Reflection surveys from northeastern New York and central Vermont, the first elements of a COCORP seismic traverse of Grenville and Appalachian terranes in the northeastern United States, indicate that an unusual zone of layered reflections characterizes the lower crust beneath the eastern Adirondacks and that Grenville-age “basement” of the Green Mountains may actually be allochthonous, perhaps thrust over Paleozoic shelf rocks. The top of the Adirondack deep layered sequence is found at about 20 km beneath the Marcy meta-anorthosite massif. It appears to dip about 10° northwestward and wedge out against more horizontal reflections at about 28 km. This intracrustal sequence may correspond with a previously inferred increase in electrical conductivity. Possible interpretations range from its being a layered igneous complex to its representing a tectonically buried metasedimentary assemblage. Although graphitic schists and iron or sulfide segregations in such an assemblage might contribute to its electrical characteristics, the problematical issue of water in the lower crust—if and how it got there and remained—is again raised, if not resolved. Other intrabasement features include major east- and west-dipping reflections, possibly corresponding to faults; arched reflections that may represent folds; a well-defined lens-shaped zone virtually devoid of reflections; and possible Moho reflections. Unequivocal sub-Moho reflections are absent. Superimposed at shallower levels on the eastern part of the traverse are Appalachian structures. Reflections down to about 4.5 km beneath the Taconic Mountains are interpreted to represent Paleozoic shelf sediments of the Middlebury synclinorium underlying the Taconic allochthons. Possibly correlative, although less well defined, reflections appear to extend in disrupted fashion beneath the Precambrian rocks of the Green Mountains, suggesting a major basement overthrust. The inferred décollement for this overthrust steepens rapidly beneath the east flank of the Green Mountains. East-dipping reflections at the east side of the Green Mountains may represent another major thrust zone involving basement and overlying metasedimentary slices. The basement overthrusting suggested by this study is similar in certain respects to the style, if not the extent, of that demonstrated by previous COCORP surveys in the southern Appalachians.