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.

Silt grains are rock particles even smaller than sand (less than 0.06 mm or 0.002 inch wide), so water and wind can transport them farther than sand grains. When the water in a pond appears brown and difficult to see through, it may be carrying large amounts of silt, which is slow to settle to the bottom. Quartz and feldspar, both silicates, are the most common minerals in silt. Once silt hardens into rock, scientists usually cannot see the individual grains without the aid of a microscope. Mud is a mixture of water, silt, soil, and clay (the tiniest of all rock particles). Mudstone, the sedimentary rock made of mud, often breaks into random blocks, and it may contain cracks that first appeared when the original mud was drying out.