Abstract Occurrences of algal mats at Plum Island along the northeastern coast of Massachusetts at 43°N latitude consist of laminated quartz silt and sand bound by mucous-secreting, filamentous blue-green algae (Cyanophyta). Recent discoveries of modern algal mats in quartz sands along cool, temperate coasts further confirms the fact that algal mat-forming organisms have wide temperature and salinity as well as substrate tolerances. At Plum Island, algal mats occur along the high intertidal to low supratidal margins of a metahaline pond in a swale on a recurved spit at the southern end of the island. They are also found along the margin of the marsh next to the edge of the backdune area of the island's dune field. The algal mats on Plum Island are vertically stratified into three major zones. The upper 1 mm of the mat in the spit area is dark brownish green due to filaments of Lyngbya, Microcoleous, and other smaller filamentous blue-green (?) algae. Coccoid cyanophytes (including Enthophysalisl), diatoms, Euglena, and nematode worms also are present. Below this upper green layer, there is a thinner, pinkish layer containing anaerobic, photosynthetic purple sulfur bacteria. Underlying the pinkish layer, there usually is a third layer composed of organic-rich black sand, 1-10 cm thick, indicating anaerobic conditions. This black layer contains the remains of buried older surface mats whose laminae can be recognized as alternating layers of decaying organic matter and layers of water-laid sands and wind-blown silts. Gelatinous material aiding filament-binding of sediment extends 1-7 mm below the upper surface. No carbonate cementation was found. The larger structures associated with these algal mats include gas domes, desiccation ploygons, elongated ridges, rolled mats, mounds, and distorted mats redeposited after flotation. Algal mat growth affects physical sedimentation in part of the interdune area of the spit by stabilizing silt and sand after erosion and/or sediment deposition. By stabilizing each new sediment surface, these mats aid upward growth of low, ponded areas of the spit. Overturned and rolled mats produce mounds that become colonized by vascular plants and thus initiate dune development on low, flat areas. Ancient quartzose algal mats may be difficult to recognize in the stratigraphic record, but occurrences are known. Preservation potential is low because of the lack of carbonate cementation. Possible criteria for recognition include fine sand and silt laminae, desiccation cracks, elongated ridges, mounds, and overturned mats.

The Newbury Volcanic Complex of northeasternmost Massachusetts is encompassed, except for one small occurrence, in a northeast-trending wedge-shaped belt bounded by regional faults. Ten stratified members, aggregating more than 4,400 m in thickness and intruded by subconcordant podlike masses of micrographic rhyolite, are disposed in a homoclinal sequence that is overturned and broken into blocks by minor faults. In contrast to previous descriptions, this report proposes that the sequence has been inverted and that it neither unconformably overlies the granitic pluton that borders it on the southeast nor is intruded by that pluton, as previously has been suggested. The lower 2,900 m of the complex are entirely volcanic materials. In this interval, the great volume of strata is contained in three of the members: a fine-grained basalt flow member; a thick flow-banded rhyolite vitrophyre member that locally includes at its base as much as 120 m of ash-flow tuff, and a porphyritic andesite member that contains numerous flows but is dominated by flow breccias and tuffs. Most of the andesites accumulated subaerially, but andesitic detritus was also laid down in a shallow marine environment. Two lesser members of rhyolitic tuff, a minor lens of dense basalt(?), and a fine-grained andesite(?) member partly of epiclastic materials complete the lower part of the Newbury. The upper 1,500 m of the complex is composed of three very poorly delineated sedimentary members: a siliceous siltstone, a red sandy mudstone, and a calcareous mudstone. The red mudstone member contains some volcanic detritus. The calcareous mudstone member is ostracodal, and the porphyritic andesite member contains at several sites a shelly fauna that provides specific correlation with the Leighton Gray Shale Member of the Pembroke Formation of the Eastport area of Maine. In the latter locality, ostracodes indicate affinities with fauna of the Baltic-British region rather than those of the Appalachian region and provide the basis for suggesting that the porphyritic andesite member of the Newbury may be wholly of Pridoli (latest Silurian) age. Ostracodes of the calcareous mudstone member may be longer ranging; therefore, the possibility that higher strata of the Newbury are as young as Gedinnian (earliest Devonian) cannot be ruled out. Recent paleontologic and radiometric dating indicates that the uppermost part of the Ames Knob Formation and the Thorofare Andesite in the southern Penobscot Bay area and the Castine Volcanics in the northern Penobscot Bay area (all more than 200 km up the coast in Maine) are also temporal equivalents. The Newbury was not affected by mild regional metamorphism, nor was it invaded by granitic plutons during the Acadian orogeny, as were some of the Silurian-Devonian terranes of coastal Maine. The lithologically comparable Lynn and Mattapan Volcanic Complexes, of the nearby Boston area, apparently were similarly unaffected by metamorphism; correlation of the Newbury with these formations is not yet proved.

Stratified rocks of northeastern Massachusetts that are known or considered to be of pre-Silurian age are (1) a sequence of nonfossiliferous marine metasedimentary and metavolcanic rocks at least 2,800 m thick, including the Westboro Formation (formerly known as the Westboro Quartzite) and the Middlesex Fells Volcanic Complex (new name), that probably are of Precambrian age; (2) a sequence of nonfossiliferous eugeosynclinal rocks having an aggregate thickness of about 18,800 m, which includes the Greenleaf Mountain Formation (new name), Burlington Formation (new name), Marlboro Formation, Shawsheen Gneiss (new name), Fish Brook Gneiss, Nashoba Formation, and Tadmuck Brook Schist (new name), all of undetermined age; and (3) some scattered remnants of unmetamorphosed, partly fossiliferous marine sedimentary rocks of Cambrian age. The Westboro Formation, Middlesex Fells Volcanic Complex, and some older unnamed gneiss and quartzite strata were domed and locally subjected to minor folding during emplacement of the Dedham Granodiorite, which intrudes them but does not intrude any of the stratigraphically higher metasedimentary rocks. These intrusive and structural features, together with local omissions of the upper part of the above-named formations, suggest a significant unconformity. These stratified rocks are tentatively correlated with the Blackstone Series of Rhode Island. The eugeosynclinal rocks constitute a generally homoclinal sequence, except where parts of the section are locally cut out or are repeated by faults or are repeated by minor folds. This sequence consists of at least 80 percent metamorphosed volcanic wackes; units characterized by strata of metamorphosed pelitic and carbonate sediments are interspersed through it. The stratified rocks of Cambrian age form pendants in plutonic rocks and possibly also outliers. The Weymouth Formation of Early Cambrian age and the Hoppin Slate that possibly is correlative with it consist mainly of marine shale and limy shale interspersed with minor thin limestone strata. The Braintree Argillite, conformably overlying the Weymouth Formation, consists of marine shale, silty shale, and siltstone. The remnants of Cambrian rocks do not have any contacts with other stratified rocks.

A section comparable in general aspect to that presented by Bell (1971) for northeastern Massachusetts is present in the New London area, southeastern Connecticut (Fig. 1). Disregarding a fossiliferous sequence, which is possibly in a different structural block, Bell described a 12,800-m-thick metamorphosed sequence of interbedded volcanic, volcaniclastic, and sedimentary rocks overlying a 1,520-mthick sequence of metamorphosed quartzose and calcareous sedimentary rocks containing some volcaniclastic material. In the New London area, an upper, primarily volcanic sequence as much as 3,100 m thick (composed of amphibolite, hornblende and biotite gneisses, and some schist) overlies a primarily sedimentary section at least 1,860 m thick (consisting of quartzite, quartz-rich schist, pelitic and feldspathic schists, and some calc-silicate rocks, gray biotite gneiss, and amphibolite). The upper sequence in the New London area consists largely of metamorphosed volcanic and volcaniclastic rocks and related meta-intrusive rocks as well as small amounts of sedimentary rocks; the lower sequence primarily consists of metamorphosed quartzose and calcareous sedimentary rocks with lesser amounts of intercalated volcanic and volcaniclastic material. The two sequences have been complexly folded and are intercalated with granitic gneiss. Primary sedimentary features useful in determining tops of beds are rarely seen. Individual units show appreciable differences in thickness in different areas. Both sequences are pre-Silurian in age, and most probably the rocks range in age from late Precambrian or Cambrian to Ordovician. Accurate estimates of thicknesses in the New London area are difficult to achieve because of the complex structure and numerous granitic intrusions. Sections have been measured...

Duxbury Beach, Massachusetts, is a retreating, transgressive barrier that is effectively managed to meet a range of competing land uses. While the barrier is heralded as a natural coastal setting, the entire landform is methodically engineered on an ongoing basis to best accomplish the goals established for the beach within a context of natural processes. Historical and geological data indicate that the natural barrier form includes numerous ephemeral tidal inlets (some of which have migrated) and overwash channels, and low discontinuous dunes. At present, the managed barrier has a continuous vegetated foredune and broad backdune. Management techniques have evolved over the past several decades based on growing experience and understanding of the coastal processes and of wildlife habitats. Although the foredune crest is reconstructed each spring, the entire beach is gradually being allowed to retreat to remain in equilibrium with rising sea level. The lagoonal shore is being widened through beach nourishment and through proposed creation of back-barrier salt marshes using silty dredge spoil. Uses of the barrier include town and public recreational beaches, off-road vehicle access, a right-of-way to isolated communities, flood protection of landward areas, and shorebird nesting habitat.