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Stop #4 on Goodwin and Johnson’s (1970) field trip was a road cut exposing ~9 m of upland gravels
“which overlie a saprolite of the Petersburg granite in a nonconformity.” I reproduce their sketch of this
exposure as Figure 9A. Most striking are the facts that the granite is first contacted by a “thin cobble zone
varying from [~8 cm to 30 cm]” overlain by “a pronounced [1.3 m] thick clay zone,” and this bilayer
conforms perfectly to the concave-upward curvature of the eastward-sloping bedrock contact!
Immediately above this bilayer, “...discoidal cobbles are oriented parallel to the contact but higher in the
gravel sequence the cobbles have their maximum dimensions nearly horizontal...” (Note how carefully this
revealing feature is drawn in Fig. 9A.) And, just as for the upland deposits of Southern Maryland, most of
the cobbles and pebbles in this ~8-m-thick stratum were reported to comprise “massive quartz, vein quartz,
quartzite, or quartz sandstone...” and are “matrix supported” by “a medium-to-coarse sand matrix.”
“which overlie a saprolite of the Petersburg granite in a nonconformity.” I reproduce their sketch of this
exposure as Figure 9A. Most striking are the facts that the granite is first contacted by a “thin cobble zone
varying from [~8 cm to 30 cm]” overlain by “a pronounced [1.3 m] thick clay zone,” and this bilayer
conforms perfectly to the concave-upward curvature of the eastward-sloping bedrock contact!
Immediately above this bilayer, “...discoidal cobbles are oriented parallel to the contact but higher in the
gravel sequence the cobbles have their maximum dimensions nearly horizontal...” (Note how carefully this
revealing feature is drawn in Fig. 9A.) And, just as for the upland deposits of Southern Maryland, most of
the cobbles and pebbles in this ~8-m-thick stratum were reported to comprise “massive quartz, vein quartz,
quartzite, or quartz sandstone...” and are “matrix supported” by “a medium-to-coarse sand matrix.”
In Figure 9B I provide a profile of part of the Midlothian gravels based on a segment of the field trip of
Goodwin and Johnson (1970) that followed U.S. Route 60 West through the tiny town of Midlothian (with
all distances measured from the crater center, rather from a car odometer). This profile (hollow squares)
can be compared with the southernmost radial section of Figure 7C, repeated here as the small black
squares. On a westerly stretch of Route 60, the road is perfectly parallel to a crater radius; elsewhere it is
slightly sub-radial. Where the two profiles overlap, Route 60 is ~8 km north of the linear, though slightly
sub-radial section represented by the black squares.
We see that the Midlothian gravels rest at much higher elevations than any of the upland deposits designated
in Figure 7. But they are nowhere close to being the champion. According to Schlee (1957) that honor
belongs to Tysons Corners, Va., at 158 m! I find it impossible to believe that rivers could have laid down
such heavy gravel loads at any one of these elevations, much less all of them, without leaving the slightest
trail of boulders leading back to their source.
My interpretations: The Midlothian “Carolina Bays,” with the major axes of their ovate rims nearly radial
to the Chesapeake Bay crater are surely results of secondary impacts. But one may well wonder how this
could be possible if the upland deposits themselves are assumed to comprise primary ejecta. Well, I argue
that the gravel member of the upland deposits comprises interference-zone ejecta and, as such, these
materials would have been ejected early and fast and thus might be expected to have been the first ejecta to
re-impact. However, whereas the (coarse) gravel member comprises the base of the upland deposits of
Southern Maryland and Northern Virginia, the Midlothian road-cut site of Figure 9A reveals that the coarse
gravels conformably overlie a ~1.5-m-thick gravel-and-clay bilayer. Clearly, if the gravels are
impactoclastic, then this basal bilayer must also be crater ejecta – ejecta that preceded the interference-zone
materials to this site. With total confidence that rivers couldn’t have emplaced any of these deposits, I
therefore conclude that this basal gravel-clay bilayer must be a jetting-phase deposit.
Now returning to what would have created those “Carolina Bays,” it should be noted that the very last, and
slowest moving, materials to be ejected from any impact crater are the dregs of the excavation-flow ejecta
(Melosh, 1989) – which should include a large melt component. For a crater the size of the Chesapeake Bay
structure, these last-out ejecta would have been launched ~90 seconds later than the last interference-zone
ejecta (Melosh, 1989, eq. 5.5.2) – allowing plenty of time for the pre-arriving jetting and interference-zone
ejecta to have settled.
Still, Goodwin and Johnson (1970) report nothing that could be interpreted as the remains of the large ejecta
fragments that would have been required to excavate secondary craters of the order of 1 km in diameter ...
or do they?
Kastner et al. (1984) were the first to make the case that many impact glasses subjected to meteoric or
marine weathering are likely to be converted to smectite. Therefore, I envision a coherent blob of largely-
molten excavation-flow ejecta arriving at one of those Midlothian sites with just the right kinetic energy to
excavate the upper sand member of the until-then-flat upland deposit all the way down to its lower, more
resistant, gravel member. Whereupon, I see this blob flattening itself against the top of the gravel and
rapidly quenching to solid glass. Then, over the eons this glassy lens would have been digenetically altered
by rain water inevitably trapped in the “Carolina Bay” of its own creation. So, if my vision has any
correspondence to reality, the remains of the original blob now rest inconspicuously as the “massive,
brownish-gray, silty clay [smectite?] ...in direct contact with the underlying gravels” (Goodwin and
Johnson, 1970).
ANOTHER CANDIDATE JETTING-PHASE DEPOSIT: A NORTHERN-VIRGINIA DIAMICTON
YIELDING REMARKABLE CLASTS
Geologic setting. Springfield, Va., is located just south of the Washington Capital Beltway at its southerly
junction with Interstate 95. It rests on a remarkably flat clay terrace, which remains within ~61-64 m above
present sea level all of the way to our former home, about 10 km east-southeastward and 2 km shy of the
Potomac River. According to a 1960s-vintage U.S. Geological Survey map, these upland flats used to be
everywhere dotted with gravel pits. If my recollections are correct, the cobble-size distributions I’ve seen at
some of these sites were commonly similar to that of Figure 3B, though frequently including a minor boulder
component.
Near our home was the spring-plus-shopping-center-runoff-fed headwaters of the Paul Spring Branch of
Little Hunting Creek, which empties into the Potomac River at Mount Vernon. Before turning south, Paul
Spring Branch flows southeasterly through a 33-m-deep valley in a 3-m-wide quartzite-cobble-lined channel
with a gradient of ~1 m/km. Our neighborhood of Hollin Hills was innovatively developed on what was
widely considered to have been undevelopable terrain, thanks in part to the developer’s decision to preserve
the existing natural drainage gullies as parklands. Brickelmaier Park slopes downward at ~66 m/km
northward to reach Paul Spring Branch. One result of this terrific gradient has been the incision of a gorge
~3 m deep part way downslope. But this occasionally vigorous rivulet levels out near a point (38°45'37.2"
N, 77° 3'56.2" W) where it has managed to cut only ~30 cm into a resistant layer of whitish-gray “clay”
supporting a random assemblage of mostly quartzose pebbles and cobbles (Fig. 10A). Hence forward, I will
refer to this stratum as “the Hollin Hills diamicton.” Microscopic examinations and X-ray diffraction
eventually revealed that what I had initially thought to be a “clay” matrix actually comprises nearly pure
quartz silt. Griscom et al. (2003a) show a photomicrograph of a quartz grain collected from this stratum
which displays at least 3 intersecting sets of planar deformation features.
During my aperiodic visits to this site, I began to notice that the rivulet was in the process of exposing
several very unusual clasts. Figures 10B, C, D, and E display a subset of my finds, down-selected to those
with stories I think I understand well enough to relate in useful detail.
Goodwin and Johnson (1970) that followed U.S. Route 60 West through the tiny town of Midlothian (with
all distances measured from the crater center, rather from a car odometer). This profile (hollow squares)
can be compared with the southernmost radial section of Figure 7C, repeated here as the small black
squares. On a westerly stretch of Route 60, the road is perfectly parallel to a crater radius; elsewhere it is
slightly sub-radial. Where the two profiles overlap, Route 60 is ~8 km north of the linear, though slightly
sub-radial section represented by the black squares.
We see that the Midlothian gravels rest at much higher elevations than any of the upland deposits designated
in Figure 7. But they are nowhere close to being the champion. According to Schlee (1957) that honor
belongs to Tysons Corners, Va., at 158 m! I find it impossible to believe that rivers could have laid down
such heavy gravel loads at any one of these elevations, much less all of them, without leaving the slightest
trail of boulders leading back to their source.
My interpretations: The Midlothian “Carolina Bays,” with the major axes of their ovate rims nearly radial
to the Chesapeake Bay crater are surely results of secondary impacts. But one may well wonder how this
could be possible if the upland deposits themselves are assumed to comprise primary ejecta. Well, I argue
that the gravel member of the upland deposits comprises interference-zone ejecta and, as such, these
materials would have been ejected early and fast and thus might be expected to have been the first ejecta to
re-impact. However, whereas the (coarse) gravel member comprises the base of the upland deposits of
Southern Maryland and Northern Virginia, the Midlothian road-cut site of Figure 9A reveals that the coarse
gravels conformably overlie a ~1.5-m-thick gravel-and-clay bilayer. Clearly, if the gravels are
impactoclastic, then this basal bilayer must also be crater ejecta – ejecta that preceded the interference-zone
materials to this site. With total confidence that rivers couldn’t have emplaced any of these deposits, I
therefore conclude that this basal gravel-clay bilayer must be a jetting-phase deposit.
Now returning to what would have created those “Carolina Bays,” it should be noted that the very last, and
slowest moving, materials to be ejected from any impact crater are the dregs of the excavation-flow ejecta
(Melosh, 1989) – which should include a large melt component. For a crater the size of the Chesapeake Bay
structure, these last-out ejecta would have been launched ~90 seconds later than the last interference-zone
ejecta (Melosh, 1989, eq. 5.5.2) – allowing plenty of time for the pre-arriving jetting and interference-zone
ejecta to have settled.
Still, Goodwin and Johnson (1970) report nothing that could be interpreted as the remains of the large ejecta
fragments that would have been required to excavate secondary craters of the order of 1 km in diameter ...
or do they?
Kastner et al. (1984) were the first to make the case that many impact glasses subjected to meteoric or
marine weathering are likely to be converted to smectite. Therefore, I envision a coherent blob of largely-
molten excavation-flow ejecta arriving at one of those Midlothian sites with just the right kinetic energy to
excavate the upper sand member of the until-then-flat upland deposit all the way down to its lower, more
resistant, gravel member. Whereupon, I see this blob flattening itself against the top of the gravel and
rapidly quenching to solid glass. Then, over the eons this glassy lens would have been digenetically altered
by rain water inevitably trapped in the “Carolina Bay” of its own creation. So, if my vision has any
correspondence to reality, the remains of the original blob now rest inconspicuously as the “massive,
brownish-gray, silty clay [smectite?] ...in direct contact with the underlying gravels” (Goodwin and
Johnson, 1970).
ANOTHER CANDIDATE JETTING-PHASE DEPOSIT: A NORTHERN-VIRGINIA DIAMICTON
YIELDING REMARKABLE CLASTS
Geologic setting. Springfield, Va., is located just south of the Washington Capital Beltway at its southerly
junction with Interstate 95. It rests on a remarkably flat clay terrace, which remains within ~61-64 m above
present sea level all of the way to our former home, about 10 km east-southeastward and 2 km shy of the
Potomac River. According to a 1960s-vintage U.S. Geological Survey map, these upland flats used to be
everywhere dotted with gravel pits. If my recollections are correct, the cobble-size distributions I’ve seen at
some of these sites were commonly similar to that of Figure 3B, though frequently including a minor boulder
component.
Near our home was the spring-plus-shopping-center-runoff-fed headwaters of the Paul Spring Branch of
Little Hunting Creek, which empties into the Potomac River at Mount Vernon. Before turning south, Paul
Spring Branch flows southeasterly through a 33-m-deep valley in a 3-m-wide quartzite-cobble-lined channel
with a gradient of ~1 m/km. Our neighborhood of Hollin Hills was innovatively developed on what was
widely considered to have been undevelopable terrain, thanks in part to the developer’s decision to preserve
the existing natural drainage gullies as parklands. Brickelmaier Park slopes downward at ~66 m/km
northward to reach Paul Spring Branch. One result of this terrific gradient has been the incision of a gorge
~3 m deep part way downslope. But this occasionally vigorous rivulet levels out near a point (38°45'37.2"
N, 77° 3'56.2" W) where it has managed to cut only ~30 cm into a resistant layer of whitish-gray “clay”
supporting a random assemblage of mostly quartzose pebbles and cobbles (Fig. 10A). Hence forward, I will
refer to this stratum as “the Hollin Hills diamicton.” Microscopic examinations and X-ray diffraction
eventually revealed that what I had initially thought to be a “clay” matrix actually comprises nearly pure
quartz silt. Griscom et al. (2003a) show a photomicrograph of a quartz grain collected from this stratum
which displays at least 3 intersecting sets of planar deformation features.
During my aperiodic visits to this site, I began to notice that the rivulet was in the process of exposing
several very unusual clasts. Figures 10B, C, D, and E display a subset of my finds, down-selected to those
with stories I think I understand well enough to relate in useful detail.
Figure 9. A: Sketch of
an exposed outcrop of
Midlothian upland gravels
(Goodwin and Johnson,
1970). B: A profile of
the Midlothian gravels
along U.S. Route 60
west of Richmond, Va.
(hollow squares), with
comparison to a straight-
line profile roughly 8 km
southward (black
squares). All distances
are relative to the center
of the Chesapeake Bay
impact structure. The
dotted areas represent
the author’s general
understanding of the
approximate lateral and
vertical extents of the
upland deposits along
these profiles, based on
maps (Frye, 1986) and
written accounts
(Goodwin and Johnson,
1970).
an exposed outcrop of
Midlothian upland gravels
(Goodwin and Johnson,
1970). B: A profile of
the Midlothian gravels
along U.S. Route 60
west of Richmond, Va.
(hollow squares), with
comparison to a straight-
line profile roughly 8 km
southward (black
squares). All distances
are relative to the center
of the Chesapeake Bay
impact structure. The
dotted areas represent
the author’s general
understanding of the
approximate lateral and
vertical extents of the
upland deposits along
these profiles, based on
maps (Frye, 1986) and
written accounts
(Goodwin and Johnson,
1970).
Figure 10. A: The Hollin Hills diamicton: an unsorted whitish-gray siliciclastic stratum ~30 cm
high above the stream bed and an unknown, but probably not great, depth below it. B: An
autobrecciated chalk ball recovered from the stream bed at site A. C: A massive “cinder”
composed of iron oxide, possible glass, quartz silt, and 5-10 mm quartz clasts, recovered near
site A. D: Rock fragment (tentatively greenstone) with a multi-layer external coating also
recovered near site A. E: A multiply-fractured quartzite cobble found still nested in this stratum.
high above the stream bed and an unknown, but probably not great, depth below it. B: An
autobrecciated chalk ball recovered from the stream bed at site A. C: A massive “cinder”
composed of iron oxide, possible glass, quartz silt, and 5-10 mm quartz clasts, recovered near
site A. D: Rock fragment (tentatively greenstone) with a multi-layer external coating also
recovered near site A. E: A multiply-fractured quartzite cobble found still nested in this stratum.
In Plain Sight
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