NATURAL BRIDGES IN EASTERN WYOMING 441 
bed of the gulch erosion is slight at the present time, so that it will 
probably be a very long time before the ends of the bridge are 
undercut sufficiently to cause either end to tumble down. 
The formation of natural bridges of this type is easily under- 
stood but the origin of the indurated masses whose existence makes 
the bridges possible is not so readily explained. The indurated 
masses appear to fall into two classes, the one taking a more or 
less regular and uniform outline (seen in Fig. 1) and the other a sort 
of irregular lense of the rock (Figs. 2 and 3). In the first case it is 
probable that a part of the mass has been replaced by chemical 
solution, while in the second case the cementing material is already 
contained in the mass and only requires time to become indurated. 
Todd’ in describing similar occurrences in rocks of this age in South 
Dakota states that they probably mark ancient shore lines. It is 
in fact quite common to find the indurated masses at one level 
along an outcrop for several miles and it is not improbable that 
the waves of one storm, for instance, might throw together along 
a shore line or in a stream channel materials which would contain 
in themselves constituents that afterward become indurated. 
Barnum Brown? has stated in a discussion of the “‘ Hell Creek 
beds” (Lance formation) of Montana that “‘it is not an infrequent 
sight to see several parallel concretions, circular in cross-section 
and a hundred feet in length, like fallen trees.’”’ He says further 
that ‘‘they are not, however, true concretions but centers of 
solidification. Cross-bedding in the surrounding sandstone is 
frequently carried through the concretions line for line.” These 
indurated masses are invariably darker color than the more friable 
surrounding sandstones. Probably they were originally about the 
same color as the friable material, but either percolating waters 
have leached the iron out of the friable sandstone or else more 
rapid erosion prevents oxidation. 
tJ. E. Todd, Am. Geol., XVII (1896), pp. 347-49. 
2 Barnum Brown, Am. Mus. Nat. Hist. Bull., XXIII (1907), pp. 829-32. 
