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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
&2, Mr. Leopold as the climate of the Southwest has always been relatively dry. The analysis given above applies only to the relatively large drainages. The through flowing rivers have silted on their lower courses recently and doubtless have had reversed. phenomena with each climatic swing. I have been teaching here at Harvard for the past three years but have spent the past two summers in New Mexico with much pleasure and scientific profit. Yours cordially, KB:D 2ý