Tag Archives: Fluvial

Geo-pic of the week: Herringbone Cross-Bedding



Pictured above is sandstone displaying classic herringbone cross-beds.  Cross-bedding results from either sediment transport by flowing water, such as in this example, or by wind flow, as in the case of dunes.

Cross-beds form by the migration of sediment, and tilt in the direction of flow.  As sediment grains are carried by the current, they migrate up the gentle ramp of previously deposited cross-beds.  When they reach the end, they tumble down the steeper face there and are deposited to become part of the next cross-bed.  In this way the sediment migrates in the downstream direction.

Each group of similarly tilted cross-beds is known as a set.  In herringbone cross-bedding, the sets are oriented contrarily, which gives the outcrop a fishbone appearance.  These differently oriented cross-bed sets indicate changing flow directions.    


Geo-pic of the week: Pebble Molds

pebbles-great(photo courtesy of Angela Chandler)

The sedimentary rock in the picture above is a sandstone with pebble molds. If the pebbles were present, this rock would be considered a conglomerate. Conglomerates consist of 2 mm or larger rounded fragments of rock, or clasts, surrounded by finer-grained sediment which geologists call “matrix”. The clasts in the rock above were pebble sized, 2-64 mm, and the matrix is sand sized.

Even though many of the clasts have been removed by erosion, we can tell that they were primarily shale pebbles. The sandy matrix was more resistant to erosion than the softer shale pebbles, so we are left with cavities where the pebbles were (pebble molds) on the rock’s surface. This creates an interesting optical illusion. Did you see the cavities as pebbles or as molds when you first looked at the picture?

This type of conglomerate is deposited by energetic and dynamic water, such as is found in rivers and waves. During higher flow periods, only large clasts are deposited. When flow is lower, finer-grained sediment settles in between the larger clasts.