|The claim of sedimentary origin is usually easy to substantiate. The presence in a rock layer of primary structures, such as fossils, ripple marks, or mudcracks and the absence of features indicative of igneous origin, such as vesicular glass or of metamorphism, such as slaty cleavage, serve as strong evidence.
Investigation commonly focuses on determining the specific environments in which sediment was formed, transported, deposited and lithified. Here is one example in which field relationships play an important role in developing a hypothesis as to the origin of a particular sedimentary rock.
|The diagram on the left is a three-dimensional block diagram showing a cliff rising above a fairly flat plain. The cliff is made of sedimentary rock: a cemented mass of unsorted boulders, pebbles, sand and clay that displays no layering (Rock 2). The sedimentary rock rests on metamorphic rock (Rock 1). In general, the plain is covered by vegetation and underlain by a soil layer that thickens away from the cliff. Near the foot of the cliff, the soil is absent and the underlying rock is exposed. On the surface of exposed ROCK 2, there are are series of parallel grooves cut into the rock (shown as black lines).|
|1. The grooves cut into the surface of Rock 2 resemble those that are the result of recent glacial erosion (indicated by red arrows).|
|The unlayered, unsorted sediment out of which Rock 2 is made resembles recent, unlithified glacial deposits.|
|Here's the story the geologist proposes:|