Holly Ridge Fault Zone

The Holly Ridge fault zone is located in Stoddard County, in southeast Missouri. The fault zone is exposed in the wall of an excavated site near the small community of Idalia, about six miles northeast of Dexter. It apparently trends generally northeast and is located on the northeast flank of Crowley's Ridge, which also trends northeast. Crowley's Ridge is a remnant upland surface that is surrounded by flat Quaternary alluvial plains of the present and ancestral Mississippi rivers. The Crowley's Ridge surface includes exposures of Tertiary, Cretaceous, and Ordovician deposits. Those surface deposits generally parallel the northeasterly trend of Crowley's Ridge: youngest deposits to the southeast, oldest to the northwest.

In the fall of 1997, the Holly Ridge fault zone was first identified and named by Dave Hoffman, a geologist with the Missouri Division of Geology and Land Survey. Two years earlier, Hoffman and two other DGLS geologists, Jim Palmer and Jim Vaughn, were working with US Geological Survey geologist Richard Harrison about 35 miles to the northeast, near Commerce, Missouri. There they identified the English Hills fault zone, located on the south flank of the Benton (or Commerce) Hills. The Holly Ridge fault zone may be a part of a larger fault system that includes the English Hills fault zone exposures as well.

The Holly Ridge fault zone is significant for several reasons. Like the English Hills fault zone, it has been active within the Holocene epoch (10,000 years). This is shown in its displacement of geologically young materials. Deposits that are less than 10,000 years old are displaced along the fault plane. This indicates that past earthquakes have released enough energy to raise or lower the earth's surface on one side of the fault relative to the other. Vertical displacement of at least several tens of feet is obvious. The fault zone may also include significant horizontal displacement.

Also like the English Hills fault zone, the Holly Ridge fault zone is coincident with the Commerce Geophysical Lineament (CGL). The CGL is a distinct linear feature that is apparent in regional geophysical surveys, such as magnetics and gravity, as well as in the alignment of surface features. Interpretation of satellite imagery shows that surface features such as natural drainages, changes in soil and rock types, changes in vegetation and topography, and other characteristics of the earth's surface, are aligned along a narrow trend from mid-Arkansas into eastern Illinois.

There are many more questions to explore about the Holly Ridge fault zone.