450 N. H. DARTON — LATER FORMATIONS OF VIRGINIA AND MARYJvAND. 



altitude of this terrace and the Columbia terraces eastward about Washing- 

 ton suggests some displacement in post-Columbia or intra-Columbia times. 

 Since Appomattox times the scarp west of the displacement has been cut 

 down in the drainage depressions, and in the larger streams has given rise to 

 a special series of gradually receding rapids, of which the Great falls of the 

 Potomac is an example. 



Geologic History. 



The principal events in the history of the coastal plain region, so far as 

 noAV determined, are as follows : 



1. Irregular surface and shore line of crystalline rocks, overlapped in cer- 

 tain areas by the Newark formation. 



2. Submergence and deposition of the Potomac formation. 



3. Emergence ; degradation of the Potomac to an unknown extent. 



4. Submergence ; deposition of Cretaceous greeusand series, including the 

 Severn deposits. 



5. Emergence ; degradation of the Cretaceous and, toward the south, of 

 the Potomac to an unknown extent. 



6. Submergence ; deposition of the Pamunkey formation on the Severn 

 and Potomac surfaces. 



7. Emergence ; degradation of the Pamunkey and its shores westward. 



8. Submergence ; deposition of the Chesapeake formation. 



9. Emergence ; terracing and cutting of basins now occupied by the 

 estuaries. 



10. Submergence ; deposition of the Appomattox formation, overlapping 

 far upon the Piedmont region. 



11. Emergence; tilting northward ; subaerial development of the outlines 

 of the present topographic configuration; widespread lateral degradation, 

 increasing in amount northward; displacement along the great dislocation. 



12. Submergence and deposition of the Columbia materials. 



13. Emergence somewhat greater in amount than the submergence of 12; 

 laud tilted gently southward ; excavation of channels in the Columbia and 

 underlying beds in greater part to somewhat below present tide-level. 



14. Slight submergence ; marsh, silt and shore formations of recent age. 



Of course, this list really comprises only the headings of chapters, which 

 record a vast number of minor events and complications of geologic history, 

 only to be ravelled by a great amount of careful study. At present it would 

 be premature to discuss the events in greater detail, for the data at hand 

 are incomplete and in large part unsatisfactory. In the continuation of the 

 investigation, however, it is hoped that there may be attained a full elucida- 

 tion of geologic history and relations in the coastal plain region. 



