The soil often varies across a site or by depth. If an unsuitable soil is found by excavating at a specific location, a better soil may be located either above or below it or elsewhere on the site. Soils of varying properties can be selected in this way for blending to give the desirable properties for a daub. If a suitable earth cannot be created from materials on site, then appropriate materials can be bought in. Suitably graded aggregates can be selected using an appropriate guide, such as the English Heritage Directory of Building Sands and Aggregates.[61]