Continued compaction by overburden then converts lignite into bituminous coal and finally, anthracite coal.
Conversely, in an extensional basin, older strata would be expected to develop higher grades beneath an overburden of younger rocks.
The overburden comprises sands and shales with thin coal interbeds, which passes upwards into sandstones and shales without coals.
It occurred to Greene that this channel resembled the area several hundred yards downstream where they had been digging - an area now filled with rock and buried under 4 feet of overburden .
Overpressured prodelta shales may act as a decoupling zone between the structural basement and deltaic overburden .
The workings were confined to several, steep-sided gorges from which the miners extracted fibre with some difficulty, mostly either by blast-removing overburden to expose benches or by underground mining.
the kaolin is mined by removing the overburden and digging a pit
In-situ methods make more economic sense for deposits covered by more than 75 m of overburden .
The economic cost of these factors could be reflected by an overburden of social medical response that could eventually deny health services for the masses, in many nations.
Contracts have been let to South Australian firms for site preparation, removal of overburden and the installation of a major mining and heavy mineral separation plant.