Two types of rocks are found in Santa Rosa: clastic and alluvium rocks.
Clastic rocks consist of inter-bedded shale and sandstone with occasional thin lenses of limestone, as well as tuff and reworked sandy tuffs and partly tuffaceous shale. These formations are located in the southwestern portion of the municipality where Sto. Domingo, Don Jose, and portions of Pulong Sta. Cruz and Malitlit are situated.
Alluvium rocks are found in the remainder of the municipality including the Poblacion. These rocks consist of an unconsolidated mixture of gravel, sand, silt and clay. Figure 4-2 shows the types of rocks in the city.