The tiniest rock grains - less than 0.06 mm or 0.002 inch wide - may stick together to form siltstone or mudstone. The fine grain of a siltstone or mudstone is an indication that it formed from mature sediments - silts that have been weathering for a long time, have been well sorted, and may have traveled far from the rocks from which they were broken off. Generally, humans need a microscope to see the mud or clay grains that went into the formation of this sedimentary rock. Sometimes siltstone contains mica and other minerals that become aligned in a preferred direction during the formation of the rock. The resulting rock, called shale, easily breaks into pieces that look like parallel sheets.