Recently, someone came to me and asked me if I could help them identify some features on some rocks. The features have been shown to a number of people with no definitive answer. Perhaps you can help. If you think you know what this is, email ( sjtaylor@mail.inhs.uiuc.edu ) me!

Excerpts (in smaller font) from a letter that accompanies these photos of strange grooves on limestone boulders discovered in 1969 around a sinkhole pond about 10 miles from Mammoth Cave, Kentucky:

He found these grooves on several limestone boulders in a small karst pond not over 100 meters in diameter. The markings were on only one type of limestone although there were other types of limestone boulders in the pond without the grooves.

The boulders with grooves are described as belonging to the Upper Mississippian series and the Ste. Genevieve limestone formation (limestone, white to light-gray, thick-bedded, oolitic interbedded with non-oolitic, light-gray lithographic to course-grained limestone).

The letter describes the grooves in some detail:

Most of the grooves or gouged out areas were on the upper surface of the boulders and had an orientation with the water level of the pond. The level of the water in the pond fluctuated as was noted at a later visit when all of the stones were covered with water. The boulders were small measuring at the most about .5 by .25 meters and not over .25 meters in height. The randomly spaced grooves or gouged out areas were 2 cm long, .5 cm wide and .25 cm deep. The literally covering the surface of the boulders but stopping short just above the mud in which boulders were embedded. The surface of the boulders were also covered with a multitude of one to two cm long superficial marks that were almost scratches. These scratches were randomly placed with no particular relationship to the grooves. Some of the smaller boulders had only a very few grooves scattered on the upper surface.

Note that some of the above is slightly inconsistant with the pictures - there does seem to be some alignment of the scratches relative to the grooves or gouged out areas, and the grooves or gouged out areas do not seem to me to be completely random in distribution. The letter goes on to say: We have reached several conclusions-.
First, the grooves are relaitive recent (speaking of real time not geologic time). The markings within the grooves and the edges are quite sharp. This particular limestone weathers rather rapidly and the grooves would be blunted in a few years. The specimens were passed around at a meeting of professional geologist and all agreed that the markings were recent.
Second, the orientation of the grooves corresponds to the plane of the surface of the pond indicating tha they were made with the boulders in their present location.
Third, the grooves are only on the surface, when the rock is broken the limestone is found to be very homogeneous, descrived as being lithographic by the geologist.

So, that's what we know:

The first image shows the general surroundings and some of the rocks involved:

Image 2 shows one of the boulders:

Image 3 is a closer view of the same boulder:

A closer view of another boulder (image 4) scale is marked off in centimeters:

The final image is a very close view. The dark material that appears cracked like mud (light colored cracks) seems to be an algal growth or perhaps a lichen - I suspect this has grown on the rock since the marks were made, but I could be wrong.: