When looking at this map of bedrock geology of the Illinois and Ozark basins (sourced from Geological and Geophysical Maps of the Illinois Basin-Ozark Dome Region),

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I can't help but notice that Missouri—and only Missouri—shows a large number of small mounds and craters:

Example from the Ozark plateau:

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Example from around Kansas city:

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What am I seeing here?

  • Surely these aren't impact craters (cool as that would be)—as nearly none of them show up in lists I can find of known or suspected craters (which seem limited to the 38th parallel structures and the Weaubleau structure).
  • Are they an artifact in how the data was collected (ex. perhaps these features do not really exist?)?
  • Are patterns like this common in bedrock maps of other regions of the world?

I do not see similar structures in Illinois, for instance:

enter image description here

  • $\begingroup$ Peatbogs? Sinkholes? $\endgroup$ Commented Oct 27, 2022 at 10:29
  • $\begingroup$ @Jean-MariePrival: seems sinkholes are common in Missouri (dnr.mo.gov/land-geology/hazards/sinkholes). But in bedrock?? $\endgroup$
    – SigmaX
    Commented Oct 27, 2022 at 18:40
  • 2
    $\begingroup$ Well, if the bedrock is limestone, sinkholes are part of the karst system. $\endgroup$ Commented Oct 28, 2022 at 6:56
  • 1
    $\begingroup$ Sinkholes might explain the dimples, but not the "pimples", sorry, mounds. $\endgroup$
    – Fred
    Commented Oct 29, 2022 at 8:51
  • $\begingroup$ Could also just be some sort of artifact in the generation of the terrain model. $\endgroup$ Commented Oct 29, 2022 at 14:59

1 Answer 1


While I can't confirm that it correlates perfectly to the points in the map, the answer may be that at least a subset of them are the same structures that Bretz proposed a theory for in 1950 (in "Origin of the Filled-Sink Structures and Circle Deposits of Missouri").

I learned this fromthe following passage in Charles Spencer's Roadside Geology of Missouri (Missoula: Mountain Press, 2011), which I came across after posting the question (p. 137–9):

The Ordovician dolomite beds on the northern and northwestern flanks of the Ozark Dome are peppered with hundreds of small, circular or elliptical structures in which the rocks inside are younger than those surrounding them. They vary in size from tens to hundreds of feet across. Most are found in the Jefferson City Dolomite, but some are in the Roubidoux Formation...

Many explanations have been proposed for how these structures formed. The earliest hypothesis was that they were stream channels eroded into the surface of the host rock during the Late Ordovician time when sea level temporarily fell. That explained the presence of crossbedded sandstone... but not the circular shapes or the Presence of Pennsylvanian coal or clay beds.

A second explanation was that they were ancient sinkholes fromed by collapse into a cavern. This idea accounted for the circular shapes and the different types and ages of sediment filling them. Some of the structures may indeed have formed in this way. But in many of the structures, the in-filling rocks are not broken as would be expected if collapse had occurred. In fact, the beds inside some of the structures are horizontal.

In 1950, Missouri geologist J. Harlen Bretz proposed another, perhaps more unusual explanation for how the structures formed, and he named them filled-sink structures, sometimes shortened to filled sinks. In Bretz's model, the structures were indeed the result of groundwater erosion, but without the formation of caves. Rather, microscopic grains of dolomite were dissolved slowly over time, creating tiny spaces in the rock. The loss of rock allowed the dolomite bed to compact under the weight of overlying beds. As it did so, beds above it sank very slowly into the structure. The amount that younger beds settled varied, depending on the amount of dolomite rock mass removed: the more rock removed, the greater the subsidence... Once uplift of the Ozark Dome began in earnest in Late Pennsylvanian time, the water table across the Salem Plateau dropped, ending the process...

Bretz's model accounts for the observed characteristics of many of the structures...

Be that as it may, some circular features found in Ordovician dolomite in the Ozarks are indeed attributed to collapse of deeper caves. These structures, called "circles," are similar in shape to filled sinks but always contain broken, tilted, and shattered beds rather than horizontal strata.

  • 1
    $\begingroup$ Nice find, +1. $\endgroup$ Commented Oct 31, 2022 at 9:53

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