580 THE JOURNAL OF GEOLOGY. 



deposition occupied a very long period of deep submergence; but 

 how deep and how wide-spread are not known. The western edge 

 of the Pamunkey formation was widely overlapped, and it is prob- 

 able that the older Miocene formations were also overlapped far to 

 the eastward. The conditions of erosion and sedimentation dif- 

 ered considerably from those of Pamunkev times, for the fine 

 sands and clays, diatomaceous in part, have had a different history 

 from the glauconitic sands and marls of the Pamunkey forma- 

 tion. The uppermost members of the formation, which are 

 found in the southeastern corner of Virginia, were laid down' 

 during the uplift of the Chesapeake formation, for the mingled 

 sands and shell fragments indicate proximity to a shore. They 

 are very young Miocene and contain a large number of Pliocene 

 shells. This emergence was probably part of the general uplift 

 and planing which followed Chesapeake deposition. It was a 

 general base-levelling precisely similar to that of the Post- 

 Pamunkey emergence, and there are similar limitations to our 

 knowledge of the extent of uplift, amount of tilting, and depth 

 of denudation. In both uplifts there was slight tilting from the 

 westward. It was during Pamunkey and Chesapeake times that 

 much of the base-levelling of the Piedmont region was effected, 

 but if these formations were originally spread far westward over 

 the surface of the crystalline rocks, their presence retarded the 

 base-levelling for the time being. 



Following Post-Chesapeake erosion there came a moderate 

 amount of submergence and the deposition of the Lafayette 

 formation. Another change had taken place in the conditions of 

 erosion and sedimentation, for coarse sands and gravels were 

 spread about by waves and currents over a wide zone in the 

 vicinity of the shore. Farther eastward there were deeper waters 

 and the sands and finer materials were deposited in them. How 

 far west the waters spread is not known, but the shore deposits 

 along the base of Catoctin Mountain indicate general submergence 

 of the peneplain for at least a portion of the time. The great 

 sheet of typical sediments was apparently not spread far beyond 

 its present limits on the divides, but it widely overlapped the 



