NATURAL BRIDGES BY STREAM EROSION 321 



where it crosses the Arbuckle Mountains 8 miles south of Davis, Okla- 

 homa. 



"The cliffs on the east side of the stream are from 150 to 300 feet high and 

 are composed of solid limestone. At some point between 'C and *D,' as indi- 

 cated in this map, the stream evidently found a course along a folding plane 

 or joint along the narrowest part of the interposed wall. This condition con- 

 tinued until all of the stream ran through this channel and the old channel 

 was abandoned. Later the bridge which evidently spanned the stream fell in." 



2. BY POT-HOLE ACTION 



Theoretically bridges of small size should result by the enlargement 

 of the diameter of pot-holes near their bottoms until the walls of two or 

 more such holes are worn through, thus permitting the water of the 

 stream to pass under an arch. At Shelburne Falls, Massachusetts, and 

 in many other stream beds, partial bridges of this origin occur, but com- 

 plete bridges of this type are rare. 



a. Bridge spanning Kiching Horse River, British Columbia. — The 

 rock of which this bridge, near Field (plate 20, figure 1), is composed is 

 an impure limestone in which a strong vertical cleavage has been pro- 

 duced, the cleavage being so strong that the synclinal structure of the 

 rock of the ledge is often rendered indistinct. 



Formerly a ledge of rock produced a fall or rapid in the river at this 

 place. In the course of time two series of pot-holes were developed (as 

 is shown in the diagram, figure 6), which gradually enlarged, especially 

 near their bottoms, until they finally opened into one another. The 

 openings were small at first and only a small quantity of water poured 

 through, but when the wall was broken through they rapidly enlarged, 

 until now the entire volume of the stream flows under the ledge. The 

 height and width of the bridge (figure 6) are difficult to estimate, be- 

 cause of its irregularity and also because of the great volume of water 

 which pours through and fills the opening. The average width of the 

 arch is about 10 feet and the length between 5 and 8 feet. The height 

 of the top of the bridge above the stream is in places not more than 3 

 feet and the thickness of the arch is probably between 6 and 8 feet. 



h. Lamoille River Bridge, Vermont. — A bridge which spans the 

 Lamoille River at low water (shown in the accompanying illustration, 

 plate 20, figure 2) appears to have been formed by pot-hole action, judg- 

 ing from the information furnished by Mr. M. A. Gibson.'^ The width 

 of the ledge through which the opening has been made varies from 6 to 

 10 feet. The mica schist of the stream bed is full of pot-holes, both 



' M. A. Gibson in a letter to the writer, 1906. 



