San Pedro Valley - Geology

Cochise College    
Geology of Southeastern Arizona
Geology Home Page

Roger Weller, geology instructor    last edited:  3/15/18

San Pedro Valley
Cochise County, Arizona

Geology of Southeastern Arizona with Emphasis on the San Pedro River-Roger Weller


General Geology

Late Tertiary Surfaces and the Valley Fill Deposits of the San Pedro River


The portion of the San Pedro River Valley in Cochise County contained a large freshwater
lake between about two and four million years ago.  The lakebeds characteristic of that
environment are seen well exposed along the road from Benson nearly to Tombstone, and can
be seen lapping up onto the flanks of the Rincon Mountains to the north.  The lakebeds contain
abundant fossils of such large animals as ground sloths, camels, large bears, mammoths, bison,
turtles, early horses, many rodents, and numerous plants.  A few miles south of Saint David , on
the east side of the road are exposed some white beds which are marls, composed of a mixture
of limestone and clays.  These suggest a fairly large body of quiet water with not much drainage
by way of a river from the lake.  In this same area is exposed a record of the lake drying up before
the last major glaciation, because a thick paleosol (fossil soil) is contained in the section which is
interpreted as forming during a time with a temperate or moist climate with probably rather lush
grasslands or scrub oak forests growing in the area.  The lake's maximum depth is hard to estimate
because old shorelines are destroyed, but it could have been 200 or 300 feet deep. 

Following the deposition of the lakebeds, a whole series of alluvial sediments was washed
down to the valley floor from the highlands.  These came first from the Dragoon Mountains to the
East, and then at a slightly later time, from the newer mountains, which lie to the west- the
Whetstones.  The gradual accumulation of these sediments with very shallow dips leaves behind
alluvial surfaces that can in this case be correlated from the area of Tombstone around to the
North side of the Santa Rita Mountains.  This surface, which is called the Whetstone surface, is now
slowly being dissected by erosion, but remnants are easily visible from I-10 between Tucson and

            Following this alluvial deposition in the San Pedro Valley, the land, which had been rising
slowly for quite some time, was elevated to a point where the through-flowing San Pedro River
began cutting away and removing the deposits in the center of the valley, and thus exposing them
to the eyes of the curious geologists.  The actual age of the lakebeds has been determined by
studies of the fossils contained in them, and by potassium-argon dating of volcanic as beds found
mixed in with the lakebeds.


Maps of the San Pedro Valley

Views of the San Pedro River and San Pedro Valley

overview article on the geology of the San Pedro River area
Geology of the San Pedro River, Roger Weller, Cochise College, 2008

major link on geology of the San Pedro River Valley

directions to the St. David selenite location
St. David selenite roses

articles on the San Pedro River Valley
The Lehner Mammoth Site, Southeastern Arizona,
     by Emil W. Haury, E.B. Sayles, and William W. Wasley
     American Antiquity, vol. 25, no. 1, 29 pages

Windows to the Past: Fossils of the San Pedro Valley
     by Everett H. Lindsay
     Fieldnotes from the Arizona Bureau of Geology and Mineral Technology
     vol. 14, no. 4, Winter 1984, 12 pages

Geomorphology and Surficial Geology of Garden Canyon, Huachuca Mountains, Arizona
     by Gary Huckleberry
     Arizona Geological Survey, Open-File Report 96-5, 1966 (selected portions)

Climatic influences on continental deposition during late-stage filling
of an extensional basin, southeastern Arizona (selected portions)
     by Gary A. Smith
     Geological Society of America Bulletin, v.106, p. 1212-1228, Sept. 1994

Published references on the geology of the San Pedro Valley

student presentations  on the San Pedro Valley
Murray Springs, Clovis Site-Jenna Backinger  (Fall 2007)