50 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



the Appalachian region the seas were often contained in narrow troughs 

 which connected at some point with the Atlantic, although occasionally 

 these troughs communicated at both ends with the ocean. 



The complete changes in the source and direction of the faunal in- 

 vasions are well shown in some of the maps of North America in Early 

 Paleozoic time. For example the Gulf invasion of Lower Black Eiver 

 (Lowville) time is superseded by an incursion from the Arctic in the 

 Middle Black Eiver (Decorah) and this is followed by one which seems to 

 have come in from the west. 



STEATIGEAPHIC AND PALEONTOLOGIC CHAEACTEEISTICS 



Throughout the Appalachian provinces the Early Paleozoic strata com- 

 prised in the Cambrian and Ordovician systems may be conveniently 

 arranged into three great phases of sedimentation the lowest of sand- 

 stone, quartzite, and sandy shales of Lower Cambrian age, next limestone 

 deposits extending from uppermost Lower Cambrian to the lower part of 

 Middle Ordovician times, and last a shale phase covering the remaining 

 Middle and Upper Ordovician. In Appalachian Maryland each of these 

 three phases is well developed, their combined thickness reaching 16,000 

 feet. Of this total, the lower division comprises over 3300 feet, the middle 

 limestones over 10,000 feet, and the upper shales 2400 feet. These thick- 

 nesses vary in different parts of the Appalachians. As a rule they arc 

 greatly diminished to the north of Maryland and much increased in the 

 states to the south. Columnar sections of the Cambrian and Ordovician 

 rocks of Maryland and neighboring states are presented on pages 48 and 

 49. while a correlation table of these strata is given on page 51. 



These three quite different lithologic divisions outcrop in equally dis- 

 tinct geographic areas. The siliceous rocks are confined to the Blue Eidge 

 province, the limestones form the floor of the Appalachian Valley, and the 

 shales, although sometimes occurring as a great infold or syncline in the 

 limestone in the middle of the Valley, are best developed in the eastern 

 ranges of the Allegheny ridges. 



