The Pico Formation in this area was deposited in a marine-delta environment, and, as I mentioned in one of my earlier posts (8/15/2014), fossil pine cones can be found (rarely) in these beds. The pine cones were derived from pine trees that grew in the adjacent, ancient San Gabriel Mountains east of the delta. Some of the pine cones eventually floated down a braided river (full of coarse debris consisting of pebbles and cobbles) and were deposited in fine-grained sandstone near an ancient shoreline, and mixed with shallow-marine fossils (e.g., seashells and shark teeth). It seems likely that presumed yucca remains could have also floated into this marine-environment setting.
When I first saw the presumed yucca fossil, I thought it was a pine cone. Upon closer inspection, however, I realized the yucca? fossil is not like the pine cones from this formation. As I walked back to my car, I came across a modern yucca plant (see photo and comments below). I was immediately struck by the fact that the vertical-striations on some of the woody part of the trunk of both the modern and presumed fossil yucca are very similar. If you have knowledge of the bases of fossil yucca plants, please let me know if you think my identification is correct or not.