NATURAL BRIDGES BY SOLUTION 329 



present course. The gorge from the dam to the preglacial valley is a 

 -succession of broken pot-holes var3dng in size up to 6 or 8 feet in diam- 

 eter, showing that after the tunnel was made the gorge was largely exca- 

 vated in this way. The preglacial valley in which the Hudson Brook 

 flows below the gorge is broad, but to some extent choked with glacial 

 drift. 



d. Illustrations of bridges formed ly the widening of a joint. — By 

 the widening of a joint near the source of a spring a portion of the coun- 

 try rock may be separated from the parent ledge so as to form a bridge. 

 A good example of a bridge of this origin is the Natural Bridge at Chat- 

 tanooga, Tennessee. The bridge (plate 24, figure 2) is a small one, being 

 8 or 10 feet in height, and spans a stream which has its source in a spring 

 a few yards away. It is clearly formed by the widening of a joint, which 

 thus separated the rock of which it is composed from the parent ledge. 

 The bridge is used to some extent by foot passengers. 



From Shepherd^s^^ descriptions of two bridges in Green County, Mis- 

 souri, it seems probable that they also are of this type, being formed by 

 the widening of a joint or fissure near the mouth of a cavern. 



One of these bridges is described as follows : This beautiful bridge 

 (township 28 north, range 21 west, section 3, northeast % of north- 

 east %) abruptly heads a narrow gorge about 100 feet wide, which ex- 

 tends up from the bottom lands of the James Eiver. The country road 

 formerly passed over it. This bridge is 50 feet long, 15 feet wide, and 

 12 feet high. 



Another bridge described is situated 4 miles east of Springfield, Mis- 

 souri. 



2. BY THE COMBINED ACTION OF 8UBAERIAL AND SUBTERRANEAN EROSION 



The following statement is one which, with a slightly modified phras- 

 ing, has done service in the geologies and geographies for a generation or 

 two as an explanation of the origin of natural bridges : 



"As a cavern enlarges and the surface of the land above it is lowered by 

 weathering, the roof at last breaks down and the cave becomes an open ravine. 

 A portion of the roof may remain, forming a 'natural bridge.' " ^ 



The accompanying familiar diagram (figure 8) illustrates the theory. 



Theoretically one would expect to find a large number of bridges of 

 this origin, but investigation has failed to bring to light any well-defined 

 examples. 



21 B. M. Shepherd: Geology of Green County, Missouri, vol. xii, 1898, pp. 117-118. 

 22 Norton : The elements of geology, 1905, p. 46. 



XXIV — Bull. Geol. Soc. Am., Vol. 21, 1909 



