ABSTRACT The Berkshire massif in western Massachusetts is one of several external basement massifs in the New England Appalachians. The Day Mountain thrust is a segment of the western frontal thrust of the Berkshire massif that carried Mesoproterozoic basement gneisses and unconformably overlying cover rocks of the Neoproterozoic (?) Dalton Formation and Cambrian Cheshire Quartzite over the Cambrian to Ordovician Stockbridge Formation. The basal unit of the Dalton Formation is a distinctive deformed quartz-pebble conglomerate. We made 27 strain estimates at 18 locations using the deformed conglomerate to investigate the strain field in the Day Mountain thrust sheet and test the plane-strain model of thrust emplacement. Although the strain ellipsoids vary from prolate to oblate shapes over distances as small as 200 m, and the orientations of the principal directions of strain range widely, a remarkably simple strain pattern, broadly consistent with simple shear, emerges when the strain data are plotted on contoured stereograms. The preferred orientation of the maximum elongation direction plunges gently and approximately coincides with the west-northwest transport direction of the thrust sheet, the preferred orientation of the intermediate principal strain axis is nearly horizontal and perpendicular to the transport direction, and the preferred orientation of the short axis plunges steeply. Most of the strain ellipsoids fall in the prolate field, which is indicative of constrictive flow, especially in the northern part of the thrust sheet. We suggest that the steep gradients in three-dimensional strain type were caused by flow of the more ductile conglomerate over an irregular surface of relatively rigid basement rocks, which were little affected by Paleozoic deformation. The constrictive flow conditions that dominate the strain field in the northern part of the thrust sheet may reflect the irregular paleotopography of the unconformity surface and/or a lateral ramp oriented at an oblique angle to the transport direction that impeded west-northwest–directed thrusting.

An intensive search for fossils in Taconic sequence strata in the region of Columbia County, New York, produced three Lower Cambrian, two Middle Cambrian, and two Upper Cambrian trilobite faunas, of which only the oldest Early Cambrian fauna was previously known from the area. All these faunas contain diagnostic genera and species, allowing precise correlation with Cambrian strata of other regions of North America and Europe. The trilobites occur in limestone units which are generally lenticular and interstratified with a much greater thickness of grey, green, or black unfossiliferous shales. These findings indicate that there is no unconformity between the Lower Cambrian and the Lower Ordovician strata of the Taconic sequence in the southern portion of the Taconic allochthon. It is likely, instead, that an essentially complete sedimentary sequence extends from the Cambrian system upward into the Lower Ordovician. Several sections were measured, and are described in detail, with lists of the fossils observed at various stratigraphic positions. For the first time in the study of the Cambrian of the Taconic sequence, several diagnostic faunas in unquestionable stratigraphic order have been collected from a single section. Aspects of the stratigraphy of the Columbia County region are compared with those determined by recent investigations of the northern portion of the Taconic allochthon, and sedimentary aspects are briefly described.