Miners Lettuce (Claytonia perfoliata)
A member of the Portulaca Family, this annual grows in very moist, shady places -- photographed above (and below) in Hot Springs Canyon above the Yellow Cliffs in March 2005, after good spring rains had brought some flooding to a number of sidewashes (this one at the base of North-facing cliffs). (Typically it is found around springs in Arizona.) The flower-bearing leaves are "dark green, fleshy, circular... to 2" wide Other leaves are lance-shaped on narrow stalks" (Epple, cited on Wildflowers page, p.62). The flowers, shown more clearly below, are "White to pinkish; to 1/4" wide, in tiny raceme rising above center of circular leaf" (Epple, op. cit.). (Click on each image to enlarge it.)
This plant was used by Native Americans (and later, by pioneers) as a potherb and for salads.