by: Charles O’Dale

aThe basin fill consists of two members, the lowermost being the Rock Elm Shale. Fossils in the Rock Elm Shale include brachiopods, molluscs, trilobites, crustacea, conodonts, and annelids (in the form of Scolecodonts). Fossils identified to the genera level for both conodonts and scolecodonts, aided in narrowing the age of the basin fill to the middle Ordovician. Genera identified were Lumbriconereites, Arabellites, Staurocephalites, Protarabellites, Leodicites?, and Oenonites for scolecodonts; and Polycaudus?, Curtognathus?, and Chirognathus? for conodonts. (PETERS et al, 2002).

Personal correspondence with Dr. Cordua, 2020.

[Older structure age documentation: The maximum age of Rock Elm is ~470 Ma, the age of the youngest exposed rocks involved in the structure. Average values of rs (6–7 m/Ma) and Äh (200–300 m) indicate that the thickness of eroded sediments represents 30–50 Ma of time, giving an age for the structure of 420–440 Ma. (French 2004). The ring basin fill consists of a shale-sandstone sequence of probable Ordovician age not found outside of the structure (Cordua 1985)].

North American Middle Ordovician impact craters. Key: 1: Ames crater, 2: Decorah crater, 3: Rock Elm Impact Structure, Wisconsin, 4: Slate Islands, Ontario.
LIDAR image of the Rock Elm Structure.
The Rock Elm Structure is a geologically anomalous, nearly circular region in west-central Wisconsin, USA. At the surface, the deformed structure does not have an obvious topographic signature. It is deeply eroded and poorly exposed, with less than 100 meters of relief from highest point to lowest. But the circular structure has several components all indicative of an impact structure, such as a ring boundary fault, deformed blocks of sedimentary rocks and a central uplift of older rocks.
The Rock Elm structure imaged from my chariot GOZooM from 5000 feet AGL. The 6 kilometre diameter crater is approximated with the superimposed circle. The structure has a ring boundary fault with at least 50 m of vertical displacement, a sediment-filled ring basin, and central uplift.

Video: Explore Rock Elm Impact Crater

The red dot represents the approximate area of the possible multiple impact in the late Ordovician Period. At that time CO2 was at 17 times that of present levels and the first terrestrial moss-type (bryophyte) fossils appear.
Trees at the skyline mark the central uplift of the Rock Elm structure in western Wisconsin. Elevation from the highest point of the crater to the lowest point is about 80 meters. Courtesy of Bevan French.



Researchers discovered the mineral, called reidite, at the Rock Elm impact structure in western Wisconsin.
Reidite is a dense form of zircon, one of the hardiest minerals on Earth. Zircon morphs into reidite when shock waves from meteorite impacts hike up pressures and temperatures to extreme levels, equal to those deep inside the Earth where diamonds form. The pressure makes minerals tightly repack their molecules into denser crystal structures. Reidite has the same composition as regular zircon but is about 10 percent denser.
Undergraduate students from the University of Puerto Rico study outcrops of shocked Mt. Simon sandstone at the Rock Elm meteorite impact structure during a field trip to the Pierce County, Wis., site in May 2013. A sample from the site revealed reidite, a mineral that has been found at only three other locations on Earth. (Photo courtesy of Aaron Cavosie)
Reidite is a rare mineral that has been found only in four crater impacts: the Chesapeake Bay Crater in Virginia, Ries Crater in Germany, Xiuyan Crater in China, and Rock Elm Crater in Wisconsin (Wiki).


Reidite is a high-pressure polymorph of ZrSiO4 that forms >30 GPa and is an important accessory mineral in studies of shock metamorphism as its formation conditions have been experimentally constrained. However, naturally occurring reidite is rare; it has only been reported from three impact structures, where it occurs in ejecta and impact melt-bearing breccias that record shock pressures from 35 to 60 GPa (shock stages II-III). Here we report a new occurrence of reidite in polymict breccia from Rock Elm, a deeply eroded, 6.5 km diameter, Middle Ordovician impact structure in Wisconsin, USA. The breccia contains lithic clasts and quartz grains with planar fractures, formed during shock deformation of the underlying Cambrian Mt. Simon sandstone. Reidite was documented in zircons using electron backscatter diffraction (EBSD), and has two habits: (1) crystallographically oriented sets of micrometer wide lamellae, and (2) randomly orientated sub-micrometer granules along margins and within intragrain voids. Both habits are inferred to have formed simultaneously, and record the variable response of zircon at the grain scale to high-pressure shock deformation in a porous sedimentary target rock. A shocked zircon with impact microtwins in {112} was also documented in the same sample by EBSD. The presence of reidite at Rock Elm provides a record of significantly higher pressures for exposed rocks (>30 GPa) than previous reports based on shocked quartz (<10 GPa), and along with shock-twinned zircon, further confirms a high pressure impact origin for the breccia. Rock Elm is the fourth impact structure where reidite has been recorded, and the first occurrence of reidite in a porous target rock, the Mt. Simon sandstone. With an inferred minimum age of ~450 Ma, Rock Elm contains the oldest preserved reidite in the geological record thus far described.

The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz, (French, Cordua,Plescia 2004.)

Abstract The Rock Elm structure in southwest Wisconsin is an anomalous circular area of highly deformed rocks, ∼6.5 km in diameter, located in a region of virtually horizontal undeformed sedimentary rocks. Shock-produced planar microstructures (PMs) have been identified in quartz grains in several lithologies associated with the structure: sandstones, quartzite pebbles, and breccia. Two distinct types of PMs are pres ent: P1 features, which appear identical to planar fractures (PFs or cleavage), and P2 features, which are interpreted as possible incipient planar deformation features (PDFs). The latter are uniquely produced by the shock waves associated with meteorite impact events. Both types of PMs are oriented parallel to specific crystallographic planes in the quartz, most commonly to c(0001), ξ{11 2}, and r/z{10 1}. The association of unusual, structurally deformed strata with distinct shock-produced microdeformation features in their quartz-bearing rocks establishes Rock Elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact. Preliminary paleontological studies indicate a minimum age of Middle Ordovician for the Rock Elm structure. A similar age estimate (450–400 Ma) is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region. Such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks.

In 1942 a UW-Madison graduate student spotted differences in soil and quartz compared to the slightly reddish limestone outcroppings found locally in flat and horizontal layers. He mapped out the area for more study and it became known as the Rock Elm Structure site. Unfortunately this discovery, containing some of the worst soil in Pierce County, was forgotten.

Recent study of the Rock Elm Structure determined that it is a circular disturbed area about 6 km in diameter, located in otherwise undisturbed flat-lying Paleozoic sandstones and limestones about 300 m thick, which overlie Precambrian metasediments and felsic volcanics. The structure shows many geological features suggestive of “cryptoexplosion” structures produced by meteorite impact:

(1) a generally circular outline;

(2) a ring boundary fault with >50 m displacement;

(3) a “central dome” of possible Cambrian (Mount Simon?) sandstones with >250 m of structural uplift;

(4) a 5-mgal negative gravity anomaly;

(5) an annular deposit of post-structure shales and sandstones around the central dome. An unusual breccia containing basement rock fragments occurs as float in the central area. The presence of two types of unusual planar microdeformation structures in quartzite pebbles and larger (>0.5-1 mm) single quartz grains from the (Mount Simon?) sandstone exposed in the uplifted central dome.

The similarity of micro-deformation features in quartz from Rock Elm samples to those from established impact structures provides strong evidence that Rock Elm is also an impact structure (French, Cordua 1999).

The structure consists of a circular, 6.5 km-diameter area of anomalously deformed rocks in a region of otherwise flat-lying Ordovician and older sediments, and it contains a central uplift in which normally deep-seated Cambrian sandstone is exposed. These geological features appear typical for a meteorite impact structure in sedimentary rocks, and there is no evidence for origin by endogenic mechanisms such as igneous activity or salt dome intrusion (Cordua 1985).

This schematic cross-section of the Rock Elm structure, Wisconsin (French et al. 2004) shows the idealized geology (right side) and typical crater parameters (left side); the vertical exaggeration is 3:1.
Geological units involved in the structure (right side) are: 1) uplifted Precambrian basement rocks (not exposed); 2) overlying Paleozoic sandstones and carbonates; 3) crater fill breccias (not exposed); 4) crater fill sediments. The line G-G’ indicates the original ground level at the time of impact; B-B’ indicates the original contact between the Precambrian basement and the overlying sediments; and P-P’ indicates the present erosion surface. The dashed line indicates the original rim of the structure. Typical crater parameters are indicated at left and above: D0 = original diameter; D = present diameter at the current ground level; Dtc = transient cavity diameter; dtc = transient cavity depth; de = depth of excavation zone; da = apparent depth of final (present) structure, from the original ground surface to the top of the crater fill breccias; dt = true depth of the final structure, from the original ground surface to the original crater floor; SU = stratigraphic uplift. The point A0, originally in the center of the crater just beneath the excavation zone, is driven downward (to point A1) during formation of the transient cavity and then rebounds with the central uplift to point A2. The point B0, located beneath the center of the transient cavity at the basement-sediment contact, moves in a similar manner, being driven downward (to B1) during transient cavity formation and then rebounding (to B2) with the rest of the central uplift. (For details on these parameters, see Melosh [1989], chapters 2, 5, and 7; Grieve 1991). A key parameter for a deeply eroded structure like Rock Elm is Äh, the vertical amount of erosion of the target stratigraphy from the original ground surface since the crater formed. (French 2004)
Diagram showing the calculated age of the Rock Elm structure (Wisconsin) from estimates of pre-impact sedimentation rate and postimpact erosion (French et al. 2004).
The diagram plots estimated values of the thickness of sediments eroded after impact (Äh) against geological age for various values (rs) of the rate of pre-impact accumulation of the eroded sediments. The maximum age of Rock Elm is ~470 Ma, the age of the youngest exposed rocks involved in the structure. Average values of rs (6–7 m/Ma) and Äh (200–300 m) indicate that the thickness of eroded sediments represents 30–50 Ma of time, giving an age for the structure of 420–440 Ma. (French 2004)

Unique microscopic deformation features in tiny mineral grains have led three scientists to the discovery of a large and ancient meteorite impact structure in western Wisconsin. As reported in the Bulletin of the Geological Society of America, the recognition of the Rock Elm, Wisconsin, structure by Dr. Bevan M. French of the Smithsonian Institution in Washington, D.C., Prof. William S. Cordua of the University of Wisconsin at River Falls, and Dr. Jeff B. Plescia of the U.S. Geological Survey at Flagstaff, Arizona, was based on the discovery of microscopic fractures and other deformation features in grains of quartz in rocks from the center of the structure, features identical to those observed in many established impact craters. Their geological studies show that the Rock Elm structure, 6.5 kilometers (4 miles) in diameter, was formed by the impact of a comet or asteroid about 200 meters across, traveling at possibly 30 km/sec (67,500 mph). The impact event released more than 1000 megatons of explosive energy, instantly lifted the central part of the structure more than 500 meters (1650 feet), and sent intense shock waves through the target rocks, crushing and deforming their minerals. The structure, with an age estimate of 420–440 million years, is now so deeply eroded that the scientists were fortunate to find the shock-deformed rocks still preserved. The Rock Elm structure is too small to have caused a major extinction of life as did the large dinosaur-killing impact event 65 million years ago, but the local geological effects could have been important, and the structure provides an example of new deformation features in quartz that might be used to identify other impact structures in the future. (Bevan M. French, Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560, USA; et al. Pages 200-218.)



Arana, A. and Cavosie, A.J. (2014) A study of shocked quartz in breccia from the Rock Elm impact structure. 45th Annual Lunar and Planetary Science Conference, LPI Contribution 001777, 2185.

Cavosie, A.J., Roig, C.I., McDougal, D.J., Ushikubo, T., Spicuzza, M.J., Fournelle, J., Valley, J.W., Cordua, W.S., Mattson, C. (2013). The Sedimentary Record of a Small, Deeply Eroded Impact Structure: A Search for Detrital Shocked Minerals and Extraterrestrial Chromites in Sediments Eroded from the Ordovician Rock Elm Impact Structure (USA). 44th Annual Lunar and Planetary Science Conference, LPI Contribution 1719, 2028.

Cordua, W. S., “The Rock Elm Structure, Pierce County, Wisconsin, a possible cryptoexplosion structure”, Geology, vol. 13, p. 372-374. 1985.

Cordua, W. S., “The Rock Elm impact structure, Wisconsin: Recent findings and relevance to the local non-geologic community”, University of Wisconsin (United States), 2004.

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

Roig, C.I., Cavosie, A.J., McDougal, D., Cordua, W.S., Mattson, C. (2013) Detrital shocked quartz in modern sediments eroded from the Rock Elm impact structure, Wisconsin, USA. 44th Annual Lunar and Planetary Science Conference, LPI Contribution 1719, 2685.


French, B. M., Cordua, W. S., Intense fracturing of quartz at the Rock Elm (Wisconsin) “cryptoexplosion” structure: evidence for meteorite impact, Lunar and Planetary Science. 1999.

French, B. M.,The importance of being cratered:The new role of meteorite impact as a normal geological process. Meteoritics & Planetary Science 39, Nr 2, 169–197. 2004.

Bevan M. French, William S. Cordua, J.B. Plescia, “The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz”, 2004.


University of New Brunswick, 2012.