Tag Archives: Chalk

Geo-pic of the week: Slickensides

Slickensidedshale_edited

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).

Geopic of the week: Coprolites

Poop_coprolite

Ok, I know what you’re probably thinking, and well….. you’re right!  Coprolites are fossilized feces.  The sample on the right is from out of state, but the fine specimen on the left came from good ole Arkansas, near the town of Saratoga.

Because poop has no hard parts (such as a skeleton or shell) it’s rarely fossilized, but under the right circumstances it can get preserved.  Often, it isn’t certain what animal was responsible for the coprolite, unless it’s discovered in the abdominal area of a fossilized animal.  In cases where it can be identified, it tells us something about the animal, that um…made it.  By closely examining  coprolite, it’s possible to recognize fragments of animals or plants, which give us information about the diet of that species.