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striking, in fact, that the tributaries to the Downstream Continuation of the
Mississippi River immediately north and
south of the confluence with the
Wisconsin River are locally referred to by
the etymologically distinct term “coulee.”
While these characteristics could be
attributed to incision through the bedrock
escarpment formed by resistant Ordovician
dolostone in the area, they are consistent
with a stream that has experienced recent
and pronounced down-cutting driven by
base-level adjustment following stream
piracy. Within the context of recognizing
a major reversal on the nearby lower
Wisconsin River valley, it should not be
surprising that the Mississippi River val-
ley contains geomorphic features that
reflect such a significant reorganization of
drainage patterns.
DISCUSSION
The Ancestral Wyalusing River and the
Continental Drainage Divide
Recognition of an eastward-flowing
river occupying the modern lower
Wisconsin River valley necessitates con-
sideration of the larger drainage pattern
required to achieve this configuration. We
propose that a river that we herein refer to
as the “Wyalusing River” (named for the
town of Wyalusing, Wisconsin, USA,
immediately south of the confluence of
the Wisconsin and Mississippi Rivers; Fig.
1C) developed through the late Cenozoic
flowing eastward to incise the valley now
occupied by the lower Wisconsin River.
The high, east-west–trending ridge to the
south of the lower Wisconsin River valley,
known locally as Military Ridge (Fig. 1C),
is formed by the resistant dolostone of the
Ordovician Galena and Platteville
Formations; the topographic ridge formed
by this bedrock structure represents a log-
ical location for a major drainage divide
separating southward flow to the Gulf of
Mexico from northeastward flow toward
the Gulf of St. Lawrence. In this configu-
ration, the numerous barbed tributaries
along the modern lower Wisconsin River
are explained; the curve of the valley wall
at the modern confluence of the
Mississippi and Wisconsin Rivers is sim-
ply the inside of a bend in the Wyalusing
River; and the width of the valley along
this reach broadens in the downstream
direction as would typically be expected.
8 GSA Today | July 2018
Wyalusing River
Late Quaternary glacial deposits
obscure direct evidence for the course of
this river east of the Baraboo Hills (Fig.
1C). However, depth-to-bedrock maps
(Trotta and Cotter, 1973) and previous
studies (Stewart, 1976) show a deep, bur-
ied bedrock valley that trends southwest-
to-northeast in the east-central portion of
the state. To evaluate this buried valley as North American drainage systems
a potential downstream continuation of
the Wyalusing River system, Bates and
Carson (2013) assessed 115,176 logs of
water wells in east-central Wisconsin. As
needed, logs were geo-located in ArcGIS
to accurately identify ground surface ele-
vation and sorted to remove logs that
lacked relevant depth-to-bedrock informa-
tion. After this processing, a total of
60,186 logs were used to generate a buried
bedrock elevation map for east-central
Wisconsin extending from the eastern-
most extent of the Bridgeport strath in the
lower Wisconsin River valley to the shores
of Green Bay. The resulting bedrock
topography map identifies the presence of
a buried bedrock valley trending to the
more than 300 km northeast toward the
Lake Michigan/Huron lowlands at the
appropriate elevation and grade to be the
continuation of the Wyalusing River (GSA
Data Repository Fig. 2 [see footnote 1]).
Having been traced into the Lake
Michigan basin, we conclude that the
Wyalusing River was the westernmost
tributary of a major river system that
drained the North American mid-conti-
nent through the St. Lawrence lowland to
the Atlantic Ocean.
As such, this represents a significant
drainage area that evolved through the late
Cenozoic as part of the St. Lawrence
drainage basin that has been pirated and
converted to the headwaters of the
Mississippi drainage basin. Reversal of the
Wyalusing River and, as a result, redirec-
tion of the mainstem Mississippi River
upstream of the modern confluence with
the Wisconsin River, added 205,000 km2
to the modern Mississippi River basin.
This is 6.9% of its total watershed area.
This event likely occurred sometime dur-
ing the early to middle Quaternary as con-
strained by the reversed paleomagnetism
of fine-grained sediments within sand and
gravel that were deposited while the river
still drained to the Gulf of St. Lawrence
(i.e., the deposits identified by Knox and
Attig [1988] that are associated with east-
ward-dipping foreset beds). As interpreted
by the data presented here, the conversion
of the basin from the St. Lawrence to the
Mississippi drainage involves shifting the
continental drainage divide northward
across Wisconsin and Minnesota. As an
independent verification, our field-based
interpretation of this drainage reorganiza-
tion is consistent with the evolution of
through the Cenozoic as inferred by the
volume and geometry of sediment pack-
ages deposited in the Gulf of Mexico
(Galloway et al., 2011).
Reorganization of North American
Mid-Continent Drainages
Having traced the ancestral Wyalusing
River into the Lake Michigan basin, and
thus into the St. Lawrence drainage, it is
possible to consider the larger drainage
patterns that are implicated by such a con-
figuration of this river. The evolution of
the ancestral Wyalusing River from head-
waters of the St. Lawrence drainage sys-
tem to its modern configuration as head-
waters of the Mississippi drainage system
is likely intimately associated with
Quaternary glaciations. While this is a
new observation in the upper Mississippi
basin, it is not unique in the greater
Mississippi basin. It has long been recog-
nized that the ancestral Pittsburgh and
Teays Rivers were rerouted to become the
upper and middle Ohio River when
Quaternary ice centered in the Hudson
Bay region advanced far enough south to
block the lower portions of the St.
Lawrence valley (e.g., Chamberlin and
Leverett, 1894; Tight, 1903). This caused
large proglacial lakes to form: glacial
Lake Monongahela in the ancestral
Pittsburgh River valley (White, 1896;
Leverett, 1934) and glacial Lake Tight in
the ancestral Teays River valley (Janssen,
1953; Goldthwait, 1983). While there is a
lack of consensus as to whether the ances-
tral Teays system drained to the St.
Lawrence or into the now-buried
Mahomet River system in Illinois (flow-
ing toward the Gulf of Mexico) prior to
Quaternary glaciations, it is certainly via-
ble that the Teays River developed as a
tributary of the St. Lawrence drainage and
was pirated multiple times (e.g., Coffey,
1958; Gray, 1991). Spill-over of those
lakes at the lowest drainage divide