcanic rocks, which interfinger with it, and fossils found in the overlying Cambridge Argillite, the Roxbury Conglomerate accumulated between 570 and 595 million years ago. The Roxbury Conglomerate is underlain by circa 610 Ma Dedham Granite and circa 599 Ma Westwood Granite. It also overlies circa 596 Ma Mattapan Volcanic Complex. The Roxbury Conglomerate interfingers with upper part of the Brighton Volcanic rocks (c. 580–650 Ma). It is overlain by the Cambridge Argillite, which contains autochthonous Ediacaran microfossils (Bavlinella cf. faveolata).Thompson, M.D., S.A. Bowring (2000) ''Age of the Squantum ‘tillite’, Boston Basin, Massachusetts: U–Pb zircon constraints on terminal Neoproterozoic glaciation.'' American Journal of Science. 300(8):630–655

Cultural and economic importance

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 Back Bay is an officially recognized neighborhood of Boston, Massachusetts, built on reclaimed land in the Charles River basin. Construction began in 1859, as the demand for luxury housing exceeded the availability in the city at the time, and t ...

neighborhood.Drake, Samuel Adams
''A book of New England legends and folk lore in prose and poetry''
Boston : Little, Brown and Co., 1906 Roxbury puddingstone is the official rock of

Massachusetts Massachusetts (Massachusett language, Massachusett: ''Muhsachuweesut assachusett writing systems, məhswatʃəwiːsət'' English: , ), officially the Commonwealth of Massachusetts, is the most populous U.S. state, state in the New England ...

.Mass. Gen. Laws ch. 2, § 22
/ref>

Puddingstone Park Kevin W. Fitzgerald Park (formerly Puddingstone Park) is a neighborhood park near Brigham Circle in the Mission Hill neighborhood of Boston, Massachusetts. It was built as part of the redevelopment of the "ledge site", a former Puddingstone ...

is a neighborhood park built as part of the redevelopment of a former puddingstone quarry in the Mission Hill neighborhood of Boston.

Notes

{{reflist, 35em

External links

*Anonymous (nd
''Mission Hill, Boston, Massachusetts.''
Massachusetts Institute of Technology, Cambridge, Massachusetts. (accessed Internet via Wayback Machine) *City of Boston (2006
''Bedrock Geology.''City of Boston Open Space Plan 2002-2006
City of Boston, Boston, MAssachusetts. *Peragallo, T. (1989

ttp://nesoil.com/ NEsoil.com*Share, J. (2011a) [http://written-in-stone-seen-through-my-lens.blogspot.com/2011/03/architectural-geology-of-boston-roxbury.html ''Architectural Geology of Boston: The Roxbury Conglomerate (Puddingstone) Part I – The Tectonic Evolution and Journey of Avalonia.''
Written In Stone...seen through my lens
*Share, J. (2011a

[http://written-in-stone-seen-through-my-lens.blogspot.com/ Written In Stone...seen through my lens]. *Stamm, N. (2013
''Geologic Unit: Roxbury''
National Geological Map Database, U.S. Geological Survey, Denver, Colorado. Conglomerate formations Geologic formations of Massachusetts Geography of Boston Back Bay, Boston Roxbury, Boston Ediacaran United States Ediacaran stratigraphic units of North America