Salt River Terraces

I just read an interesting article [1] regarding ancient terraces on the lower Salt River. I was well acquainted with four of the terraces; Sawick, Mesa, Blue Point and Lehi. The first three are easily found in the Lower Salt River Recreation Area.  The Sawick terrace is the oldest and is found on top of the Mesa Terrace, both of which are situated on the Blue Point terrace.

The Coon Bluff and Phon D. Sutton parking areas lie on the Blue Point terrace. The three oldest terraces are accessible from the Phon D. Sutton parking area and from Bush Hwy southeast of the Usery Rd. intersection.  The terraces are considered “scarp” terraces because they were formed by cutting through bedrock. The cutting through bedrock may be related to uplift of the mountainous transition zone to the east. The gravels on the Sawick and Mesa terrace are well cemented by caliche. The Sawick terrace is estimated to have been excised 1-2 million years ago [2].

On Mesa Terrace with higher Sawick Terrace in immediate background

Looking down from Mesa Terrace to Blue Point Terrace

Gravels cemented with caliche on Mesa Terrace

The 2010 article reports findings describing another older and higher terrace named the Stewart Mountain Terrace. Characteristics of the gravel hint at different flow of the Salt River when it was formed. The new terrace was discovered at 1870' on the north side of the Salt River southwest of Stewart Mountain and east of the Verde River. The types of gravels found on the new terrace suggest ancient river flow different from the current floodplain.

The most interesting fact for me is that the proposed new terrace suggests that the river was once at a much higher elevation (approx 500' higher) and much wider than the current river. That's a lot of river.

Another mechanism discussed is that the terrain may have been 500' lower millions of years ago. As the terrain rose, the river could have cut successively lower terraces.


[1] STEWART MOUNTAIN TERRACE: A NEW SALT RIVER TERRACE WITH IMPLICATIONS FOR LANDSCAPE EVOLUTION OF THE LOWER SALT RIVER VALLEY, ARIZONA, September 2010, PHILLIP H.LARSON,RONALD I.DORN, School of Geographical Sciences and Urban Planning, Arizona State University, PO Box 875302, Tempe, AZ 85287-5302; JOHN DOUGLASS, Department of Geography, Paradise Valley Community College, 18401 N 32nd St,Phoenix, AZ 85032; BRIAN F. GOOTEE, Arizona Geological Survey, 416 W Congress St., Ste 100, Tucson, AZ 85701-1381;and RAMON ARROWSMITH, School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287-1404

[2] TERRACES OF THE LOWER SALT RIVER VALLEY IN RELATION TO THE
LATE CENOZOIC HISTORY OF THE PHOENIX BASIN, ARIZONA, by Troy L. Pewe, Guidebook to the Geology of Central Arizona, 1978.