Geo-pic of the week: Pebble Molds

(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.

Geopic of the week: Rosie boulders

Pictured above are gigantic sandstone boulders in a gravel pit near Rosie Arkansas.  While these house-sized boulders are not unusual, their location is: there is no nearby source for them.  Some geologists speculate that they were transported  to this location by a great tsunami generated during a major meteorite impact.  Perhaps the same meteorite that caused that tsunami created the 110 mile diameter Chicxulub crater on the Yucatan peninsula about 65 million years ago.

For more views of the Rosie boulders look here