Other Evidence Marking a Fault

There are times, especially when rocks are not neatly layered sedimentary rocks, where it is not obvious whether a fault is a fault or just a mere joint across which no movement took place. Clues such as those displayed below would help tip the interpretive scales in favor of a fault.

The vertical grooves on this outcrop are called 'slickenlines'. Such grooves are made when rocks gouge each other as they are dragged along a fault surface. In this case, the last fault movement was vertical--either along a reverse fault or a normal fault (can't tell from this photo). Photo by Norris W. Jones
These slickenlines show movement that is diagonal (combined vertical and horizontal). This type of motion indicates an oblique slip fault. Photo by Norris W. Jones
Since faults are often badly fractures, they may weather quickly. Here is just a scrap of slickenlines preserved in a granite near Spirit Mountain, Nevada.
Breccias often form along faults as a result of the grinding between the two fault blocks. (A breccia is a conglomerate with highly angular pieces.) This breccia was found along a nice waterfalls in a woods. Photo: Amnicon Falls, by Norris W. Jones
Heated fluids full of dissolved solids often flow along faults. Where the faults pull apart, the fluids may precipitate ore deposits or other interesting mineral deposits. In this case, several rows of vertical calcite crystals formed. Photo by Norris W. Jones

Next page: Fault scarps and other large-scale fault features

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