Abstract: A method for in situ processing of oil shale in which rubblized shale is removed from the retort and subsequently returned to the retort. A section of shale in a subterranean formation is first rubblized so as to form a retort chamber filled with rubble. The rubblized shale is then removed from the retort chamber, followed by crushing of the rubblized shale into shale particles of various sizes within an overall size range. Subsequent to the crushing step, the shale particles are separated according to size into a plurality of shale particle groups, each of which includes shale particles within a predetermined group size range. Each group size range makes up a portion of the overall size range. Substantially all of the shale particle groups are then sequentially reloaded into the retort chamber so that the particle groups are graded according to particle size within the chamber, wherein the largest particles are at bottom end of the chamber and the smallest particles are at the top end of the chamber.