Not sure were those are although I live near there. In addition to Junper Trees and duck islands, There are some natural features that give that appearance in this part of the state. The high clay soils a prone to freezing and thawing and over time they develope a mound appearance with surface rock accumulating around the mound perimiter. There is a discussion of this in Ca bureau of mines publication on the geology of the Macdoel quadrangle 1949. Look approximately 4 miles SE of the intersection of the Tennant Rd. and Highway 97.