THE PENNSYLVANIAN PERIOD 



381 



Now and again the swamps were inundated, allowing sand, 

 mud, and even lime ooze to be spread over them. If the 

 movement soon ceased, the sea bottom was gradually built 

 up by the sediments until the water again became so shallow as 

 to favor the growth of marsh plants. That the sea was not 



COALi FIELDS 



OF 

 EASTERN V1QIED STATES 



SCALE OF MILES 



\ 



FIG. 401. The coal fields of eastern United States. Those east of the 

 Great Plains are largely of Pennsylvanian age, but in part Triassic and 

 younger. (U.S. Geol. Surv.) 



always encroaching on the land is shown by the presence of 

 unconformities in the Coal Measures. While some of these 

 are due merely to the shifting of stream channels traversing 

 the marshes, others imply temporary land conditions during 

 which the rocks were eroded slightly. In short, the land 

 was very near sea level, but was sometimes above it and 

 B. & B. GEOL. 22 



