This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
9. Rock, in its original geological formation, is called bed rock, and as a rule, makes the best support for foundations. It requires, however, good judgment to determine its value, and careful handling to secure the best results, for it very often happens within the area of a proposed building - especially if it is to be a large one - that the rock will present uneven surfaces, so that some parts of the foundations will rest on rock and others on loose gravel or clay. The methods followed, where these irregularities of surface are met with, will be found under the heading "Footings."
The sandstones and limestones are often found in strata, beds, or layers, one on top of another. If these layers are not separated by clay, and the beds are even, they make good foundations. The strata or beds of rock may shelve, or dip at varying angles. This formation is especially found in hilly sections. The method of laying foundations when shelving rock is encountered will also be described under "Footings."