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Given that uniformitarian geology was “de rigueur” in those days, it is understandable that Schlee (1957)
incorrectly described the slopes of those three curves of Figure 2B as “of the same general order of
magnitude.” In fact, the cobble-size-reduction-rate slope that he was forced to attribute to the ancestral
Potomac River is well over one order of magnitude greater than those of the modern Rhine and Muir Rivers,
each of which has cut channels into crystalline rock. The contradiction grows worse when one considers
that the ancestral Potomac River envisioned by Hack (1955) was now proven to have ground down the
upland cobbles at the absolutely fantastic rate implied by Figure 2B, while at the same time failing to cut a
channel down to bed rock in the “soft easily eroded Coastal Plain sediments (Schlee, 1957).”
Still, Schlee (1957) deserves credit for realizing that, whenever data follow an exponential law as well as
they do in Figure 2B, it should be possible to safely extrapolate them forward or backward by at least one or
two factors of two. So he did indeed extrapolate the upland gravels of the Patuxent-Potomac peninsula and
the District of Columbia to a modal cobble size of 128 mm in an area ~18 to 30 km northwest of the U.S.
Capitol Building. However, he found this putative “source area” (roughly within the dashed circle in Fig.
2A) to comprise the “granites, diorites, schists, gneisses, and quartzites of the Piedmont province,” in sharp
contrast to the coarser fraction of upland deposits, which “contain little or no gneiss, schist, diorite, or
granite.” Although Schlee (1957) does not explicitly state that he found no 128-mm sized quartzite cobbles
in that area, one may safely assume that, if he had found them, his story line would have been very different.
So, where was the source of the quartzite component of the gravel member of the upland deposits? Well,
Schlee (1957) remarked that “Though generally unfossiliferous, a few of the pebbles display fossil
brachiopod impressions which indicate a Devonian age: the source rock was probably the Oriskany
sandstone.” He noted, however, that the Oriskany formation is ~130 km distant from the upland gravels,
whereas the nearest outcrop of Devonian quartzite is the Weaverton formation at ~58 km.
However, given the total absence of a trail of ever-larger quartzite boulders leading back to either of these
putative sources, Hack’s (1955) fluvial model is seen to completely fail the central test of uniformitarian
geology: We do not see today’s rivers transporting cobbles dozens of kilometers before depositing the first
one. Although such things could happen in the case of superfloods (Baker, 2002), this scenario would
surely have resulted in the stripping of the soft coastal plain sediments before the deposition of the first
cobble. Thus, it is truly difficult to imagine any single river behaving in the manner that Hack (1955)
imputes to the ancestral Potomac River ...much less the ancestral Potomac, Rappahannock, York, and
James Rivers all performing this same “ballet fantastique,” as though choreographed by Nijinsky!
ADDITIONAL UPLAND-DEPOSITS FACIES BEST EXPLAINED BY IMPACT
With reference to properties of the upland deposits already mentioned
Regarding Schlee’s wavy bands locally cementing the gravels, I note that those ~1-cm-thick “wavy bands”
are better described as “wavy sheets” and that, in all cases I have inspected, such sheets have been
terminated by fresh conchoidal fractures – implying that they were fractured during and/or just before
emplacement. Many examples of such fracture surfaces are illustrated in Griscom et al. (2003a).
Here I propose an explanation for why, after investigating fully 98 locations, Schlee (1957) reported the
largest “irregular mass” cemented by “fraction of an inch thick” bands “which parallel and transect the
bedding at a low angle” to be only ~1 meter across (as opposed to many meters across as might be expected
under the canonical notion that they were precipitated from water solution). If, based on the observations of
Griscom et al. (2003a), it is assumed that all such “irregular masses” were not much thicker than the
average pebble or cobble webbed together by those hard ferric-oxyhydroxide(ferrihydrite?)-matrix sheets,
then Schlee’s largest object of this nature would have had an aspect ratio of ~10 to 1. It turns out that such
large aspect ratios are expected of interference-zone impact ejecta. Melosh (1989) has pointed out that “At
low speeds (up to a few hundred meters per second) the near surface ejecta consist of spall plates. These
plates are several to ten times broader than they are thick (emphasis added).”
Melosh (1989) further instructs that “[Spall plates] are the largest and least shocked fragments thrown out at
any given velocity” but that they “...contain so much elastic energy from the interfering stress waves that
they themselves break up into smaller, Grady-Kipp fragments immediately upon ejection.” Thus, the
maximum size of an interference-zone ferric-oxyhydroxide-welded-quartzite-pebble spall plate that will
commonly survive both launch and re-impact may well turn out to be of the order of just 1 meter. N.B. An
impact-based mechanism for creating such “peanut-brittle-form” objects has been sketched by Griscom et
al. (2003a).
incorrectly described the slopes of those three curves of Figure 2B as “of the same general order of
magnitude.” In fact, the cobble-size-reduction-rate slope that he was forced to attribute to the ancestral
Potomac River is well over one order of magnitude greater than those of the modern Rhine and Muir Rivers,
each of which has cut channels into crystalline rock. The contradiction grows worse when one considers
that the ancestral Potomac River envisioned by Hack (1955) was now proven to have ground down the
upland cobbles at the absolutely fantastic rate implied by Figure 2B, while at the same time failing to cut a
channel down to bed rock in the “soft easily eroded Coastal Plain sediments (Schlee, 1957).”
Still, Schlee (1957) deserves credit for realizing that, whenever data follow an exponential law as well as
they do in Figure 2B, it should be possible to safely extrapolate them forward or backward by at least one or
two factors of two. So he did indeed extrapolate the upland gravels of the Patuxent-Potomac peninsula and
the District of Columbia to a modal cobble size of 128 mm in an area ~18 to 30 km northwest of the U.S.
Capitol Building. However, he found this putative “source area” (roughly within the dashed circle in Fig.
2A) to comprise the “granites, diorites, schists, gneisses, and quartzites of the Piedmont province,” in sharp
contrast to the coarser fraction of upland deposits, which “contain little or no gneiss, schist, diorite, or
granite.” Although Schlee (1957) does not explicitly state that he found no 128-mm sized quartzite cobbles
in that area, one may safely assume that, if he had found them, his story line would have been very different.
So, where was the source of the quartzite component of the gravel member of the upland deposits? Well,
Schlee (1957) remarked that “Though generally unfossiliferous, a few of the pebbles display fossil
brachiopod impressions which indicate a Devonian age: the source rock was probably the Oriskany
sandstone.” He noted, however, that the Oriskany formation is ~130 km distant from the upland gravels,
whereas the nearest outcrop of Devonian quartzite is the Weaverton formation at ~58 km.
However, given the total absence of a trail of ever-larger quartzite boulders leading back to either of these
putative sources, Hack’s (1955) fluvial model is seen to completely fail the central test of uniformitarian
geology: We do not see today’s rivers transporting cobbles dozens of kilometers before depositing the first
one. Although such things could happen in the case of superfloods (Baker, 2002), this scenario would
surely have resulted in the stripping of the soft coastal plain sediments before the deposition of the first
cobble. Thus, it is truly difficult to imagine any single river behaving in the manner that Hack (1955)
imputes to the ancestral Potomac River ...much less the ancestral Potomac, Rappahannock, York, and
James Rivers all performing this same “ballet fantastique,” as though choreographed by Nijinsky!
ADDITIONAL UPLAND-DEPOSITS FACIES BEST EXPLAINED BY IMPACT
With reference to properties of the upland deposits already mentioned
Regarding Schlee’s wavy bands locally cementing the gravels, I note that those ~1-cm-thick “wavy bands”
are better described as “wavy sheets” and that, in all cases I have inspected, such sheets have been
terminated by fresh conchoidal fractures – implying that they were fractured during and/or just before
emplacement. Many examples of such fracture surfaces are illustrated in Griscom et al. (2003a).
Here I propose an explanation for why, after investigating fully 98 locations, Schlee (1957) reported the
largest “irregular mass” cemented by “fraction of an inch thick” bands “which parallel and transect the
bedding at a low angle” to be only ~1 meter across (as opposed to many meters across as might be expected
under the canonical notion that they were precipitated from water solution). If, based on the observations of
Griscom et al. (2003a), it is assumed that all such “irregular masses” were not much thicker than the
average pebble or cobble webbed together by those hard ferric-oxyhydroxide(ferrihydrite?)-matrix sheets,
then Schlee’s largest object of this nature would have had an aspect ratio of ~10 to 1. It turns out that such
large aspect ratios are expected of interference-zone impact ejecta. Melosh (1989) has pointed out that “At
low speeds (up to a few hundred meters per second) the near surface ejecta consist of spall plates. These
plates are several to ten times broader than they are thick (emphasis added).”
Melosh (1989) further instructs that “[Spall plates] are the largest and least shocked fragments thrown out at
any given velocity” but that they “...contain so much elastic energy from the interfering stress waves that
they themselves break up into smaller, Grady-Kipp fragments immediately upon ejection.” Thus, the
maximum size of an interference-zone ferric-oxyhydroxide-welded-quartzite-pebble spall plate that will
commonly survive both launch and re-impact may well turn out to be of the order of just 1 meter. N.B. An
impact-based mechanism for creating such “peanut-brittle-form” objects has been sketched by Griscom et
al. (2003a).
Correlation of the upland deposits of southern Maryland with those of eastern Virginia
If, as I believe, all of the upland deposits mapped in Figure 1 came out of the Chesapeake Bay crater, then
any given mapped locality should be lithologically similar to all others. Strong support for this notion comes
from the fact that Wentworth’s (1930) extensive field work led him to apply the name Brandywine to the
upland deposits of eastern Virginia. Moreover, note the similarity of the following descriptions of two
widely separated upland-deposit locations, the former being an average over all of Southern Maryland and
the latter pertaining to a small cluster of sites west of Richmond, Va., which I will described in greater detail
later. These sampled locals are separated laterally by an average distance of ~140 km, vertically by ~40 m,
and angularly by ~60° with respect to the crater center:
Schlee (1957), regarding the upland deposits of Southern Maryland:
(a) Describing the gravel member only: “The predominance of mature siliceous rocks is one of the unusual
features of the upland gravels...” “In the 64- to 128-mm size grade, quartzite usually far exceeds ‘vein’
quartz and chert.” “Other rock types constitute a few percent or less of the total.” “In all but two analyses
[out of 72], the size distribution is bimodal...” In addition to showing 12 of these 72 distributions in a figure
as bar graphs spanning a size range of <1/16 mm to 128 mm, Schlee (1957) presented a histogram of the
modal sizes of those 72 distributions, which is indeed bimodal, showing 81% of the modes to fall in the
gravel group (4 to 128 mm) and 19% in the sand group (1/4 to 1 mm).
(b) Footnote: “The ‘only fossil’ seen by the author was Cassius madagascariensis spinella Clench, found at
the gravel-loam interface near Silver Hill, MD... It is unlikely that a tropical marine form such as this... was
indigenous to the upland deposits; it may have been buried by Indians.” (N.B. The Earth was a “hothouse”
for ~ 500 m.y. prior to the Late Eocene.)
(c) “Pseudo-bedding in the form of wavy bands, which parallel and transect the bedding at a low angle,
occurs in many exposures. The bands are chocolate to ruddy brown, well indurated, and a fraction of an
inch thick.” “...secondarily introduced iron oxide ... locally cements the sand and gravel along definite zones
and in large irregular masses up to [1 m] across.” N.B. Griscom et al. (2003) have argued that these hard
ferric oxyhydroxide (ferrihydrite?) bands can be interpreted in no other way than as impactites.
(d) Speaking of the upland deposits of Southern Maryland as a single unit: “This sheetlike deposit, which
successively overlaps older formations to the northwest, has a fairly constant thickness of [6 to 9 m] and
dips southeastward at approximately [1 m/km]” (in the direction of the crater).
Goodwin and Johnson (1970), RE the Midlothian gravels west of Richmond:
(a) “The most striking feature of the Midlothian gravels is the thick gravel member. The gravel member,
which ranges in thickness from zero to nearly [12 m], is composed of abundant pebbles and cobbles in a
sandy matrix containing variable amounts of clay. Although cobbles up to [230 mm] in maximum diameter
were observed, the average cobble measures [50 to 75 mm] in diameter.” “The clasts are variable in
composition but most are of some variety of quartz. Vein quartz, massive quartz, quartzite and quartz
sandstone are the dominant lithologies comprising the clasts. A few clasts of other metamorphic rocks,
igneous rocks, and chert constitute a minor portion of the gravels.”
(b) “No fossils have been found within the gravels to aid in determining their age or their origin.”
(c) “The basal cobble zone is one to four cobbles thick and is commonly solidly cemented by iron oxide to
form a ferricrete zone.”
(d) “The [~60 km2] upland gravels around Midlothian [~10 km west of Richmond, Va.] are isolated and
have been truncated by erosion on all sides.” “Although locally the contact between the gravels and
underlying rocks exhibits more than [6 m] of relief due to channeling, the surface of unconformity dips
gently to the east or southeast at approximately [1.7 m/km]” (in the direction of the crater).
UPLAND DEPOSITS: A GRAVEL-SIZE GRADIENT RADIAL TO THE CRATER
Perhaps the most illuminating of Schlee’s (1957) many innovative contributions to our knowledge of the
upland deposits was his experimental demonstration and mathematical analysis of the dramatic cobble-size
gradient decreasing from 32-mm modal size in Washington, D.C., down to 4 mm midway down the
Patuxent-Potomac peninsula (Fig. 2). Figure 2A, taken from Schlee (1957), shows the isoliths of modal
gravel size that he determined for Washington and the Patuxent-Potomac peninsula. In contemplating these
isoliths, Schlee (1957) observed that “The orderly changes in modal size in the northern portion of the sheet
suggest that the size change may be a mathematical function similar to those found for the size diminution in
modern rivers.” He therefore selected “four profiles parallel to the current flow direction and as far as
possible normal to the size contours,” plotted them on the semilog graph (Fig. 2B), fit them to a straight line,
and then compared the slope of this line to those reported for the modern Rhine and Mur Rivers.
If, as I believe, all of the upland deposits mapped in Figure 1 came out of the Chesapeake Bay crater, then
any given mapped locality should be lithologically similar to all others. Strong support for this notion comes
from the fact that Wentworth’s (1930) extensive field work led him to apply the name Brandywine to the
upland deposits of eastern Virginia. Moreover, note the similarity of the following descriptions of two
widely separated upland-deposit locations, the former being an average over all of Southern Maryland and
the latter pertaining to a small cluster of sites west of Richmond, Va., which I will described in greater detail
later. These sampled locals are separated laterally by an average distance of ~140 km, vertically by ~40 m,
and angularly by ~60° with respect to the crater center:
Schlee (1957), regarding the upland deposits of Southern Maryland:
(a) Describing the gravel member only: “The predominance of mature siliceous rocks is one of the unusual
features of the upland gravels...” “In the 64- to 128-mm size grade, quartzite usually far exceeds ‘vein’
quartz and chert.” “Other rock types constitute a few percent or less of the total.” “In all but two analyses
[out of 72], the size distribution is bimodal...” In addition to showing 12 of these 72 distributions in a figure
as bar graphs spanning a size range of <1/16 mm to 128 mm, Schlee (1957) presented a histogram of the
modal sizes of those 72 distributions, which is indeed bimodal, showing 81% of the modes to fall in the
gravel group (4 to 128 mm) and 19% in the sand group (1/4 to 1 mm).
(b) Footnote: “The ‘only fossil’ seen by the author was Cassius madagascariensis spinella Clench, found at
the gravel-loam interface near Silver Hill, MD... It is unlikely that a tropical marine form such as this... was
indigenous to the upland deposits; it may have been buried by Indians.” (N.B. The Earth was a “hothouse”
for ~ 500 m.y. prior to the Late Eocene.)
(c) “Pseudo-bedding in the form of wavy bands, which parallel and transect the bedding at a low angle,
occurs in many exposures. The bands are chocolate to ruddy brown, well indurated, and a fraction of an
inch thick.” “...secondarily introduced iron oxide ... locally cements the sand and gravel along definite zones
and in large irregular masses up to [1 m] across.” N.B. Griscom et al. (2003) have argued that these hard
ferric oxyhydroxide (ferrihydrite?) bands can be interpreted in no other way than as impactites.
(d) Speaking of the upland deposits of Southern Maryland as a single unit: “This sheetlike deposit, which
successively overlaps older formations to the northwest, has a fairly constant thickness of [6 to 9 m] and
dips southeastward at approximately [1 m/km]” (in the direction of the crater).
Goodwin and Johnson (1970), RE the Midlothian gravels west of Richmond:
(a) “The most striking feature of the Midlothian gravels is the thick gravel member. The gravel member,
which ranges in thickness from zero to nearly [12 m], is composed of abundant pebbles and cobbles in a
sandy matrix containing variable amounts of clay. Although cobbles up to [230 mm] in maximum diameter
were observed, the average cobble measures [50 to 75 mm] in diameter.” “The clasts are variable in
composition but most are of some variety of quartz. Vein quartz, massive quartz, quartzite and quartz
sandstone are the dominant lithologies comprising the clasts. A few clasts of other metamorphic rocks,
igneous rocks, and chert constitute a minor portion of the gravels.”
(b) “No fossils have been found within the gravels to aid in determining their age or their origin.”
(c) “The basal cobble zone is one to four cobbles thick and is commonly solidly cemented by iron oxide to
form a ferricrete zone.”
(d) “The [~60 km2] upland gravels around Midlothian [~10 km west of Richmond, Va.] are isolated and
have been truncated by erosion on all sides.” “Although locally the contact between the gravels and
underlying rocks exhibits more than [6 m] of relief due to channeling, the surface of unconformity dips
gently to the east or southeast at approximately [1.7 m/km]” (in the direction of the crater).
UPLAND DEPOSITS: A GRAVEL-SIZE GRADIENT RADIAL TO THE CRATER
Perhaps the most illuminating of Schlee’s (1957) many innovative contributions to our knowledge of the
upland deposits was his experimental demonstration and mathematical analysis of the dramatic cobble-size
gradient decreasing from 32-mm modal size in Washington, D.C., down to 4 mm midway down the
Patuxent-Potomac peninsula (Fig. 2). Figure 2A, taken from Schlee (1957), shows the isoliths of modal
gravel size that he determined for Washington and the Patuxent-Potomac peninsula. In contemplating these
isoliths, Schlee (1957) observed that “The orderly changes in modal size in the northern portion of the sheet
suggest that the size change may be a mathematical function similar to those found for the size diminution in
modern rivers.” He therefore selected “four profiles parallel to the current flow direction and as far as
possible normal to the size contours,” plotted them on the semilog graph (Fig. 2B), fit them to a straight line,
and then compared the slope of this line to those reported for the modern Rhine and Mur Rivers.
Figure 2. A: Moving average modal-gravel-size isoliths for the gravel member of the upland
deposits of the District of Columbia and the Potomac-Patuxent peninsula of Southern Maryland
adapted from Schlee (1957). B: Modal gravel size plotted on a semi-log scale versus distance
for four separate paths chosen to be nearly as possible perpendicular to the isoliths of A
(replotted from Schlee, 1957) with comparisons to the cobble-size reduction rates for the
modern Rhine and Mur Rivers (Pettijohn, 1949). Arrows, circles, and commentary external to
the boxed graphs added by the author.
deposits of the District of Columbia and the Potomac-Patuxent peninsula of Southern Maryland
adapted from Schlee (1957). B: Modal gravel size plotted on a semi-log scale versus distance
for four separate paths chosen to be nearly as possible perpendicular to the isoliths of A
(replotted from Schlee, 1957) with comparisons to the cobble-size reduction rates for the
modern Rhine and Mur Rivers (Pettijohn, 1949). Arrows, circles, and commentary external to
the boxed graphs added by the author.
In Plain Sight
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