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NATURAL BRIDGES IN EASTERN WYOMING 439 
indurated masses when they lie in the way of a receding gulch some- 
times result in natural bridges. Their origin is easily explained. 
As the gulch receded the water first exposed the indurated mass, 
then a waterfall was formed and undercutting commenced. This 
process soon removed the softer material until the water flowed 
under the harder mass and a natural bridge was formed. In 
Fig. 1, a loglike concretion is seen at the very head of a gulch, 
which has recently been undercut. The diameter of the concretion 
is about three feet and its length is about ten feet. In this case 
Fic. 1.—Loglike concretion occurring in friable sandstone of Lance formation 
about eight miles south of Moorcroft, Wyo. 
one could step from the bridge to the bank at the head of the 
gulch. : 
Fig. 2 represents a bridge spanning a ravine 30 feet wide and 12 
feet deep. The loglike concretion is 5 feet wide by about 3 feet 
thick and is quite uniform throughout its length. It is flat on top 
and sufficiently firm to support a saddle-horse, as shown in the 
picture. The third case (Fig. 3) shows an indurated sandstone 
mass less regular in outline than either of the others. This one was 
visited by Mr. Winchester who states that the bridge is about 16 
feet long and 4 feet wide on top. The thickness of the indurated 
