Pleistocene cobbles of the San Pedro River

An AZGS geologist records cobble type, size and abundance of late Pleistocene cobbles. This deposit stands tens of feet above the modern channel and was deposited prior to the relatively narrow entrenchment of the inner valley. Older San Pedro deposits are generally composed of coarse, well-rounded cobbles, with reddened clay-rich soils and strong calcium carbonate (caliche) development. 6/2007 For a StoryMap tour of "Arizona's San Pedro River - through a geologist's eyes": http://uagis.maps.arcgis.com/apps/MapTour/index.html?appid=f6a2b2630c3b4...