

GEOLOGICAL HISTORY OF THE COUNTY. F 2 . 39 



mously great ; and there is no part of the earth's surface 

 where the erosion of the rocks can be better studied, or the 

 quantity of material can be more easily estimated. (See 

 the numerous cross-sections given in this report.) 



To adduce a single instance : — The floor of Horse valley 

 is made by the exposed outcrops of limestone No. II. On 

 each side of the limestone descend in opposite directions 

 the slates of No. III. These once arched over the lime- 

 stones, but have been worn away. Tuscarora mountain 

 and Conecocheague mountain on each side of the valley are 

 made by the sandstones of No. IV, which once arched over 

 the slates of No. III. The still higher formations from No. 

 V to No. XIII (the Coal Measures) once lay in successive 

 arches over No. IV. In other words there was once a pile 

 of formations say 25,000 feet (nearly five miles) thick arch- 

 ing over what is now Horse valley, all of which has been 

 eroded and carried away in the lapse of geological time. 



The same is true of all parts of Perry county, with the 

 difference that in the troughs there has been less erosion 

 than on the rolls. For example, in the Cove, most of the 

 formations are still preserved under ground, and only say 

 5000 feet, or about a mile of rocks, have been removed. 



If then we take 3000 yards as the average thickness of 

 rocks removed from Perry county, and multiply it by the 

 area of the county, say 350, 000 acres, or about 1,700,000,000 

 square yards, we get a quantity of over 5,000,000,000,000 

 cubic yards of rock washed away into the Atlantic. 



But very few people have any idea of the amount of work 

 done by a single river like the Juniata in transporting the 

 land into the sea. 



In ordinary weather, a gallon of Juniata water carries 

 about 8 grains of earthy sediment, or one pound for every 

 100 cubic feet of water. 



At Millerstown, the river is about 000 feet wide and 4 

 feet deep, with a current flowing about 2 miles an hour ; that 

 is, 24,000,000 cubic feet of water pass Millerstown every 

 hour, carrying 240,000 pounds (120 tons) of rock sediment. 

 In other words, 1,000,000 cubic yards of the rock waste of 

 Juniata, Mifflin, Huntingdon, and Blair counties pass 



