Sand and gravel was traditionally taken in relatively small amounts from local 'borrow' pits. Nowadays large opencast sand and gravel sites are excavated by huge mechanical diggers. Coal is also now obtained in this way, rather than by using mining techniques. It is these large scale opencast sites that appear as HLA data.
Once worked out or abandoned, opencast sites are often restored for modern use, either as farmland or as recreational areas of open spaces, wetlands and footpaths.
Opencast sites like this one at St Ninians in Fife present difficulties when mapping their extent which aerial photographs help to resolve. Many are also noted on current OS maps.