The Early Silurian–Early Devonian Arisaig Group, in the Avalon terrane of Nova Scotia, consists of a thick (∼1900 m) sequence of unmetamorphosed fossiliferous siliciclastic strata that unconformably overlies the 460 Ma bimodal Dunn Point Formation volcanic rocks and is unconformably overlain by basalts and red clastic rocks of the McArras Brook Formation. The Dunn Point volcanic rocks were deposited when Avalonia was a microcontinent, in a New Zealand–type arc setting ∼1800 km north of Gondwana and 1700–2000 km south of Laurentia. Geochemical, Sm-Nd, and U-Pb (detrital zircon) isotopic data of all Arisaig Group strata show fundamental differences from the underlying Avalonian rocks, indicating that they were not derived from Avalonian basement. These data are instead compatible with derivation from Baltica, implying that Avalonia had accreted to Baltica by the earliest Silurian and that the Arisaig Group is part of a clastic sequence that has overstepped Appalachian-Caledonide terrane boundaries. The lack of penetrative deformation and the approximately concordant nature of the contact between the Dunn Point Formation and the Arisaig Group suggest that this portion of Avalonia was located on the trailing edge of the Avalonia plate during the collision. Regional syntheses suggest that the basin was initiated by local transtension during oblique sinistral collision between Avalonia and Baltica. An overall trend toward increasingly negative ϵ Nd values in the clastic rocks toward the top of the Arisaig Group is thought to reflect increasing input from Laurentia by the time of deposition of the Early Devonian strata. The basin also preserves evidence of loading in the Late Silurian, which is thought to reflect the development of a foreland basin and the ongoing shortening across the orogen associated with the onset of the Acadian orogeny. The unconformity between the Arisaig Group and the overlying McArras Brook Formation is the local expression of the deformation associated with Acadian orogeny in the Antigonish Highlands. The orientation and style of Acadian deformation preserved in the Arisaig Group is compatible with dextral movement along major northeast-trending faults, consistent with evidence of regional dextral shear along the northern margin of the Rheic Ocean in the Middle Devonian. Late Devonian–early Carboniferous deposition of the predominantly continental clastic rocks of the Horton Group occurred around the Antigonish Highlands in a series of grabens and half-grabens, most notably represented by the St. Mary's basin, which originated by dextral shear along the boundary between the Meguma and Avalon terranes. Continued episodes of dextral shear in the late Carboniferous resulted in localized regions of transtension and basin development, and also in episodes of transpression, manifested by intense deformation, thrusting, and S-C fabric development. Taken together, Middle Devonian–late Carboniferous episodes of dextral shear reflect the local accommodation of oblique convergence and eventual collision between Gondwana and Laurussia.

Location The upper flow units of the North Mountain Basalt Forma tion extend in continuous exposure for 124 mi (200 km) along the Bay of Fundy. The massive lower unit is exposed at several places along the escarpment face of the North Mountain cuesta, and continuously along Saint Mary's Bay (Fig. 1). The five stops described here center on the remarkable exposures near Scots Bay and along the North Shore of the Minas Basin, where structuraland stratigraphic relations are well displayed. Stops are situatedon the Parrsboro and Wolfville 1:50,000-scale map sheets 21-H8and 2 i-Hi, National Topographic System, available from the Canada Map Office, 615 Booth Street, Ontario K1A 0E9. The Geology Map of The Cobequid Highlands (sheet 2), and the Geological Highway Map of Nova Scotia (Bujak and Donahoe,1980) both available from the Nova Scotia Department of Minesand Energy, Halifax, are useful for locating optional sites. Caution. The tidal range of the Bay of Fundy region attains 50 ft (15 m). Shoreline field studies are severely constrained bythe times of the tides, which vary daily. Consult the publishedcurrent tide tables or daily Halifax newspaper to plan for a dayhaving an early morning high tide. The shore should be vacatedwell before high tide to pass promontories that flood early. North Mountain and Scots Bay Syncline Stop 1 is a road cut and quarry at the base of the ridge,2.5 mi (4 km) north of Sheffield Mills on the road to Glenmont (Fig. 2). Stop 2 is on the intertidal shelf at

The relations of the Silurian and Lower Devonian rocks of Nova Scotia to those of adjacent portions of the northern Appalachians have, until recently, been uncertain. The pre-Carboniferous grain in coastal Maine and New Brunswick is northeasterly, and in southern Newfoundland it is again roughly northeast, but in the intervening area of northern Nova Scotia (excluding Cape Breton Island) it is almost east-west. Recent work in the northern Appalachians, both published (Berry and Boucot, 1970) and unpublished, helps to resolve this situation. It is also possible to interpret the Paleozoic of the Maritimes in the light of plate tectonics (Mc-Kerrow and Ziegler, 1971). The existing relations fall easily into place if it is assumed that the Silurian and Lower Devonian lithofacies units follow the regional grain; and that the northern Appalachians have a salient (in Nova Scotia) and a recess (in New Brunswick), like many other mountain belts. Once this reconstruction is allowed, the problem of setting up reasonable paleogeographic relations (taking into account both physical and biological environmental factors) becomes relatively simple for Maine, New Brunswick, and Nova Scotia, but the detailed relations of Southern Newfoundland to Nova Scotia are not clear. Figure 24 (Eastport Belt of Figure 24 equals the Coastal Volcanic Belt of Berry and Boucot, 1970) indicates three belts which have consistently distinct facies throughout the Silurian and Lower Devonian. These form the basis for the following discussion. None of the pre-Silurian rocks of Antigonish and Pictou Counties can be dated well enough to make it. . .