This section discusses a characteristic class of isolated ‘impact boulder fields’ with unusual surface features. This section suggests a catastrophic origin for ‘impact boulder fields’, formed in small secondary impacts from material sloughed off from the primary comet impact which formed the 450 km diameter Nastapoka arc of lower Hudson Bay, 12.8 ± 0.15 ka. Secondary icy-body impacts are suggested to sometimes create impact boulder fields, with boulders having characteristic surface features, such as relatively-young and uniformly weathered surfaces, where some of the boulders will exhibit deep pits and striations scoured (sandblasted) by super-high-velocity extraterrestrial material.
Younger Dryas impact hypothesis:
Impact-related proxies, including microspherules, nanodiamonds, and iridium are distributed across
four continents at the Younger Dryas boundary (YDB). Archeological material, charcoal and megafaunal remains is associated with a black mat in 5 locations, with fewer correlations at many more sites across 4 continents. (Wittke et al. 2013)
“Most Younger Dryas (YD) age black layers or “black mats” are dark gray to black because of increased organic carbon (0.05–8%) compared with strata above and below (6, 7). Although these layers are not all alike, they all represent relatively moist conditions unlike immediately before or after their time of deposition as a result of higher water tables.” (Haynes 2007)
“The spherules correlate with abundances of associated melt-glass, nanodiamonds,
carbon spherules, aciniform carbon, charcoal, and iridium” “across 4 continents”.
(Wittke et al. 2013)
“Bayesian chronological modeling was applied to 354 dates from 23 stratigraphic sections in 12 countries on four continents to establish a modeled YDB age range for this event of 12,835–12,735 Cal B.P. at 95% probability. This range overlaps that of a peak in extraterrestrial platinum in the Greenland Ice Sheet and of the earliest age of the Younger Dryas climate episode in six proxy records, suggesting a causal connection between the YDB impact event and the Younger Dryas.” (Kennett et al. 2015)
“The fact remains that the existence of mammoths, mastodons, horses, camels, dire wolves, American lions, short-faced bears, sloths, and tapirs terminated abruptly at the Allerød-Younger Dryas boundary.” The Quaternary megafaunal extinction is sometimes attributed to the ‘prehistoric overkill hypothesis’, although “The megafaunal extinction and the Clovis-Folsom transition appear to have occurred in <100 years, perhaps much less”. (Haynes 2007)
Many, most or perhaps all boulder fields worldwide of secondary impact origin may date to the ‘YD impact’, 12.8 ± 0.15 ka, which is suggested here to have formed the 450 km Nastapoka arc (impact basin) of lower Hudson Bay. But impact boulder fields and perhaps the associated Quaternary megafaunal extinction event itself may be mostly attributable to widely-disbursed secondary impacts from material sloughed off of the YD comet in its passage through Earth’s atmosphere. So while our atmosphere may protect us from most cosmic rays and small meteoroids, it may greatly exacerbate the harm to lifeforms in large icy-body impacts, due to widely-disbursed secondary impacts from comet material sloughed off in Earth’s atmosphere.
The vast 4 continent distribution of YD impact artifacts raises the question of whether fragmentation responsible for impact boulder fields et al. occurred in the atmosphere alone, or whether an earlier fragmentation occurred from a close encounter with one of the giant planets of the outer solar system.
The orientation of Carolina bays appear to point to two origins, lower Hudson Bay and Lake Michigan. (Firestone 2009) The orientation of elliptically-shaped Carolina bay appear to point back to two source locations, one in the lower Hudson Bay area (Nastapoka arc) and the second one pointing to circa Lake Michigan. Firestone et al. suggest the bays were formed by chunks of the Laurentide ice sheet, lofted into 100s to 1000s of kilometer trajectories by a dual impact (or airburst) on or over the ice sheet at those two locations. While dating the Carolina bays is difficult and controversial, the bays contain elevated levels of spherules common in the YD-impact black mat. Dual impacts on the ice sheet suggests that at least one chunk of the comet fragmentation was sufficiently sizable to loft sizable icebergs into trajectories of 100s of kilometers, but the Lake Michigan impact was apparently of insufficient size to create a Nastapoka arc counterpart.
Icy-body comet impacts are suggested here to form impact basins, whereas rocky-iron meteorites are known to form impact craters. Relatively-compressible ices are suggested to clamp the impact shock wave pressure below the melting point of silicates, largely precluding impact melt rock. PdV compression of ices may also clamp the shock wave pressure below the pressures necessary to form shatter cones, shocked quartz and high-pressure polymorphs like coesite, masking icy-body impact structures from identification as such. For instance, ices that undergo 10 times the dV compression of silicates will absorb 10 times the work energy from the impact shock wave, instantly soaring to 1000s of Kelvins which quickly melt embedded nebular dust and terrestrial sediments into molten microscopic silicate spherules. If ice compression lowers the impact power, then conservation of energy dictates that the impulse duration is commensurately extended. And a blunted but extended impact impulse may distort Earth’s crust into basins (in large impacts) rather than excavating craters, as rocky-iron meteorites are known to do. So while rocky-iron impacts may act like the sharp blow of a ball peen hammer, forming distinctive impact craters with distinctive overturned target rock, icy-body impacts may act more like the dull thud of a dead blow hammer, depressing the ground into a spherical impact basins, like Nastapoka arc. And the sustained shock wave duration of icy-body impacts (during the compression and rebound decompression of compressible ices) may tend to clamp the target rock in place, largely preventing the signature overturned rock of crater rims and the central peak rebound of complex craters.
Secondary impact boulder fields:
A number of boulder fields in the Appalachians are attributed to the suggested exaggerated freeze and thaw cycle toward the end of the last glacial period, but this gradualism approach can not account for unusual surface features in suggested impact boulder fields, nor the ability of ability of 2 diabase (Ringing Rock) boulder fields to resonate or ‘ring’ when struck sharply.
Impact boulder fields concentrated by downhill debris flows require a degree of incline to concentrate the boulders and to drain the boulder field to prevent burial by sedimentation over the intervening millennia; however, catastrophic impact boulder fields should be capable of flow down a much shallower grade than ‘talus-slope boulder fields’ formed by more gradual processes. The shear-thinning properties of phyllosilicate slurries in catastrophic impacts may lubricate a downhill pyroclastic flow or debris flow, stacking boulders many boulders deep.
Eastern Pennsylvania is suggested to have at least 3 impact boulder fields, with two Ringing Rock boulder fields composed of diabase and the Hickory Run boulder field, in Hickory Run State Park, composed of sandstone/quartzite. The sandstone boulders that compose Blue Rocks boulder field (near Hawk Mountain, Berks County Park) are too eroded to show surface scouring, which may indicate softer boulders, and/or boulders older than End Pleistocene, so the Blue Rocks boulder field can not be positively attributed to an impact origin. Talus-slope boulder fields are common along the ridges of the Appalachians. In general, boulder fields in rugged terrain and particularly along mountaintop ridge lines should be dismissed as unlikely impact boulder fields, and in any case, distinctive surface surface-feature scouring is necessary to affirm an impact origin.
The suggested Lake Michigan impact extrapolated from Carolina bay orientations likely had the protection of perhaps as much as a kilometer of the Laurentide ice sheet, whereas the three suggested impact boulder fields in Pennsylvania were presumably below the Late Wisconsinan extent of the ice sheet (although Hickory Run State Park is mapped as covered by the last substage of the Wisconsinan Stage of the ice sheet on the USGS geologic map of Pennsylvania). Could an impact have flash melted a thin tip of ice sheet, lubricating the resulting debris flow that formed Hickory Run boulder field, explaining its well-rounded boulders from extensive tumbling? The approach direction of the comet, however, is somewhat problematic, since the terrain falls away to the northwest in Ringing Rocks Park, Bucks County PA, whereas the terrain rises to the northwest of the Hickory Run boulder field.
Scoured surface features:
Pockmark, striation and pot hole surface features on boulders in impact boulder fields are suggestive of sandblasting or water-jet cutting in an industrial setting. So while a massive impulse may be necessary to fracture the bedrock into boulders, exposure to high-velocity streams of material are necessary to create the observed scoured surface features.
Impact boulder field boulders will exhibit more or less rounding of corners from a greater or lesser degree of downhill debris flow tumbling from their impact origin. The boulders in Hickory Run boulder field are significantly more rounded than those in the two Ringing Rocks boulder fields, suggesting more abrasive tumbling over a greater distance by a larger mass of boulders. The ‘terrain’ feature of Google maps is not sufficiently sensitive to positively identify secondary impact locations, even for the large Hickory Run boulder field, so it’s likely that impact fracturing by secondary impacts is only a few boulders deep at most. The size and width of 3 known impact boulder fields suggest an impact footprint on the order of 10s of meters across, as a working hypothesis. Similarly, secondary impacts on low ground may also be below the resolution of the terrain feature of Google maps. Even so, perfectly-round water-filled secondary-impact features on low ground should jump out on the satellite imagery of ‘Google Earth’, unless atmospheric fragmentation of sloughed off material typically distorts the impact footprint into non-circular shapes, and/or if secondary impacts on low ground on the order of 10s of meters will have filled in with sediment in the intervening 12,800 years.
Comet-spatter rock scale:
Additionally, the most erosion resistant of boulder-field boulders and stream cobbles may still retain secondary ‘comet spatter–’on one side only–in the form of rock scale, although boulder field boulders may exhibit more than 180° coverage due to being briefly airborne at some point. Most apparent rock scale is actually lichen, particularly if the apparent rock scale has a rounded perimeter, and most comet spatter appears to be orange or brown, whereas lichen is often white or jet black. And lichen like comet spatter will typically appear on one side only of a rock or boulder, since the algae or cyanobacteria component of lichen requires sunlight for photosynthesis. A weathering rind is another look alike, and weathered diabase boulders often exhibit a yellow or orange weathering rind that may simulate comet spatter. Ideally a cobble or boulder with a maple-leaf-shaped deficit, or some other recognizable shape which acted as a comet spatter mask, will reveal itself to a persistent or fortuitous observer.
Shoe stone with comet spatter:
A greywacke ‘shoe stone’ shaped like a human slipper was found in the Susquehanna River in Millersburg, PA. Most of the shoe stone is natural, but the sole has evidence of human modification, evidently to make it into a more-perfect slipper shape. And the stone has raised brown nodules on ‘one side only’, suggesting the stone was Clovis to have been exposed on the day of the comet, and indeed a small amount of suggested comet spatter overlays the tooled surface of the sole.
Cup marks in cairns in the British Isles:
In addition to North American boulder fields, cup marks in boulders from cairns in the British Isles are also suggested to be of secondary impact origin, where the associated boulder fields were presumably long ago scavenged for building materials
Ringing Rocks impact boulder fields:
Pennsylvania has two Ringing Rock boulder fields, Ringing Rocks Park in Lower Black Eddy, PA and Ringing Rocks Park in Pottstown, PA 40.270647, -75.605616. ‘Ringing Rocks’ refers to the propensity of diabase boulders within the two Ringing Rocks boulder fields to resonate or ‘ring’ at a characteristic frequency when struck sharply with a hard object, whereas diabase boulders elsewhere do not ring. Apparently, the super-high-pressure impact shock wave stressed the surface of diabase boulders, like prestressed glass, imparting the ability to resonate when struck. Additionally, Ringing Rock boulders variably exhibit scoured surface features, with uniformly ‘young’ subconchoidal fractured surfaces that exhibit very-shallow surface decomposition (exfoliation), indicating a relatively-young age. For Southeastern Pennsylvania to have two Ringing Rock impact boulder fields composed of diabase boulders, suggests that a large number of other boulder fields are also of impact origin, since diabase forms only a very small fraction of the terrain in Southeastern Pennsylvania.
Firestone, Richard B., 2009, The Case for the Younger Dryas Extraterrestrial Impact Event: Mammoth, Megafauna, and Clovis Extinction, 12,900 Years Ago, Journal of Cosmology, 2009, Vol 2, pages 256-285
Haynes Jr., C. Vance, 2007, Younger Dryas “black mats” and the Rancholabrean termination in North America, Proceedings of the National Academy of Sciences, vol. 105 no. 18
Kennett, James P. et al., 2015, Bayesian chronological analyses consistent with synchronous age of 12,835–12,735 Cal B.P. for Younger Dryas boundary on four continents, Proceedings of the National Academy of Sciences, vol. 112 no. 32
Wittke, James H. et al., 2013, Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago, Proceedings of the National Academy of Sciences, vol. 110 no. 23