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The features that crisscross the surface of the sandstone pictured above are deformation bands. They are micro faults that form where the bedrock is under strain. They typically develop near larger faults and the orientation of the bands is determined by the orientation of the stresses acting on the rock.
The grains along deformation bands have been crushed, rotated, and reorganized. The resultant bands are harder and less permeable than the rock they formed in, which causes them to stand in relief when the rock weathers.
Geologists look for deformation bands as indicators that a fault may be nearby. This photo was taken in Van Buren County, Arkansas on the downthrown block of a normal fault.
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Sorry about that long hiatus, but I had a couple of extra projects the last couple months that took a lot of extra time. We’ve been in the field almost every week except for March 3-5 during the 3 inch snow in Van Buren County. We’ve mostly worked on the Fairfield Bay quad during the last few weeks. This week was spent tracing a very thick-bedded, massive sandstone unit through the town of Fairfield Bay itself. It is quite an impressive bluff-former and actually underlies almost the entire Mountain Ranch golf course.
Apparently some structure or perhaps a change in depositional environment made this sandstone climb up 200 feet to the east. There it forms the cap of the ridge on which the small town of Fairfield Bay sits. Moving east again, It underlies the Indian Hills Country Club where weathering (and earth-moving equipment) has produced the famous Indian Rock House on the golf course there. Underlying that massive across the entire area is a very thick sequence of very thin-bedded sandstone/siltstone/shale. A lot of the roads built in this unit have formed deep gullies making some of them impassable. Still, there is better access in this area than most that we map, so we’re thankful for that. Only about two weeks left of the field season. We’ll probably be jumping around a lot to work out problem areas on both quads during that time.
See you on the outcrop!