July 24, 1885.] 



SCIENCE, 



67 



AN ARIZONA NATURAL BRIDGE. 



During a Aisit to Arizona, I came across a 

 natural bridge, an account of which may be of 

 interest in connection with recent notes on the 

 Virginia bridge. 



A long ridge, about six hundred or eight 

 hundred feet high, extends for some distance 

 north from a point near which the Atlantic 

 and Pacific railroad crosses the boundary be- 

 tween New Mexico and Arizona. This ridge 

 is formed by strata of dark-red sandstone un- 

 der light-red, and is capped by a stratum of 

 fine conglomerate or coarse grit. 

 These strata are broken, and pre- 

 sent an abrupt and generally per- 

 pendicular face to the west, slop- 

 ing at an angle of 14° to the east, 

 with the course of the stream, 

 until they bury themselves below 

 the alluvial sand. There are low- 

 er ridges, of corresponding struc- 

 ture, parallel to this on the east 

 and west. This ridge is cut b}^ 

 deep and narrow caiions. At the 

 mouth of one of these canons, just 

 before it dips under the sand, oc- 

 curs this bridge. It is about 

 twenty miles from the railroad, 

 and, as far as I know, has never 

 been visited by white men. I was 

 guided to it by my Indian scout. 

 The canon extends west for about 

 five miles above the bridge, and 

 becomes deep, narrow, and wild ; 

 the sides, with their growth of 

 hard- wood and pine, almost cut- 

 ting oflf the light at mid- day. 



The bridge is formed by a rem- 

 nant of the overlying grit, which 

 is continuous with it on both 

 sides. The section cut through 

 beneath it is of light and dark red sandstone, 

 the former showing very pretty cross-bedding, 

 and is non-conformable to the latter, which has 

 much less dip. The bridge is sixty-five feet 

 long, and fifteen feet wide at the narrowest 

 point. It is two feet thick in the centre, and 

 fifteen feet at the sides. The illustration, from 

 a photograph taken at the time, will give a 

 good idea of the position and proportions of 

 the bridge; our ponies, standing underneath, 

 serving for comparative measure. It will be 

 seen that the caiion is wider for a short dis- 

 tance above the bridge, which may be due to a 

 tributary caiion at that point. 



It is difficult to give an explanation of this 

 curious phenomenon ; and I shall only suggest 



a possibility, in the hope that some one will 

 find time to investigate it more thoroughl}'. 

 If above the present grit there had been soft 

 strata, capped again by grit, it might be ex- 

 plained on the principle of the Swiss pot-holes ; 

 a waterfall being formed above, which w^ore 

 a hole through the lower grit, and so under- 

 mined it, and cut out the sandstone beneath, 

 as at Trumelbach. But the grit has every 

 appearance of being a continuous cap over 

 the ridge. The grit shows, however, evidence 

 of an inclination to break into blocks ; and 

 it may be that a large crack, thus formed to 



the west of the present bridge, allowed the 

 stream to reach the soft sandstone, and so cut 

 it away beneath. 



A short distance off is another curious but 

 not so uncommon phenomenon, — a 'petrified 

 forest.' The stone tree-trunks lie just beneath 

 the soil, or half exposed, fallen in all direc- 

 tions. I procured specimens which showed 

 the bark, knots, roots, and branches. The 

 radiate arrangement of the wood-cells was 

 very evident in some cases. There are a num- 

 ber of these ' petrified forests ' in Arizona, I 

 was told. I know only of one other on the 

 Navajo reservation, and one near Flagstatf. 



Frederick Gardiner, Jun. 



