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.

[6][7][8][9][10] 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.

The conglomerates consist of grey feldspathic sand and well-rounded pebbles and cobbles of quartzite, granite, felsite, and quartz monzonite.

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.

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.

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.

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.

[10] Overlying the Squantum Member, the Cambridge Argillite consists of up to 5 km (3 miles) of laminated, dark to olive grey, graded, turbiditic siltstone and sandstone beds.

Soft sediment deformation structures, such as mega slump folds many meters in amplitude, and pinch and swell bedding, are also common.

[10][19] 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.