OLD DRAINAGE FROM UPPER TO MIDDLE ALLEGHENY. 141 



divide at the head of Pithole Creek rises to a height of l,fi40 feet above 

 tide, and the rock surface is about 1,560 feet, while the rock floor in the 

 valley on the north is less than 1,200 feet. It is scarcely probable that 

 the divide crossed by Oil Creek south of Titusville stood quite so high. The 

 gap made by the creek is bordered by abrupt bluffs up to a height of only 

 1,320 feet, which probably marks the height of the old divide. South of 

 this old divide is the small valley of the old lower Oil Creek, leading to 

 the Allegheny at Oil City. Its width, including rock shelves, averages 

 scarcely 100 rods, while the width inside the rock shelves is in places but 

 40 to 60 rods. It is similar in size to Pithole Creek Valley, which drains 

 a small district on the east. This small valley had been excavated nearly 

 to the present level of Oil Creek before the culmination of the earliest 

 glaciation, for low rock shelves on its borders 40 to 60 feet above the 

 stream are thickly covered with early glacial deposits. Its valley floor is in 

 harmony with that of the neighboring portion of the Middle Allegheny, 

 which was excavated nearly to the present stream level before the glacial 

 deposition took place. An abandoned oxbow channel west of Petroleum 

 Center has a rock floor as low as the creek level, 1,090 feet above tide, and 

 yet it seems not improbable that its excavation preceded the drift deposition.-^ 

 At the Boughton Acid Woi-ks, within a mile south of the old divide, the 

 valley floor appears to have stood only about 1,200 feet above tide, or 40 

 to 50 feet above the creek at the time the reversal took place. 



The headwater portions of French Creek and Little and Big Broken- 

 straw creeks evidently have been greatly modified by the obstruction of 

 old lines of drainage. In several places the waters now divide in the 

 valley-like lowlands which probably were formed by ancient streams. 

 It is probable that the headwater portion of Little Brokenstraw Creek 

 discharged into the valley now occupied by Lake Chautauqua, there being 

 an abandoned valley along the line of the Erie Railway from this creek at 

 Grant, N. Y., to Lake Chautauqua. Evidence that the headwater portion of 

 this creek was once distinct from the lower coui-se is found in the lower 

 altitude of its rock floor, and also in the fact that the headwater portion 

 is in a larger valley than the lower course of the creek. A boring at 

 Lottsville, Pa., in the headwater portion of the creek reached a level 150 

 feet below the rock floor at the mouth of the creek without entering rock. 



'See Carll: Second Geol. Survey Pennsylvania, Kept. I*, 1883, p. 311. 



