Mélanges interpreted as olistostromes are present in the late Precambrian Westboro Formation, Blackstone Group, and Newport Neck Formation of the Avalonian terrane of southeastern New England. All of these metasedimentary sequences were intruded by plutons dating 600 to 640 Ma and were variably deformed during late Precambrian(?) to late Paleozoic orogenic events. These Avalonian olistostromes contain quartzarenite and carbonate olistoliths, from 10 to 1,000 m in length, and are interstratified with turbidites and laminated and deformed mudstones. The Blackstone and Westboro sequences are associated with mafic volcanics, whereas the Newport Neck Formation contains interstratified felsic tuffs. Olistostromes and interstratified rocks are interpreted as submarine-slide, debris-flow, and sediment gravity flow deposits of midslope to proximal-fan environments. These deposits provide evidence for an episode of significant Late Proterozoic (650 to 800 Ma) rifting and marginal collapse of continental crust.

Location The Boston Bay Group is preserved in the Boston Basin of eastern Massachusetts. The instructions below allow the reader tolocate four different stops within the basin. For driving convenienceit is easiest to visit the areas in either regular or reversenumerical sequence (Fig. 1). Persons not familiar with the greater Boston metropolitan area should have a detailed road map orroad atlas. All of the sites are open to the public without permissionduring daylight hours and ample parking is available. Stop 1. Old Mystic River Quarry, Somerville (Fig. 2). From I-93 (traveling north) exit onto Massachusetts 28; turn leftunder interstate and immediately right onto Mystic Avenue (Massachusetts 38); after 0.3 mi (0.5 km) turn left onto Butler Drive; enter public housing project (that now occupies quarry) and turn left into parking area. Park next to basketball courts and walk about 290 ft (100 m) southwest across the field to thequarry face. The best exposures are to the northwest along therock face. If heading to Stop 2, take Massachusetts 38 onramp to I-93 south; exit onto Storrow Drive (U.S. Route 1); exit onto Beacon Street at Kenmore Square. Continue west on Beacon Street. Stop 2. Webster Conservation Area, Newton (Fig. 3). Outcrops in the vicinity of the intersection of Beacon Street and Hammond Pond Parkway may be reached by traveling west on Beacon Street (about 1.2 mi [2 km] beyond Boston College) ornorth on Hammond Pond Parkway from Massachusetts 9. Parkon north side of Beacon Street (Fig. 3) and examine outcropsalong and