446 N. H. DARTON — LATER FORMATIONS OF VIRGINIA AND MARYLAND. 



in Maryland coarser materials gradually increase in amount, and in the 

 Washington-Baltimore region and northward gravel beds predominate. On 

 Good Hope hill, east of Washington, the high terrace is capped for some dis- 

 tance by beds consisting mainly of large pebbles and sand, with a buif loam 

 matrix. Farther eastward the proportion of loam increases and the pebbles 

 decrease in size and number. In the high terraces extending westward from 

 Alexandria, in the outliers west of Washington and Baltimore, in the high 

 terraces southeast of Baltimore, and generally along the crystalline border 

 in Maryland and Delaware, the formation consists mainly of iron-stained 

 pebbles in a matrix of more or less sandy orange or buff loam. Thin layers 

 and lenses of ferruginous conglomerates are of frequent occurrence in the 

 northern Maryland belt, in the capping on Elk neck, and in the Pennsyl- 

 vania and New Jersey outliers. In some cases the formation contains some- 

 what coarser materials adjacent to the larger drainage depressions, especially 

 on the Potomac river, wdiere the pebble beds are particularly noteworthy. 



The thickness of the formation is variable, but it averages between 20 

 and 30 feet. In Maryland it is generally under 25 feet, but in Virginia it is 

 usually somewhat thicker than this. 



Stratigraphic Relations. — The Appomattox formation lies on a terraced sur- 

 face comprising in various regions all the preceding formations of the coastal 

 plain series. In Virginia and the southern part of the " western shore " of 

 Maryland it lies on the Chesapeake formation over an area of several thou- 

 sand square miles. It overlaps upon the Pamunkey formation in the Fred- 

 ericksburg region, northeast of Washington, and in the James, Pamunkey, 

 Mattaponi, Rappahannock and Potomac depressions. In several isolated 

 knobs on Elk neck it lies directly on the Cretaceous greengand series. It 

 lies on the Potomac formation in the Hanover Junction region, about Fred- 

 ericksburg, in the wide terraces west and south of Alexandria and Wash- 

 ington, in the Baltimore region, and thence northward in Maryland and 

 probably in Delaware. In the Richmond coal field and about Hanover 

 Junction it lies on the Newark formation, and all along the western edge of 

 the coastal plain region it overlaps for a greater or less distance upon the 

 crystalline rocks in Virginia, Maryland and Delaware. 



Generally the base of the formation is sharply demarked, but frequently 

 it is composed of local materials which merge more or less gradually into 

 the surface of the underlying formation. This is particularly the case in 

 some contacts with the lower Chesapeake, Pamunkey and Potomac forma- 

 tions, which have furnished much of the Appomattox materials. 



The surface on w^iich the Appomattox formation was deposited is a series 

 of gently rolling plains, separated by gentle slopes and low local terrace 

 scarps. These terraces and slopes descend successively eastward with vary- 

 ing intervals and amounts, and the plains have also a very gentle eastward 



