A portion of last weekend was spent walking through Ute canyon in the Colorado National Monument. In some areas we were walking across an eroded Wingate surface. The Wingate sandstone is early Jurassic in age and is a windblown quartz dominant sandstone with obvious crossbedding through out the formation. While walking on the Wingate (or the Navajo ss) there are sometimes collections of small marble like objects on the ground. These marbles are iron concretions.
When the rock was originally going through the rock forming process a thin patina of hematite coated the quartz grains creating a cement holding them together. Later, water (with certain reducing agents) moving through the sandstone picked up the iron. When this dissolved iron combines with oxygen the dissolved iron loses an electron and its solubility is reduced. The hematite is immediately precipitated out forming the iron concretion.
This process happens all through the sandstone creating all sorts of shapes. The marbles are the most common, but concretions can be found as buttons and towers and sheets along the ground. As the softer sandstone erodes away, the harder concretions stand out making a surface of small marbles.
A portion of last weekend was spent walking through Ute canyon in the Colorado National Monument. In some areas we were walking across an eroded Wingate surface. The Wingate sandstone is early Jurassic in age and is a windblown quartz dominant sandstone with obvious crossbedding through out the formation. While walking on the Wingate (or the Navajo ss) there are sometimes collections of small marble like objects on the ground. These marbles are iron concretions. 
When the rock was originally going through the rock forming process a thin patina of hematite coated the quartz grains creating a cement holding them together. Later, water (with certain reducing agents) moving through the sandstone picked up the iron. When this dissolved iron combines with oxygen the dissolved iron loses an electron and its solubility is reduced. The hematite is immediately precipitated out forming the iron concretion.
This process happens all through the sandstone creating all sorts of shapes. The marbles are the most common, but concretions can be found as buttons and towers and sheets along the ground. As the softer sandstone erodes away, the harder concretions stand out making a surface of small marbles. 
3 comments:
Blueberries! Well, not quite...you'd break your teeth on those. I've seen them in Big Bend, Texas as well, although I can't remember the unit.
Marjorie Chan (at the University of Utah) does a lot of work with these, and I've seen several of her talks about looking for 'blueberries' on Mars. Cool stuff.
I had forgotten about the blueberries on Mars and how the initial drilling by Spirit had some heart stopping moments because of the chance of hard chunks flying into the rover.
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