HARVARD UNIVERSITY 
     DEPARTMENT OF GEOLOGY AND GEOGRAPHY 
                GEOLOGICAL MUSEUM 
 
                   November 21, 1928         OXFORD STREET 
                                         CAMBRIDGE, MASSACHUSETTS 
 
 
Mr. Aldo Leopold 
421 Chemistry Building 
Madison, Wisconsin 
 
Dear Leopold: 
 
I saw in Science some time ago that you were on 
your own in a Game Survey - congratulations! 
 
The paper in Ecology was as you doubtless saw a 
little propaganda to induce botanists to take 
arroyos seriously. In regard to your question on 
climate: the present arroyo in our Southwestern 
valleys had a predecessor in late pre-historic 
time. This arroyo was cut and then filled up. 
The only thing in literature is an abstract in 
the Washington (D.C.) Acad. Sci. Jour. vol.16, 
pp.75-76, 1926, "Recent deposits of Chaco Canyon 
in relation to the life of the Prehistoric peoples 
of Pueblo Bonito". The full account will appear 
as an appendix to Neil M. Judd's account of his 
explorations at Chaco Canyon to be published by 
the National Geographic Society. 
 
This buried channel of late prehistoric date has 
been formed on Rio Chaco, Rio Puerco, Rio Zuni, 
Rio Galisteo and elsewhere on smaller creeks. As 
it formed when there were no domestiQ grazing 
animals in the country and then filled up again, e 
we cannot attribute either process to the influencog 
of overgrazing. It appears to have been a climatic 
swing from less dry to dry and back to less dry. 
Presumably there was a swing to dry again before 
the Spanish conquest and the conditions were ripe 
for overgrazing to be effective in timing the new 
arroyo. That is, it would have been cut any way 
but the precise time was determined by the intro- 
duction of stock. 
 
Overgrazing would imitate gullying on hillsides 
more or less regardless of the swings in climate