This presentation examines the possibility that some of the glacier ice projectiles that made the Carolina Bays could also have fallen on solid ground.
Glacier Ice Impacts on Solid Ground. The extraterrestrial impact that hit the Laurentide Ice Sheet 12,900 years ago ejected a barrage of glacier ice boulders with a range of 1500 kilometers. The secondary impacts of the glacier ice projectiles made the Carolina Bays on unconsolidated soil, but it is also possible that some ice boulders fell on solid ground. In an earlier video, I calculated that some impacts had energy of at least 50 megatons, and it is possible that these huge impacts left some evidence on hard ground.
It is reasonable to expect that the impact of an ice boulder with multi-megaton energy should have left a detectable mark on hard ground. However, ice is brittle and has low yield strength compared to hard ground, so the impacts by glacier ice boulders on hard ground could have created shallow surface features that were easily concealed in a few centuries by erosion and vegetation growth. Ice has only one third the density of rock, so craters are not expected to form on hard ground even by impacts of ice boulders with diameters of several hundred meters.
The high-energy secondary impacts by glacier ice boulders certainly had an effect on hard ground. Some landslides have been dated to the time of the Younger Dryas impact, 12,900 years ago. A barrage of secondary impacts of glacier ice had enough energy to produce seismic vibrations capable of acoustically fluidizing the Hickory Run boulder field to make it remarkably level. Acoustic fluidization occurs when fluctuating pressures induced by strong vibrations, like from large impacts, briefly release the pressure of overlying rock debris and allow a dry boulder bed to flow like a liquid.
The Younger Dryas Impact Hypothesis was introduced in 2007 by Richard Firestone and 25 co-authors with broad scientific backgrounds. They proposed that an extraterrestrial impact 12,900 years ago contributed to the megafaunal extinctions and the Younger Dryas cooling. The evidence for the impact consisted of magnetic microspherules, nanodiamonds, and other proxies that were attributed to the airburst of an extraterrestrial object. The Carolina Bays were only mentioned as locations from where soil samples were collected.
The reaction of the impact experts was aggressive and brutal to the point of declaring the Younger Dryas Impact Hypothesis dead in a requiem paper. The critics said that there was no reliable evidence of such a recent extraterrestrial impact, and that the impact may not have happened at all.
Little by little, additional evidence accumulated in support of the Younger Dryas Impact Hypothesis. A large platinum anomaly was found in the Greenland Ice Core at the Younger Dryas Boundary. Platinum is a rare element on the Earth's surface but it is associated with iron meteorites. Microspherules attributed to the Younger Dryas Impact were found across four continents. This added support to the Younger Dryas Impact Hypothesis, but the attacks against the hypothesis continued.
Professor Peter Schultz was one of the co-authors of the Younger Dryas paper. His experiments of high-speed impacts on sheets of ice found that the ice prevents the formation of a typical crater on the surface below. This has been offered as an explanation for the inability to find a crater for the Younger Dryas comet.
The experiments by Professor Schultz clearly show that a high-speed impact on a sheet of ice ejects large pieces of ice in ballistic trajectories.
The Nebraska Rainwater Basins and the Carolina Bays are elliptical geological structures separated by 1,800 kilometers and their major axes are oriented toward the Great Lakes. In 2010, Davias and Gilbride calculated the convergence point of the Carolina Bays and the Nebraska Rainwater Basins at Saginaw Bay based on their axial orientations. The calculation required using great circle trajectories and taking into consideration the Coriolis effect caused by the rotation of the Earth.
Michael Davias proposed an extraterrestrial impact that created butterfly-shaped ejecta that formed the Carolina Bays and the Nebraska Rainwater Basins. The geometry of well-preserved Carolina Bays is elliptical in the mathematical sense. This is evidence that the Carolina Bays originated as conical cavities inclined at an angle corresponding to the sine of the width-to-length ratio.
In 2017, the Glacier Ice Impact Hypothesis was published in a peer-reviewed journal. This paper described four mechanisms by which the Carolina Bays could have been created. A meteorite impact on the Laurentide Ice Sheet ejected ice boulders in ballistic trajectories. Considering the great energy of the secondary impacts by the glacier ice boulders, the emplacement of the Carolina Bays and Nebraska Rainwater basins needs to be associated with an extinction event. For this reason, it makes sense to associate the creation of the Carolina bays with the extinction of the megafauna 12,900 years ago.
Impacts of ice projectiles on a viscous medium can be used to test the hypothesis. The experiments demonstrate that oblique impacts by ice projectiles can create inclined conical cavities with raised rims. The impact cavities are elliptical when viewed from above. Additional impact experiments illustrate how adjacent impacts can become overlapping elliptical bays.
If the ejecta curtain from the impact on the Laurentide Ice Sheet was circular, instead of butterfly-shaped, then we can consider that some of the ice boulders landed on hard ground. Impacts of ice on hard ground generally destroy the ice projectile without creating any marks on the target. The contact and compression phase of the impact of an ice projectile on hard ground shows that the projectile disintegrates into many fast moving ice shards. The impact does not even create a dent on the hard surface. Erland Schulson, who is a professor of engineering at Dartmouth College, explains that the brittle failure of ice under compression is marked by sudden material collapse after shortening less than about half a percent. The failure mode is generally shear faulting on planes inclined by about 30 degrees to the direction of maximum principal stress.
When the ice projectile contacts a hard surface, it starts to disintegrate and creates a broader area of contact. As the remainder of the projectile keeps advancing toward the target, it disintegrates over the broad area of ice debris, which deflects ice shards laterally. The dissipation of energy over a broad contact area is the reason why ice impacts on hard ground cannot make deep craters. This is very different from impacts by iron meteorites that remain fairly cohesive during contact and compression and transfer their energy to a very localized area, thereby penetrating the target and creating a crater during the excavation phase.
Finding evidence of glacier ice impacts on hard ground requires identifying a location that has remained fairly undisturbed for at least 13,000 years, but it is also necessary to recognize what the impacts would look like.
Hickory Run Boulder Field is an area with a hard surface that has remained unchanged for thousands of years. As we zoom in on the image, we start to notice some discolored areas. In this close-up view, we can distinctly see some circular areas of discoloration that could potentially indicate impact sites of the glacier ice boulders. The sites with the lighter colored rocks are identified with circles. Measuring with the Google Earth ruler, we can see that the discolored areas are approximately 10 meters wide.
The United States Geological Survey has a lot of freely available LiDAR data. Using the 3DEP LiDAR Explorer we can define an area of interest to cover the Hickory Run boulder field. Clicking on the folder icon reveals five datasets that cover the area of interest. Hovering over each dataset highlights the area covered. The relevant datasets can then be downloaded.
For this video, I used FugroViewer, which is a freeware package that can be used to view the LiDAR datasets from the US Geological Survey. One display mode shows the Hickory Run boulder field with topographic contour lines. Another display mode shows the topography and the location of the laser pulses as little blue dots. The information associated with the LiDAR data set indicates that the pulse spacing is 1.8 meters. This means that it will be very difficult to discern details of the 10-meter features that we are interested in.
The LiDAR visualization of the Hickory Run Boulder Field available from the US Geological Service does not have enough resolution to see the 10-meter features. Using a graphics editor we can improve the contrast. This makes it possible to see an elliptical feature with the proper orientation to be a Carolina Bay. The elliptical feature is not well defined and may just be the result of wishful thinking. What are these discolored circular areas? Is this evidence of glacier ice impacts on the hard surface? Our questions remain unanswered. We also need to consider that these discolored areas could be the entrances to animal dens or the result of some human activity.
A paper published in 2018 by Alison Denn and 5 co-authors examined the Hickory Run boulder field to identify the age of the field and the movement of the boulders. The field is the largest boulder field in North America and the boulders have cosmogenic nuclide concentrations equivalent to at least 600,000 years of near-surface history. Boulder fields, such as Hickory Run, are dynamic features that persist through multiple glacial-interglacial cycles because the boulders resist weathering and erosion.
The boulders in Hickory Run range in length from less than one meter to more than ten meters. They consist of hard, gray-red, medium-grained sandstone and conglomeratic sandstone from the Catskill formation similar to the adjacent ridgelines. The boulders at the southwest of the field are mostly rounded and underlain by small, polished clasts with a red weathering rind. Pay attention to the coloration of the boulders where the geologists are working. These boulders are redish brown compared to the gray color of the boulders further away from the work area.
The authors report that there is a distinct subsection of the field to the southeast of Hickory Run with boulders mostly greater than 5 meters long. These appear to be bedrock shattered along bedding planes as shown in this image.
It is interesting that the authors describe "bedrock shattered along bedding planes". Is it possible that an impact by a large glacier ice boulder could have shattered the bedrock as described by the authors? LiDAR images with greater resolution might help to answer this question.
This image shows the locations where Denn et al. took samples in the upslope section of the boulder field. The red dots indicate higher concentration of beryllium-10. The authors concluded that boulders at Hickory Run have experienced widely varying and substantial near-surface exposure. Hickory Run samples have beryllium-10 concentrations equivalent of between 70,000 and 600,000 years of surface exposure. There is no significant correlation between beryllium-10 concentration and boulder lithology, size, or proximity to the edge of the field. Boulders downslope are more rounded, smaller and have more developed weathering rinds than those upslope, suggesting that boulder weathering increases downslope.
In this image we can see that some of the discolored areas that could be the location of impacts by glacier ice boulders were spared by the researchers. It may still be possible to investigate the reason for the discolorations. However, the fact that this boulder field has been explored before, emphasizes how difficult it is to find a patch of solid earth that has remained undisturbed for 13,000 years and could have preserved evidence of the glacier ice bombardment.
It will be necessary to get some boots on the ground to investigate whether the boulder field was impacted by glacier ice boulders. The use of drones equipped with LiDAR would also be helpful.
Finding evidence of glacier ice bombardment on hard ground at the onset of the Younger Dryas is a challenging task. Ice is fragile compared to rock, and the ice projectiles would just have disintegrated upon impacting hard ground. It is possible that more detailed analysis of the Hickory Run boulder field may be able to confirm this intriguing aspect of the Glacier Ice Impact Hypothesis. For now this question remains unanswered.