MARYLAND GEOLOGICAL SURVEY 



81 



emerging sea-bottom as on a new land surface; gradually they sank 

 their valleys deeper and deeper in the cover of soft sediments, and 

 finally, one by one, reached the underlying buried crystalline rocks 

 (Fig. 4). The structure of these rocks on which the streams now 

 began to flow was extremely complex and entirely different from 

 that of the formations through which they had just cut their valleys. 

 Thus, on reaching the buried rocks, the drainage was immediately 



Fig. 4. — Diagram showing origin of superimposed river. 



out of adjustment with the conditions there encountered. Neverthe- 

 less, the rivers, unable to alter their courses, continued to sink their 

 channels with the same circuitous courses which they had developed 

 when meandering across the low, featureless surface of the newly- 

 raised ocean-bottom. Thus it came about that these streams, cutting 

 on. whatever rocks they happened to uncover, would at one time erode 

 a hard and at another time a softer rock, as each chanced to appear in 

 their beds. In this way a drainage pattern which had been developed 

 on one kind of deposits, in harmony with the conditions which there 

 existed, became fixed in the underlying crystalline rocks, although 

 out of adjustment with the conditions which were there found. Thus 



