ABSTRACT We present a tectonic model for the generation of quartzofeldspathic veins and pegmatite dikes found in southern New England that proposes the following: (1) Hydraulic fracturing produces the initial fracture that opens to become a vein, and (2) oscillating differential stress produces a gradient in interstitial fluid pressure that draws the melt to the dilating walls of the fracture. Oscillating fluid pressures at the dike walls lead to oscillating rates of crystallization, and thus to alternating aplitic and pegmatitic textures in dikes as thin as a few centimeters. Excess alkalis and SiO 2 (aq) lower the solidus temperature of the granitic liquid and thus stabilize a peralkaline silicate liquid without crystallization. This silicate liquid is capable of crystallizing only quartz, only feldspars, or both. Crystallization of this liquid lowers its pH and accounts for the mineralogical zoning consisting of quartz cores, feldspathic mantles, and muscovite-bearing margins common in many quartzofeldspathic veins.

Abstract The sedimentologic development of Horseneck Beach on the northwest coast of Buzzards Bay, Massachusetts, is interpreted from 40 vibracores, 20 boreholes, 9 km of ground-penetrating radar and eight topographic profiles. The beach-ridge barrier is 4 to 10 m thick and consists primarily of fine sands with some coarse sand and gravel layers underlain by a coarsening-upward estuarine sequence (3-8 m thick) composed of silts and clays grading to fine sand. Underlying the estuarine deposits are glacio-fluvial sands and gravel up to 6 m thick, blanketing a Paleozoic bedrock surface. Ground-penetrating-radar records and backhoe trenching indicate that progradation of the barrier occurs sporadically. An inferred-process model is proposed in which the accretionary phase represents a period of abundant sand supply when the beach widens, builds vertically, and is punctuated by a period of low-sediment supply and erosion. At the eastern end of the barrier, the erosional phase coincides with an influx of gravel, steepening of the beach profile, and ridge construction, which is probably controlled by the 100-year, or longer, storm frequency. During these infrequent events, sand and gravel are released from an offshore drumloid. Segregation of the sand and gravel results from differences in longshore sediment-transport rates.

Tills that discontinuously underlie the late Wisconsinan till throughout New England represent the penultimate full glaciation of the region. In southern New England, the late Wisconsinan till and the tills that locally underlie it are informally referred to as upper and lower tills, respectively. For the most part, the ages of the lower tills are not firmly established, and regional correlations between occurrences of lower till, including those on Long Island, New York, are tenuous. Where a lower till underlies deposits having limiting middle Wisconsinan radiocarbon ages (e.g., the Montauk till member of the Manhassett Formation on Long Island at Port Washington, New York, and the lower till at New Sharon, Maine), many workers have assigned the till an early Wisconsinan age. However, lower tills throughout much of New England may be Illinoian or older in age and may correlate with a lower till exposed at Sankaty Head, Nantucket Island, Massachusetts, that is pre-Sangamonian in age. The till at Sankaty Head lies below marine beds containing marine faunas indicative of sea-water temperatures both warmer and slightly cooler than those off Nantucket today and that have uranium-thorium and amino-acid racemization (AAR) age estimates suggesting a Sangamonian age (marine oxygen-isotope stage 5). The lower till at Sankaty Head and the Montauk till member on Long Island were deposited during a full glaciation of New England that was at least as extensive as the late Wisconsinan advance of the Laurentide ice. Global ice-volume data from the marine oxygen-isotope record and the late Pleistocene eustatic sea-level record inferred from raised coral terraces support an advance of this magnitude during marine oxygenisotope stage 6, but not during stage 4. An early Wisconsinan age of the southern New England lower tills and, hence, of the penultimate glaciation there is problematic in terms of the pre-Sangamonian age of the lower till on Nantucket, and in terms of the late Pleistocene global ice-volume and sea-level records. An Illinoian age for the tills and for the penultimate full glaciation of New England is compatible with all the available evidence except some equivocal radiocarbon ages and AAR age estimates.