. Bulletin of the British Museum (Natural History), Geology. 274 S. M. BERGSTROM & A. J. BOUCOT. Fig. 1 Index map showing areas with Ordovician and/or Silurian outcrops (black) and systemic boundary sections. 1, northern Maine; 2, eastern New York and western Vermont; 3, central Appalachians (Pennsylvania and adjacent states); 4, eastern Tennessee; 5, Alabama and Georgia (southern Appalachians); 6, the Cincinnati region and adjacent areas in Ohio, Kentucky, and Indiana; 7, the Nashville dome in central Tennessee; 8, northern Arkansas (including the Batesville district); 9, eastern Missouri
. Bulletin of the British Museum (Natural History), Geology. 274 S. M. BERGSTROM & A. J. BOUCOT. Fig. 1 Index map showing areas with Ordovician and/or Silurian outcrops (black) and systemic boundary sections. 1, northern Maine; 2, eastern New York and western Vermont; 3, central Appalachians (Pennsylvania and adjacent states); 4, eastern Tennessee; 5, Alabama and Georgia (southern Appalachians); 6, the Cincinnati region and adjacent areas in Ohio, Kentucky, and Indiana; 7, the Nashville dome in central Tennessee; 8, northern Arkansas (including the Batesville district); 9, eastern Missouri and southwestern Illinois; 10, southern Oklahoma (including the Arbuckle Mountains); 11, Black Hills (South Dakota and Wyoming); 12, North Dakota; 13, Colorado; 14, west Texas; 15, New Mexico; 16, Bighorn Mountains, Wyoming; 17, Montana; 18, Idaho; 19, Nevada; 20, Utah; 21, southeastern California; 22, southeastern Alaska (inset map). The purpose of the present paper is to review briefly the biostratigraphy of the systemic boundary interval in key sections in the principal outcrop areas. Page limitations make it necessary to restrict ourselves to data essential for the understanding of the local and regional geology of this interval in the United States. For convenience, we will deal with each of the major outcrop regions separately, from the Appalachians in the east to the Great Basin in the west. For the location of these regions, see Fig. 1. Northern Appalachians In large parts of the Northern Appalachians in the United States (Maine to New York State), Silurian or younger rocks rest with a conspicuous, in many cases angular, unconformity on the Ordovician (Berry & Boucot 1970: fig. 6). This stratigraphical gap varies in magnitude both locally and regionally but includes in most cases portions of both the Ordovician and Silurian systems. Conventionally, this gap is explained as a product of the Middle to Late Ordovician Taconic orogeny, but it is evident that the appare
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