YD IMPACT BOULDER FIELDS

Hickory Run boulder field, Hickory Run State Park Pennsylvania

Hickory Run boulder field, Pennsylvania

Abstract:

    Isolated boulder fields attributed to the last glacial maximum (LGM) contain many boulders with incised surface features, such as cup marks and striations, that appear to be abrasively scoured out, pointing to a catastrophic origin. These suggested Younger Dryas ‘YD impact boulder fields’ are suggested here to have the same catastrophic origin as Carolina bays.
    Both phenomena are suggested to have formed from secondary impacts of Laurentide ice sheet fragments (‘icebergs’) launched into ballistic trajectories by a YD bolide impact, circa 12,900 BP, on the Laurentide ice sheet in the Great Lakes region.
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Introduction:

    A Younger Dryas (YD) impact or airburst, circa 12,900 BP, has been all but proven with the finding of platinum spike in the Greenland ice sheet, with the location and size of the bolide impactor open to question. Additionally, a thin layer of magnetic grains, microspherules, nanodiamonds, and glass-like carbon have been unearthed at various locations across North America, generally overlain by a ‘black mat’ with a high carbon content, both of which are nearly coincident with megafauna extinction in North America and the disappearance of the Clovis civilization (Firestone et al., 2007; Firestone, 2011).

Magnetic glass spherule from Pennsylvania

Magnetic glass spherule from Pennsylvania

    The impact hypothesis for Carolina bays goes to the 1940s. Carolina bays are a series of 500,000 oval depressions that range in size from 50 m to 10 km in length which are concentrated along the Atlantic seaboard and along the coastal plain of the Gulf of Mexico. Their impact origin has been more recently revived, suggesting a secondary impact origin. Triangulating the Carolina bay orientations points back to a primary bolide impact location in the Great Lakes region on or over the Laurentide ice sheet. Presumably, a primary ice sheet impact fractured chunks of the Laurentide ice sheet and lofted them into ballistic trajectories of 1000s of km which impacted to form elongated craters in the soft wet soil of the coastal plane. These ballistic chunks of ice sheet hypothesized to have formed the Carolina bays are hereafter designated, ‘icebergs’.

    Isolated Pennsylvania boulder fields are classically attributed to a periglacial freeze-thaw process during the Last Glacial Maximum (LGM), where frost action fractured bedrock, and the boulders slid downhill by ‘solifluction’, where frozen subsoil acts as a barrier to the percolation of water.
    Alternatively, boulder fields dating to the LGM are suggested here to have been caused by secondary impacts of icebergs which brecciated the target bedrock, followed by downward debris flow which concentrated the brecciated bedrock into boulder fields.
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Boulder fields:

    It would be quite remarkable if ice sheet fragments launched into ballistic trajectories of 1000s of km by a Younger Dryas impact over the Great Lakes region only impacted along the coastline, creating Carolina bays. It would be almost as remarkable if corresponding inland impacts left no mark or had been entirely erased after the same time span.

    Upland bedrock is a much more resistant target than wet sandy soil of the coastal plane; however, bedrock is comparatively brittle, inferring that iceberg impacts may have brecciated bedrock. And if brecciated boulders weren’t sculpted into oval craters circumscribed by elevated rims, like the Carolina bays, then 12,900 years of sedimentation may have largely hidden them from view.
    While ground penetrating radar may readily reveal the brecciated bedrock shrouded by sediments of secondary impacts, nature may have provided a mechanism for concentrating brecciated boulders into boulder fields by means of downhill debris flows.
    On sloping ground, when an iceberg impact shattered bedrock into breccia and flooded the soil and breccia mixture with the flash-melted water of the impacting iceberg, the resulting soup gushed downhill as a debris flow, concentrating the breccia into a ravine in the form of a boulder field. And coarse boulder breccia in a sloping gully would tend to act as a French drain, flushing sediments through, rather than trapping them, with the lack of vegetation making them conspicuous on Google Maps, satellite mode.
    Isolated impact boulder fields should not be confused with talus slopes below high cliffs or on steep terrain, which form by gradualism.

Two Ringing Rocks boulder fields, PA:
   Triassic diabase of the Central Atlantic magmatic province (CAMP) provided a particularly tough target rock to secondary iceberg impacts, with many boulders exhibiting sharp subconchoidal fractures suggestive of impact fracturing, compared to the gradual weathering of diabase bedrock creating jointed diabase boulders with spheroidal exfoliation characteristic of Devil’s Den in Gettysburg, PA.
    The diabase boulders of the two Ringing Rocks boulder fields have unusual acoustic properties, in that many of the boulders ring like a bell when struck sharply, as with a hammer, hence the name, ‘Ringing Rocks’. Diabase boulders outside these two discrete boulder fields do not exhibit this ringing property. Presumably this ability to resonate was imparted by the super-high-pressure conditions of the iceberg impact which brecciated the rock, presumably by prestressing the outer surface.
    While weathering of many kinds of rock is highly variable, diabase weathers slowly and predictably, first attaining a rust-like patina, followed by forming a brown-orange rind which exfoliates to expose virgin rock underneath. And like granite, diabase tends to undergo spherical exfoliation to form rounded boulders. In the two Ringing Rocks diabase boulder fields, weathering has barely progressed beyond forming a rust-like patina on the exposed boulders, and indeed the Ringing Rocks boulder field in Lower Black Eddy, PA is recognized to date to around the Last Glacial Maxima. Diabase boulders outside the two Ringing Rocks boulder fields exhibit active exfoliation of a discolored weathering rind along with distinctive rounding caused by spheroidal weathering, indicating weathering exposure times greatly in excess of 12,900 years. The smaller and less well known of the two Ringing Rocks boulder fields is in Pottstown, PA (40.270647, -75.605616), where the less tumbled boulders of the smaller diabase boulder field exhibit sharper subconchoidal-fractured corners.

Hickory Run boulder field, PA:
    Devonian sandstone, Catskill Fm, Duncannon Member constitute the pink boulders of the Hickory Run boulder field. The sandstone sand-grain size appears to be inversely related to boulder size, which suggests that in the upward-fining cycle, fine-grained sandstone was at the surface, which was brecciated into smaller boulders than the underlying sandstone with coarser sand-grain size. While finer-grained sandstone will naturally attain a higher polish than coarser-grained sandstone from the same degree of tumbling, the difference in roughness between large and small boulders seems exaggerated, with many of the smooth smaller boulders appearing to have a chemically indurated surface. In tumbling, the large boulders rise to the surface, presumably above the churning sandy-muddy matrix which gave the smaller boulders a high initial polish. Then presumably the muddy matrix indurated the surfaces of the smaller boulders by crystallization from saturated mineral species. Finally, the sandy-muddy matrix washed away through the porous French drain of the boulder field, revealing the curiously polished and indurated smaller boulders.
    Curiously, the terminal moraine of the Wisconsin glaciation appears to surround the Hickory Run boulder field in a semicircular ring of more than 180° from west-southwest to due east.
    Hickory Run boulder field is much larger than the two Ringing Rocks boulder fields, and its boulders are considerably more rounded as well. The greater corner rounding in the larger boulder field is no doubt mostly attributable to a greater degree of tumbling, although the tougher diabase boulders may have offered a small degree of relative protection from flaking; however, no quantitative evaluations were made on the degree of rounding between boulder fields, and not even a qualitative evaluation was performed of the uphill vs. downhill ends of any of the boulder fields.

Blue Rock boulder field at Hawk Mountain, PA:
    The heavily weathered Tuscarora quartzite boulders of Blue Rock boulder field at Hawk Mountain, PA make age determination problematic, although an official signpost attributes the Blue Rock boulder field to periglacial mechanisms, and the fact that the boulder field spills down the south side of Hawk Mountain makes a ballistic iceberg impact from the the west-northwest, from the Great Lakes region, somewhat improbable.

    While there are numerous boulder fields apparent in Google Maps satellite mode in the Appalachians, only 4 were examined here for a secondary YD impact origin, but if the 500,000 Carolina bays correspond to a similar density of upland impacts, then highly-visible boulder fields are only the tip of the iceberg.
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Incised surface features in impact boulder field boulders:

    Boulder field boulders exhibit a high incidence of incised surface features, such as cup marks, striations, and ‘pot holes’ (large, deep cup marks especially with ‘handle’ striations, or small cup marks with handles), which are presumably caused by abrasive scouring by super-high-velocity slurries (as in abrasive water jet cutting) and/or by super-high-velocity particle streams (as in sandblasting).

    Incised surface features are common in 3 of 4 Pennsylvania boulder fields examined for this phenomenon, namely the two Ringing Rocks boulder fields, and Hickory Run boulder field. In the fourth boulder field, the Tuscarora quartzite boulders of Blue Rock boulder field at Hawk Mountain, PA appear to have experienced such severe weathering that incised surface features may have been erased.

    Super-high-velocity slurries or particle streams responsible for scouring incised surface features would have rapidly decelerated due to air resistance after sloughing off from the main iceberg. Air resistance would not only slow the sloughed off material, but also cause it to fall short of ground zero of an impacting iceberg following a shallow ballistic trajectory. Therefore scouring material must have sloughed off quite shortly before iceberg impact.

Hickory Run boulder field
Deeply-incised sandstone boulder

Hickory Run boulder field
Sandstone boulder with cup mark, domed in center

Ringing Rocks boulder field
Diabase boulder with broad parallel striations

Ringing Rocks boulder field
Diabase boulder with deep pot holes and striations

    Cup marks are also common in European boulders and exposed bedrock. In Europe, cup marks are often enhanced with concentric ring art, artistically highlighting the presumably naturally formed cup marks.

Cup marks in cairn boulder, Inverness Scotland

    Some presumably naturally-incised cup marks have been enhanced with ring art as in the following IMAGE from Fowberry Cairn, UK

    In the following IMAGE, presumably only the cup-marked top portion of the rock was exposed above the soil line at the time of a local iceberg impact, from Farnhill Moor, UK.

    Note the granular cup marks in the following image from Val Camonica, Italy. Note the pronounced pitting in the cup marks and meandering striations. To be artificially fabricated, the pits comprising the cup marks and striations would have had to drilled with different diameter drills, since percussive hammering on chisels or picks would produce a very different effect. And the random placement, varied relief and departure from circularity and linearity of the cup marks and striations are not the work of an artist or artisan.

Rock with granular cup marks and striations, Val Camonica, Italy
Image credit: Luca Giarelli / CC-BY-SA 3.0

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Dark-brown, nodular, ‘impact splatter’ rock scale as primary bolide material:

    Dark-brown nodular rock scale associated with YD impact boulder fields is suggested to result from ‘impact splatter’. The typical dark-brown coloration presumably presumably indicates high iron-oxide content, since a magnetized needle suspended from a thread deflects toward this type of rock scale. And the nodular aspect of the rock scale presumably arises from the additive nature of high-velocity splattering.
    Impact rock scale has only been noticed outside impact boulder fields, and not yet on boulders within impact boulder fields.
    Nodular impact splatter is typically only be found on ‘one side only’ of a boulder or cobble, coating only the side of the rock exposed to the incoming impact splatter.

    The high iron content and moderate velocity of the impact-spatter rock scale found outside impact boulder fields suggests that the impact splatter is primary material sloughed off of the bolide itself, which slowed to terminal velocity before reaching the ground. Larger chunks of bolide, however, may have been traveling fast enough to have start widespread fires which created the carbon of the black mat.

    The following two images show nodular rock scale on Devonian conglomerate and sandstone on Stony Mountain, north of Fort Indiantown Gap, PA.

Stony Mountain north of Fort Indiantown Gap
Nodular rock scale
GPS (40.48301, -76.62908)

Stony Mountain north of Fort Indiantown Gap
Nodular rock scale
GPS (40.48116, -76.62837)

    The greywacke ‘shoe stone’ from the Susquehanna River at Millersburg, PA exhibits nodular rock scale in millimeter-scale nodules on one side only (left side and bottom). One small area on the sole, circled in rod, exhibits numerous chip marks, presumably indicating human modification to appear more like a human shoe. If the nodular rock scale is indeed YD impact splatter, then its presence at the surface 12,900 years ago raises the probability that the sole modification was Clovis.

(Clovis?) shoe stone from Susquehanna River, Millersburg, PA, left side with nodular rock scale

(Clovis?) shoe stone from Susquehanna River, Millersburg, PA, bottom side with minimal nodular rock scale. The area circled in red has faint chip marks, presumably indicating human modification.

(Clovis?) shoe stone from Susquehanna River, Millersburg, PA, right side, no nodular rock scale

Close up of shoe stone nodular rock scale

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Conclusion:

    An impact origin for Carolina bays requires an inland equivalent, where a small percentage of inland impacts are suggested to stand out as boulder fields. The discrete nature of these impact boulder fields is difficult to explain in the context of widespread periglacial freeze-thaw cracking of bedrock, accompanied by solifluction (gradualism), whereas secondary impacts (catastrophism) predicts it, and it’s unclear why the periglacial freeze-thaw cracking of bedrock wouldn’t be strongly correlated with the most susceptible type of bedrock, rather than occurring in igneous diabase and in clastic quartzite/sandstone. And gradualism would seem to have greater difficulty explaining the concentration of boulders into deep boulder fields with such a high degree of corner rounding, since talus slopes (definitely formed by gradualism) exhibit almost no corner rounding.

    Secondly, incised surface features, such as cup marks and striations, are significant evidence of a catastrophic primary mechanism, whereas gradualism dismisses the features as ad hoc secondary mechanisms, such as differential weathering or human artistry.

    Finally dark-brown, nodular, impact-splatter rock scale is suggested to be part of the primary bolide that rained down at terminal velocity across North America, and likely to a lesser extent across Central America, Western Europe and beyond. Since no cache of meteorites is known from the End Pleistocene, the material was presumably unlike any known meteorites, and therefore unrecognizable as such.
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Ringing Rocks boulder field
Diabase boulder with deep pot holes

Hickory Run boulder field
Sandstone boulder with cup marks

Ringing Rocks boulder field
Diabase boulder with deep crossed striations

Ringing Rocks boulder field
Diabase boulder with cup marks and striations

References:

Firestone, Richard B., Analysis of the Younger Dryas Impact Layer, (2007), Lawrence Berkeley National Laboratory https://escholarship.org/uc/item/03q2r98x

Firestone, R.B.; West, A.; Kennett, J.P. et al., (2007), Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling, PNAS October 9, 2007, vol. 104, no. 41
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2 thoughts on “YD IMPACT BOULDER FIELDS

  1. I am interested in your position on the Nastapoka Arc as impact site. I have compiled an article on the same subject from material on the Web. If you are interested I could E-mail it to you.

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