30 Introduction 



Chemung time, following a shallowing of the sea. This recurrence was, 

 however, of brief duration and the Chemung species again became domi- 

 nant, occupying the region until the close of marine conditions. The 

 shifting and migrating of these faunas with the varying physical condi- 

 tions constitutes one of the most remarkable aspects of Devonian history. 

 The relations of the sediments and faunas in western Maryland and 

 adjacent areas in Upper Devonian time is shown in the accompanying 

 text figure. 



Toward the close of the Devonian the continent was elevated on the 

 northeast. Upon the surface of the low western land were numerous lakes 

 and embayments in which thick deposits of red clays and sands were 

 formed, which progressively encroached southward and westward upon the 

 waters of the neighboring seas. These sediments are termed the Catskill 

 formation in the Appalachian Province because of their extensive develop- 

 ment in the Catskill Mountains of New York. The formation has variable 

 significance in different localities and is expressive of sedimentation rather 

 than of a definite unit of time. It is a significant fact that land conditions 

 were ushered in at the beginning of Devonian time in extreme north- 

 western Europe, in Middle Devonian time at Gaspe on the St. Lawrence 

 Gulf, and in Upper Devonian time in Maryland and eastern New York. 

 Continental conditions appear later and later in passing westward until 

 west of the Appalachians marine conditions prevail entirely. The transi- 

 tion from marine to non-marine conditions was not abrupt but was at- 

 tended by numerous oscillations. In this manner the red sediments of the 

 eastern United States thin towards the west where they are interpenetrated 

 and replaced by marine deposits. In all these areas the continental deposits 

 are similar red, and occasional green, beds bearing macerated remains of 

 plants and fragments of fishes. The Devonian was finally terminated by a 

 subsidence of the continent and a new invasion of the Mississippi seas in- 

 troducing Lower Carboniferous time. 



The accompanying table shows the correlation of the Devonian deposits 

 of Maryland with those of Pennsylvania, New Jersey, and New York. 



The State of Maryland is divisible into three main topographic prov- 

 inces. The eastern division is a flat coastal plain but recently elevated 



