What exactly do geologists mean by “mud”?

We usually think of “mud” as sticky, wet dirt. So you might wonder what geologists mean when they talk about “mudstone,” and how they distinguish it from other rocks such as “sandstone.” The answer is that it depends on the sizes of the sediments that go into these rocks. In fact, geologists typically divide small particles into three main categories:

• Mud is the name geologists use for sediments that have very small grain sizes — so small that, visually, they all blend together. Mud is often subdivided into silt for larger mud grains (typically 0.004 to 0.06 millimeters across) and clay for the smallest mud grains (less than 0.004 millimeters across).
• Sand represents grain sizes typical of what you might find on a beach or in a sand box (which range in size from about 0.06 to 2 millimeters across).
• Gravel represents any “grain” sizes larger than sand, so it includes pebbles, cobbles, and even small boulders.

Keep in mind that nature makes sediments and sedimentary rocks from across the complete range of different grain sizes. The classification that we humans make to separate grains by whether they are mud, sand, or gravel is simply a convenient way of organizing the rocks we see.