How Sedimentary Rocks are Structured

HOW SEDIMENTARY ROCKS ARE STRUCTURED

When you look at a sedimentary rock, look for the way it is layered. You can get different patterns of stratification or bedding in these rocks. Different layers can be a different color, grain size, or chemical composition.

Look for rhythmic layering, where you see an alternating pattern because of seasonal changes in deposition. Lake deposits will vary in the summer and winter months. A varve is a pattern of lake deposition seen in a single year's time. You will see these layered out evenly.

Other patterns you will see include cross-bedding, which involves a series of beds sitting at an incline to other beds. Beaches, rivers, and sand dunes all have cross bedding features, especially in areas that have erosion.

Graded bedding involves changes in velocity of current in a river so that grain size will decrease as you go up the riverbed. If you see multiple sequences of these, they are called turbidities, reflecting a series of changes over time. Sediment can be completely unsorted or you might see sediment affected by ripples where wave action has occurred. Cracks in mud will expose surface within the cracks; this fills in with sediment that can look interesting. Other things you'll see is sediment left within raindrops or in troughs called flutes.

Sedimentary rock will often be colored based on impurities or other things within the sediment. Iron oxide makes the sediment red and indicates sediment that did not form in a marine environment. Sulfides get buried along with organic material to give rock a dark coloration.