The Grover Gravel in Eastern Missouri and Its Implications for Tertiary
Tectonic (Non)Events in the Midcontinent Region.
Charles Rovey (’80)
Dept. of Geography, Geology, Planning
Missouri State University
Springfield, Missouri 65897
The Grover Gravel in St. Louis County, Missouri and adjacent Illinois is an
enduring enigma. The Grover is one of the “Lafayette-type” gravels that cap
isolated upland remnants throughout the Mississippi Basin, but its age is poorly
constrained. Most workers have considered the Grover to be a pre-glacial stream
deposit, based on its presence south of the (current) pre-Illinoian till boundary,
supermature composition and topographic position. Nevertheless, the Grover
contains large boulders of purple (Baraboo) quartzite along with granite, banded
iron formation and Lake Superior Agate. Thus, early workers were perplexed by
the presence of obvious glacial erratics within a “preglacial” deposit.
Classically, deposition of the “Lafayette-type” gravels was attributed to
widespread uplift during the late Tertiary, which rejuvenated streams meandering
across an old-age, low-relief landscape. Rapid entrenchment isolated the initial
gravel deposits in upland positions, leading to a topographic reversal as the
present dissected landscape developed; this interpretation seems to have survived
the near demise of peneplain theory.
The recent advent of Cosmogenic-Nuclide Burial Dating has enhanced our
understanding of early glaciations and forced a reinterpretation of the Grover’s
origins. The oldest continental-scale glaciation culminated at ~2.4 Ma, leaving a
very distinctive till (Atlanta Fm. in Missouri) with erratics and stable-mineral
ratios that are essentially identical to those within the Grover. Recent excavations
in western St. Louis County have exposed the Grover directly above this till.
Thus, the oldest (2.4 Ma) continental glaciation extended beyond the currently
recognized limits and was the main source of the Grover Gravel. Elsewhere, the
Grover rests directly atop (and within) ice-wedge casts, meaning that deposition
began while permafrost was present. Moreover, careful examination shows that
most of the Grover is not actually a fluvial deposit. Instead it is a diamicton with
clasts supported by a sandy clay-rich matrix. Thus, the Grover generally
originated as mass-flow deposits within a periglacial environment during a later
less-extensive glaciation and does not necessarily indicate late Tertiary tectonic
uplift.