OUTWASH FROM THE OLDEST DRIFT. 245 



remnants of the gravel were found east of Logansport and back of White 

 Rock at an altitude of about 220 feet above the river. 



On the west side, near Natrona, there is one of the most extensive 

 terrace remnants on the river, fully a square mile being preserved at an 

 altitude of about 250 feet above the stream. Jillson has determined the 

 altitude by Locke level and found it to be 953 feet above tide at the reser- 

 voir and 963 feet at the fair grounds.' This may have been reduced about 

 50 feet from the original level, as indicated by deposits at Arnold, discussed 

 below. The surface portion to a depth of several feet is mainly sand, but 

 at greater depths fine gravel occurs. The gravel has a depth of more than 

 50 feet and may in places reach 100 feet. 



An extensive remnant of gravel filling is found on the east side of the 

 Allegheny below Natrona, near Arnold and Kensington stations. It borders 

 the river for. a distance of nearly 3 miles. Just east of Arnold gravel is 

 found up to an altitude about 200 feet above the station, or 1,000 feet above 

 tide. Jillson has determined its altitude by Locke level to be 1,011 feet 

 above tide. There is an island-like hill with this altitude bordered by a 

 terrace 60 to 65 feet lower. An old weather-stained gravel covers the slope 

 from this terrace down to the level of the railwa)^ station, about 800 feet 

 above tide, where the Wisconsin gravel sets in. If the valley trenching 

 had reached down to the level of the railway station before the first gravel 

 filling occurred, a filling of 200 feet would have been required in the deepest 

 part of the trench to build it up to the level of the island-like knoll. This 

 is, however, one of the cases in which the relation of the gravel to valley 

 trenching could not be clearly made out. 



The gravel appears on the west side of the river below Kensington, 

 and is preserved in a tei'race I'emnant, about 2 miles in length, back of 

 Springdale and Acmaton. The altitude of the upper terrace, as determined 

 by Jillson, is 954 feet above tide at Springdale and 948 feet at Acmaton. 



' The late Dr. B. C. Jillson, of Pittsburg, made many accurate determinations of the altitudes of 

 terraces and rock shelves in the vicinity of that city on the Allegheny, Monongahela, and Ohio rivers, 

 which have been published by the Pittsburg Academy of Science, in a pamphlet of 25 pages issued in 

 December, 1893, and entitled "River Terraces in and near Pittsburg." It should perhaps be 

 explained that the writer's studies preceded the publication of this pamphlet and were carried on 

 without the knowledge of Jillson's work. The studies were entirely independent of each other, yet 

 the interpretations are in essential harmony so far as the old or high-level gravels are concerne<l. The 

 title of Jillson's paper, as. well as his discussion, indicates that he recognized the deposits to be the 

 product of a stream. 



