Roxbury Conglomerate

Roxbury Conglomerate Stratigraphic range: Ediacaran: 595–570  Ma
Roxbury Conglomerate in a monument on the Gettysburg Battlefield.
Type	Geological formation
Unit of	Boston Bay Group
Sub-units	Brookline Member, Dorchester Member, and Squantum Member
Underlies	Cambridge Argillite
Overlies	Mattapan Volcanic Complex and Middlesex Fells Volcanic Complex
Thickness	1,310 meters (4,300 ft) approximate maximum
Lithology
Primary	sandstone, conglomerate, and diamictite
Other	argillite
Location
Country	United States of America
Extent	Boston Basin, eastern Massachusetts
Type section
Named for	Roxbury, Massachusetts
Named by	Hitchcock (1841) and Shaler (1869)

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.

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.