8. Dikes associated with hydraulic fracturing are often accompanied by sills, mainly in the form of the horizontal intrusions that are shown in the slide. Sills often form within the portion of the cap that has fine-grained strata and are especially common along the base of the lowermost fine-grained stratum. Sills normally are very thin in relation to their length. The thickness of a sill typically ranges from a few to several centimeters but can exceed a half meter where liquefac tion has been severe.