MARYLAND GEOLOGICAL SURVEY 79 



other streams; thus, Conowingo Creek develops two ox-bows in its 

 course just before it passes over into Cecil county from Pennsylvania, 

 and Principio, Northeast, Little and Big Elk creeks all exhibit mean- 

 dering courses, particularly toward their headwaters. 



The third feature is that none of the streams have confined their 

 courses to any one sort of rock, but pass abruptly from one kind to 

 another, and back again, and run indiscriminately through rocks of 

 varying hardness and solubility. Octoraro Creek shows this feature 

 also most strikingly. The wavering course of this channel runs 

 abruptly off of one kind of rock to cut an ox-bow in another, and 

 suddenly returns to the first, only to leave it again a few miles further 

 down. Thus the stream enters the State from Pennsylvania, eroding 

 on a serpentine rock; this it leaves abruptly to cut through a dike 

 of pegmatite, then flows athwart a mass of hypersthene-gabbro avoid- 

 ing two other pegmatite dikes. After leaving this rock, it cuts 

 abruptly through a mass of meta-gabbro and into a formation of 

 granite-gneiss. In this rock it erodes an ox-bow, then turns and 

 runs suddenly into the meta-gabbro once more. After flowing along 

 the border between the meta-gabbro and granite-gneiss for about half 

 a mile the stream re-enters the mass of the latter and remains in it 

 until it reaches the Susquehanna river. The crystalline rocks of 

 Cecil county are not strikingly different in their powers of with- 

 standing erosion, and the anomaly of a stream crossing from one sort 

 of rock to another and back again is not as well brought out in this 

 county as further south, where streams along the Piedmont Plateau 

 run abruptly from very soft to very hard rocks and back again with 

 utter disregard of the great differences in the varying powers of the 

 rocks to withstand erosion. 



It has been demonstrated by countless field observations that nor- 

 mal streams, when they have a steep grade and rapid current, tend 

 not only to flow in direct courses, but also to avoid the harder rocks, 

 and to establish their channels in the softer ones, thus moving along 

 the lines of least resistance. The streams of the Piedmont Plateau 

 are therefore not normal, but the contrary, and as they all possess 

 these abnormal features, it wonld seem to point to some cause which 



