The Dendritic or Scenic Shale appears to be fragments of concretions weathered from the underlying Cretaceous shales and would only have travelled a short distance to this site of deposition.  Many of the polishable pieces are contained in an outer shell of iron and manganese oxides, these minerals causing the penetration inward of dendritic and colored patterns.  They occur in shades of yellow, tan and gray.  Specimens do take a respectable polish due to the high silica content adding to their hardness.  It works quite will into pendents and bolo ties.