By definition, sills are concordant, tabular sheet-like bodies of igneous rock with the Great Whin Sill of northern England often quoted as a primary example. Analysis of surface and subsurface records of the Great Whin shows that it is seldom concordant over wide areas and usually rises and falls in the stratigraphical succession in gentle transgressions and abrupt jumps to new levels. The term ‘sill’ is not used here for the Great Whin complex, but is used for the related Little Whin Sill of Weardale, Co. Durham. Studies of the petrology of the dolerite Whin complex have brought out significant differences between the Little Whin and the Great Whin. The Little Whin Sill is olivine-bearing and believed to be composed of an early differentiate of the Whin dolerite magma. The Great Whin, non-olivine-bearing and slightly density graded, is a later differentiate of the Whin magma. Two separate periods of Whin dolerite injection are confirmed by studies of vitrinite reflectance over the Alston Block where two periods of Whin contact metamorphism have been recognized. The two periods of Whin dolerite emplacement form part of the end-Carboniferous earth movements in northern England. They can be shown to have occurred between a period of compression from a W-SW direction and later gentle doming of the Alston Block near the Westphalian-Stephanian boundary, dated about 300-295 Ma.