76 THE PHYSIOGRAPHY OF PEINCE GEOEGE^S COUXTY 



plain is so well developed the flat-topped divides are in several 

 instances as much as two miles in width, while in the northern por- 

 tion they are seldom more than one-half mile wide. The difi^erence 

 is due to the relative ease with which the difl'erent strata are eroded 

 as the geologic map shows the narrow divides in the Potomac strata 

 and the wide ones in the regions where the Tertiary formations out- 

 crop. The asymmetry of the drainage is shown in the greater length 

 of the secondary streams flowing into the Potomac as compared with 

 the Patuxent tributaries. In Charles and St. Mary's counties the 

 streams on the Patuxtent Piver slope are very short and numerous 

 as compared with the fewer, longer, and larger tributaries on the 

 Potomac side, and in Calvert County the Chesapeake drainage slope 

 receives the small streams while the Patuxent River the large ones. 

 Thus the west erh' -flowing streams seem to have been able to advance 

 their headAvaters faster than the ones flowing in the opposite direc- 

 tion. In Prince George's (younty the same conditions prevail, though 

 in a much less exaggerated form. However, the sinuous divide 

 between the Patuxent and Potomac drainage basins is readily seen 

 to lie adjacent to the Patuxent River. This asymmetry would be 

 more marked were it not for the Western Branch, which extends its 

 head so far from the parent stream. 



At the present time because of the tributaries of the Patuxent 

 being shorter and more direct than those of the Potomac, erosion is 

 more vigorous in the former basin than the latter, with the result 

 that the divide is being pushed rapidly toward the Potomac River. 



Tide-water Estuaries. — The lower courses of almost all the larger 

 streams emptying into Chesapeake Bay have been converted into 

 estuaries througli a submergence which has permitted tide water to 

 pass up the former valleys of the streams. In the early development 

 of the country these estuaries were of great value, as they are nav- 

 igable for many miles from their mouths and thus afford means for 

 ready transport of the produce of the region to market. Even the 

 advent of railroads has not rendered them valueless and much grain 

 and fruit are now shipped to market on steamers and small sailing 

 vessels which traverse these estuaries. Chesapeake Bay and its 

 tributary estuaries also furnish good flshing grounds, and during 



