yo ulrich: major causes of oscii<lations 



tions are wholly or mainly confined to one or more narrow, 

 trough-like, longitudinal divisions of the Appalachian geosyn- 

 cline and commonly to one or another of three divisions of the 

 geosyncline that are more or less effectively separated from 

 each other by low transverse axes. The most northerly of these 

 broad axes passes across the valley between Carlisle and Leba- 

 non, Pennsylvania. It is known as the Harrisburg axis. The 

 next to the south intersects the valley of Virginia between 

 Staunton and Harrisonburg. The third or Wytheville axis 

 passes across southwestern Virginia, which is today the highest 

 and narrowest part of the great valley. The fourth axis crosses 

 in a more northerly direction than the others through the belt 

 lying between Rome, Georgia, and Gadsden, Alabama. 



These transverse axes do not cross the longitudinal troughs 

 of the geosyncline in continuous direct lines. On the contrary, 

 their course zigzags within the varying limits of a broad band 

 so that the northern head of a bay in one trough may extend 50 

 miles or more beyond the latitude of the southern head of an- 

 other, younger or older, bay in an adjacent trough. The band 

 is wide enough and was always low enough so that regional 

 tilting occasionally permitted overlap of edges of formations 

 transgressing from opposite directions. Often the axis formed 

 an efficient barrier in one trough and was much less effective 

 in the one next to the west or east. More rarely, a bay, terminated 

 at the north by a transverse axis, connected laterally with waters 

 in an adjoining trough in which the submergence was not stopped 

 by the axis. Finally, at other times the axis offered no serious 

 obstacle to the passage of the marine invasion. Of course, the 

 individual troughs were submerged over and over again, but in 

 none do we find representatives of all of the formations known 

 to have been deposited in the Appalachian Valley. 



Var^dng geographic expressions Uke these could have been 

 made possible only by differential vertical movements in the 

 concerned parts of the lithosphere, and these Appalachian 

 oscillations in sea level were by no means small affairs. Most 

 of them are measured by hundreds of feet and some by thousands. 



Excellent and very interesting oscillations occurred about 



