SEDIMENTARY ROCKS by Richard (Dick Gibson)

As soon as land is formed—whether by volcanic eruption or mountain uplift or a drop in sea level—the forces of erosion begin to tear it down.  Wind, ice, and especially water break rocks into smaller and smaller particles—just think of cobbles in a river clunking against each other as the water moves them downstream.  Chemical weathering also attacks rocks, changing hard minerals into softer clay and breaking tightly interlocked grains from their neighbors. Freezing and thawing crack rocks.  And when the pieces become small enough, wind and water transport them and pile them up in a wide range of settings.  Lakes, river banks, beaches, and the ocean floor all are traps for sediment, as the pieces are known.  Pebbles, sand, silt, and mud accumulate, and eventually—usually after hundreds of thousands or millions of years—they solidify into rocks.

SEDIMENTARY ENVIRONMENTS

The grains of sedimentary rocks may be packed together so tightly that they form solid rocks on their own, but often, larger sediments like sand and pebbles must be cemented together.  Iron, silica, calcite, and other materials can cement grains together to form hard rocks. 

Because they are often deposited very gradually in ocean or lake waters, sediments pile up into distinct layers.  Other rock types have layers, but sedimentary rocks often bear the hallmarks of their environments—ripples, mud cracks, fossils, and other features that tell us how they were laid down. 

Sedimentary rocks abound in all the local mountain ranges and valleys. Gray ridges and crags in the northern and western Tobacco Roots are limestones, most commonly about 320 million or 500 million years old.  Valleys are filled with sandstones and conglomerates, usually hidden beneath modern soils and river alluvium.

Conglomerate

Sedimentary Rock Types  Conglomerate is made of pebbles cemented together.  Sandstone is just what it says—cemented sand.  Shale is made of particles that are too fine to see, even with a magnifying glass.  The original sediment was mud.  Limestone is made of calcite (calcium carbonate) and may crystallize directly on the sea floor, or may be secreted by animals like coral to build large reefs.

Characteristics of sedimentary rocks tell us a lot about their history. For example, cross bedding, or layers at an angle to the main surface of deposition, may indicate flowing water pushing sand along and down sloping surfaces. Dune sand blown by winds shows similar features, so we may infer the presence of deserts or beaches at the time such sediments were laid down. Mud cracks mean the surface was once wet, but it dried out.  Chaotic deposits, with little in the way of layering, can imply that a glacier or landslide was the agent that moved the sediment.  

Alluvial fans, like the Cedar Creek Fan south of Ennis, are piles of sediment deposited by streams at mountain fronts.

Cross bedding (photo by Dick Gibson)