LOWER ALLEGHENY DRAINAGE SYSTEM. 143 



the old Muddy Creek (Upper Oil Creek) drainage. But as yet no line 

 has been traced out to the lake and it is not certain that the constricti6n 

 on French Creek Valley near the Erie-Crawford county line marks an old 

 divide. 



THE LOWER ALLEGHENY AND ITS TEIBUTAEIES. 



In the discussion of the old Monongahela system (p. 88), attention was, 

 called to the broad gradation plain found on the Lower Allegheny, and com- 

 parison was drawn between the breadth of that gradation plain and the much 

 narrower rock shelves, abandoned ox-bows, and gradation plains found on 

 the Middle Allegheny system. The Lower Allegheny and all its tributaries 

 lie in large part, if not entirely, outside the glacial boundary. Consequently 

 the several fluvial plains are better displaj^ed than in the Middle and Upper 

 Allegheny where much of the drainage area lies within the glacial boundary 

 and where the old fluvial plains have been greatly concealed by glacial 

 deposits. But even on the Lower Allegheny there have been quite heavy 

 deposits of glacial gravel (60 to 100 feet in depth), made by streams that 

 led down this valley from the ice field to the north, after the old divide 

 which separated the Lower from the Middle Allegheny had been cut away. 



The main gradation plain, as determined b)' aneroid, has a height of 

 1,020 to 1,040 feet in the vicinity of the mouth of the Clarion River or 150 

 to 170 feet above the present stream. It falls to about 900 feet at the 

 junction of the Allegheny and Monongahela at Pittsburg and stands at that 

 point very nearly 200 feet above the river. The glacial gravel which caps 

 the gradation plain has an upper limit at about 1,135 feet near the mouth 

 of the Clarion and about 975 feet near Pittsburg. The original lower limit 

 is not known, inasmuch as the amount of trenching prior to the gravel 

 deposition has not been settled. 



In the western part of the city of Allegheny a feature was observed which 

 supports the view that the streams had cut somewhat below the level of the 

 old gradation plain before the gravel filling took place. Upon ascending to 

 the old gradation plain along California avenue north of Woods Run the 

 contact between the glacial gravel and the underlying shales is well exposed. 

 The shales are found to be deepl)'^ weathered, so that for 1^ to 2J feet from 

 the surface only a brown residuary clay remains. The surface of the shale 

 here stands about 890 feet above tide and very nearly at the general level of 

 the old gradation plain. The amount of weathering which it had undergone 



