Location Stop 1. The most spectacular exposures of any ductile faultin southeastern New England are those of the Willimantic fault inroad cuts along the westernmost interchange of the 5 mi (8 km) segment of unfinished I-84 at its junction with U.S. 6 and Connecticut 66, 3.5 mi (5.7 km) west of Willimantic, Connecticut (Fig. 1). The outcrops at 41°43'24„ north latitude and 72°16'west longitude are in the town of Coventry, Tolland County, east-central Connecticut (Fig. 2). They lie in the northwest quadrantof the Columbia 7½-minute Quadrangle, but are not shownon Snyder's (1967) bedrock map because the cuts were firstopened in 1972. This interchange maybe reached from Hartford, Connecticut by following I-84 east to U.S. 6, then east to theinterchange from Middletown, Connecticut by following Connecticut 66 east to the interchange; from Providence, Rhode Island, by following U.S. 6 west to the interchange; or from Norwich, Connecticut by following Connecticut 32 north to U.S.6, then west to the interchange. Once at the interchange, park in the commuter parking lot between U.S. 6 and Connecticut 66, and walk up the median of I-84 to the cuts (see Fig. 1). These cutsare in part of a federal highway system! Stay well off the pavementand away from traffic! Please do nothing to jeopardize theuse of these exposures by future geologists.

The age of the late Wisconsin maximum of the Laurentide ice sheet off the coast of New England and on Long Island, New York, is not closely designated. Radiocarbon and stratigraphic evidence from Martha’s Vineyard suggests that the glacier margin may have been close to its maximum position as late as 15,300 yrs ago; indirect evidence from Long Island infers that the ice sheet had reached a maximum and had begun to recede prior to 17,000 yrs ago. In any case, by at least 14,200 yrs ago the glacier margin had retreated from its maximum late Wisconsin position at the Ronkonkoma Moraine on Long Island, had constructed recessional frontal deposits, and had retreated north of Rogers Lake on the southern Connecticut coast. Subsequent readvances culminated near Middletown, Connecticut, some time after 15,000 yrs ago, and in Cambridge, Massachusetts, after 14,000 yrs ago. Whether these readvances were synchronous is unknown because of the absence of close limiting dates and because of the lack of evidence for readvance in the intervening area. The northwestward recession of the glacier margin from the present coast in eastern Maine was accompanied by a marine transgression and deposition of hundreds of submarine moraines between 13,500 and 12,500 yrs ago. This general recession was interrupted by a readvance which culminated at the Pineo Ridge Moraine approximately 12,700 yrs ago. Although it may have resulted from general climatic change, the Pineo Ridge readvance just as likely may have been caused by a vastly decreased calving rate associated with isostatic uplift and marine recession from coastal Maine. This is well documented as having occurred simultaneously with the Pineo Ridge readvance. Thereafter, the ice sheet thinned and separated over the highlands of northwestern Maine leaving residual ice to the southeast. Active ice, receding into the St. Lawrence Valley of southeastern Quebec deposited the Highland Front Moraine approximately 12,700 to 12,600 yrs ago. In summary, (1) a major amelioration of climate that began prior to 14,200 yrs ago resulted in very rapid dissipation of the ice sheet in New England at least by 12,500 yrs ago, with the exception of small glaciers that possibly persisted in the highlands; (2) no conclusive evidence has been recognized for any climatic reversals during the dissipation of the ice sheet in New England; and (3) although major events in New England compare with those of the Great Lakes region, no minor events have proven correlation with the possible exception of the Pineo Ridge and Port Huron readvances.

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.