332 H. F. CLELAI^D NORTH AMERICAJST NATURAL BRIDGES 



A small bridge of light gray marl is reported near Homosassa, Walton 

 County, Florida, the width of the stream being about 20 feet and the 

 width of the bridge, from the "sink" to the '^rise" of the stream, being 

 about one-third or one-fifth of a mile.^^ 



i. New York bridges. — The New York natural bridges in Jefferson 

 County are formed by the first method. The largest of these is ona 

 across Perch Creek (figure 9), but even this one does not fulfill the usual 

 conception of a natural bridge, since the arch does not stand much above 

 the surface of the stream at low water and is submerged at high water. 



"The water flows underground in the Black River limestone, which is full of 

 enlarged joints, and runs underground on the summit of the Lowville, which is 

 less soluble. It eats out a channel in the basal portion of the Black River, 

 and as this has no great thickness, it is not self-supporting over the arches, 

 and much of the roof slowly slumps as the process goes on. It is here entirely 

 a process of limestone solution and of slow slumping of the roofs due to non- 

 support. The water goes down a widened joint athwart the stream course, 

 and follows joint planes more or less parallel with the course, eating them away 

 until they hold a larger and larger portion of the stream's capacity. There are 

 several instances in the district where the entire flow is usually underground, 

 but where the bare limestone surface of the channel shows clearly that the 

 underground channel can not care for the entire volume during floods. Slight 

 falling of the rocks is often influential in putting the streams back to the sur- 

 face again."^* 



In one place the stream is brought to the surface, where it cuts across 

 a preglacial channel at about right angles, emerging from under one wall 

 of limestone and disappearing under the opposite wall. 



c. Iowa hridges. — In the report on Jackson County, Iowa, Savage^^ 

 describes a "series of natural bridges that have been developed by the 

 waters of a small creek eroding a subterranean passage." "The upper or 

 most northerly bridge has a length of 150 feet across the gorge and a 

 width of about 60 feet." "[The stream] has carved a passage 50 feet in 

 height beneath the span of the bridge." "About 8 rods farther down 

 the stream a second arch crosses the ravine." The bridges are stated as 

 being the result of the partial caving in of a cave roof. 



d. Missouri Natural Bridge. — A natural bridge in Miller County, Mis- 

 souri, differs from the other bridges which have been included in this 



23 Modified from descriptions given by Dr. E. H. Sellards in a letter to the writer and 

 from the Second Annual Report of the Florida Geological Survey, 1909, pp. 36-37. 



2* H. P. Gushing in a letter to the writer, June 8, 1909. See also R. Ruedemann, 

 "Types of inliers observed in New York." New York State Museum Bulletin no. 133, 

 1909, p. 175. 



25 T. E. Savage : Geology of Jackson County, Iowa, Geological Survey, vol. xvi, Annual 

 Report, 1905, pp. 571-573. 



