NATURAL BRIDGES OF UTAH 
221 
bed at C. These conditions afford evidence of two interesting 
points: First, the level at which the stream broke through be¬ 
neath the narrow neck, and second, the amount of erosion that 
has subsequently taken place. 
At all of the three bridges the old deserted channels are almost 
as easily traceable as if the water still flowed in them (Plate 
xiii. A). Since the time of the formation of the bridges, how¬ 
ever, the used channels have been cut from 50 to 100 feet deeper, 
and, in consequence, the deserted sections are left stranded at 
a correspondingly higher position. 
Plainly, the elevation of the abandoned channel marks the 
horizon at which the stream broke through and initiated the 
archway. With this fact in mind it is not difficult to determine 
the relative rate at which the opening has been enlarged, down¬ 
ward and upward. In the case of the Edwin Bridge and also of 
the Augusta Bridge, the development in both lateral directions 
has greatly exceeded the upward. 
The enlargement of the archway downward is, of course, 
accomplished very largely by stream-action alone. It is alto¬ 
gether possible, however, that when the opening was small, wind 
played an important part in its upward development; but, on the 
other hand, as it increased in height this factor necessarily dimin¬ 
ished, until at present it is almost entirely replaced by spalling 
brought about by both freezing and daily changes of temperature. 
Aside from the value of this fact in the present connection, 
it is interesting to note that in a country such as this, the down¬ 
cutting action of an intermittent stream is just about equal to 
the up-cutting accomplished by spalling on the lower surface of 
the overhanging rock-mass. 
,Not withstanding the fact that wind action on the under side 
of the bridges is at present so slight as to be almost negligible, 
yet, on the upper surface, it is of far more than passing impor¬ 
tance. Circular wind-pits, ranging from one to four feet in di¬ 
ameter, are very abundant, particularly on the top of the Edwin 
Bridge, where they comprise an intimate mass of shallow borings 
covering half the entire surface. In addition to the eroding action 
in connection with the wind, these depressions act as temporary 
receptacles for water derived from precipitation, and as places 
of marked disintegration during freezing weather. 
