C EN O ZOIC HISTORY. S77 



strongly suggest that they were carried by ice and dropped in 

 their present attitudes. The loams contain scattered pebbles 

 and bowlders, and pebbly and sandy streaks. Locally the forma- 

 tions consist entirely of pebbly sands. To the eastward, there 

 is a gradual decrease in the coarseness of the materials. These 

 statements apply about equally to the earlier and later Columbia 

 deposits, but, on the whole, the materials of the earlier Columbia 

 are often coarser than those of the later Columbia. Both forma- 

 tions contain local streaks of ferruginous conglomerate, but the 

 earlier Columbia deposit presents much of this material, and east 

 of Washington it is a conspicuous feature. In some portions 

 of the regions between the valleys of the Potomac and Patuxent, 

 and northeast of Baltimore, the earlier Columbia formation 

 becomes very thin, and consists largely of local materials. The 

 thickness of the later and earlier Columbia deposit averages 

 twenty feet each. 



The earlier Columbia deposit has been widely removed from 

 the higher altitudes, and up the Piedmont gorges is represented 

 by meagre fragments. The widest areas now constituting the 

 surface are southeast of Baltimore, and south of Washington 

 west of the Potomac. In the upper part of Baltimore, and 

 about Mount Pleasant, the upper part of Washington, there are 

 moderately large areas. In the valleys north of Baltimore there 

 are many thin patches of earlier Columbia gravels, and some of 

 these widen considerably in the limestone valley of the Cocky- 

 ville region. In the Rock creek valley, near Washington, there 

 are many small areas of the deposits at from 195 feet to 205 

 feet, and an early Columbia delta at the broad intersection of 

 the 200-foot terrace level with the gorge above. 



The later Columbia deposit is not widely degraded, and it 

 has only been removed over the area occupied by tide water, and 

 narrowly trenched by the various small water ways. 



POST-COLUMBIA RELATIONS. 



The principal Post-Columbia features are the channels which 

 have been cut through the Columbia terraces, recent alluvium, 



