36 
GEOLOGY: C KEYES 
was excavated in the smooth surface of the sloping plain. By the time 
a permanent bridge was built to span the deep trench the winds had 
filled the entire excavation, so that where a yawning chasm had been 
was again as smooth as the rest of the plain, and the wing-dams also 
had melted down into the general evenness of the desert's surface. 
For several years, until it was finally replaced by an earthen grade, 
travelers were wont to express great wonderment at the possible utility 
of so fine a steel bridge resting on the smooth sands of the desert plain. 
The Socorro arroyo, in central New Mexico, presents another perti- 
nent case. There is water running in the shallow wash once or twice 
a year, the supply coming off the lofty Magdalena peak 20 miles away. 
For many years this arroyo, which divides the town of the same name, 
has given the residents an infinite amount of trouble. That its 2% 
grade really produces torrential conditions when the waters do run is 
indicated by the fact that the arroyo-bed is composed largely of pebbles 
and boulder?, many of the latter attaining a size of 2 feet. In order 
to obviate the yearly inconveniences of flooding it was determined, a 
few years ago, to divert the channel 4 miles above the town. This 
change of course was accomplished by cutting a narrow trench from the 
bed of the water-way through its bank to a point some 50 yards to one 
side, where the general plain was slightly lower than the bed of the 
wash at the head of the ditch. A low dam was thrown up obliquely 
across the arroyo by piling up boulders from the bed. The theory was 
that the first water coming down the wash would flow out the ditch, or 
spill-way, and there would soon cut a deep channel; and that eventually 
this would carry away all of the future flood-waters. Results more 
than fulfilled expectations. The first time the dry creek became a 
brook there was trenched in a single night a chasm 50 feet deep for a dis- 
tance of more than a mile down the slope of the plain. The materials 
from the great artificial canyon spread out over the railroad tracks 3 
miles away to a depth of 7 feet and to a width of half a mile, necessi- 
tating the rebuilding and raising of the grade for a distance of several 
miles. These two illustrations might be infinitely repeated. 
Observations such as these demonstrate beyond a shadow of doubt 
that whenever around the desert ranges there are alluvial fans, or ac- 
cumulations of unindurated deposits, it is possible at any time for pro- 
found and rapid dissection to take place through means of the copious 
but infrequent storm-waters. The recent notes of C. L. Baker, A. C. 
Trowbridge and others on the eastern slopes of the Sierra Nevada, 
amply confirm these conclusions. Since, however, the restricted areas 
in which the latter investigations were undertaken lie at the foot of 
