From Soil Taxonomy:
R layers: Hard Bedrock
Granite, basalt, quartzite, and indurated limestone or sandstone are examples of bedrock designated R. R layers are cemented, and excavation difficulty exceeds high. The R layer is sufficiently coherent when moist to make hand-digging with a spade impractical, although it may be chipped or scraped. Some R layers can be ripped with heavy power equipment. The bedrock may contain cracks, but these are generally too few and too small to allow roots to penetrate. The cracks may be coated or filled with clay or other material.