Non-interpolatable classification of rocks in a region as defined by rock classification maps may be digitized to generate gridded index map. For rock classification maps stored as raster images, image treatment may be applied to generate the gridded index map. The gridded index map may be associated with lithology values using a rock-lithology library, and the lithology values may be used to generate a lithologic representation of the region. Such conversion of rock classification maps for modeling the region may improve input lithologic data accuracy and consistency, while enhancing the efficiency of lithologic modeling (e.g., for basin modeling).