Geo-pic of the week: Slickensides

The grooved surface pictured above is a slickenside.  Slickensides indicate the relative direction of movement between fault blocks (hanging wall moved up, down, laterally, etc..).  

Slickensides form when fault blocks move against each other.  The natural irregularities on each scratches grooves into the other.  The grooves are parallel to movement;  for instance in this example, movement was either to the right or the left.  To tell whether it was right or left, you can rub your hand along the slickensides.  They feel smooth in the direction the fault moved and rough in the opposite direction – it’s like petting a dog from tail to head.  Slickensides are a valuable tool because determining fault movement can be a challenge when there are no easily-recognized beds that can be correlated across the fault to show the sense of offset.

The shale above was photographed in Big Rock Quarry, North Little Rock, AR.  It’s a part of the Jackfork Formation (Pennsylvanian).

Geo-pic of the week: Flute casts

Fig. A – Many flute casts that truncate one another.  Black arrow points downstream.

Fig. B – Flute casts with characteristic round heads pointing in upstream direction.  Black arrow points downstream.

 

Pictured above are beds of sandstone displaying flute casts.  Flute casts are common in channel environments (for instance river channels) where water is carrying sediment and debris (rocks, shells, sticks etc..). 

As debris is carried along, it randomly grazes the mud in the channel bottom, scouring divots.  The divots are typically deeper and narrower in the upstream direction, with a round head pointing upstream.  Flute casts form subsequently when sand in-fills these divots and later becomes a rock, preserving casts of the divots on the bottom of the sand bed. 

Flow direction indicators, such as flute casts, are one of many clues geologists use to reconstruct the history of the earth.