AGE OF TERRACE SOUTH OP DES MOINES 
231 
THE AGE OF THE TERRACE SOUTH OF DES MOINES, 
IOWA. 
JOHN L. TILTON. 
INTRODUCTION. 
Along the Raccoon river in Polk county, along the Des Moines 
river, and along the tributaries of the Des Moinesi from Warren 
county there is a terrace, previously noted by geological work- 
ers. An important question with the writer has been this : Do 
the gravels in this terrace form one continuous deposit, or do 
they form two separate deposits, an upper of Wisconsin or post- 
Wisconsin age, and an underlying portion that is older than the 
upper deposit, and possibly of Aftonian age? This question 
was suggested by a division in the sand recorded in well rec- 
ords, and by the presence of fossils of large mammals that had 
been found at a depth of perhaps twenty-five feet below the 
surface of the sand. The identification of these fossils in the 
spring of 1914 by Professor Oliver^sP. Hay of the United Statea 
National Museum, and the publication of topographic maps of 
regions along the Des Moines river, make a review of local evi- 
dence seem desirable. 
THE TERRACE. 
The terrace, or second bottom, as it is locally called, is very 
noticeable along the valleys named in southern Polk and north- 
eastern Warren counties, where it forms a low plain four or 
five feet above the flood plain of the rivers. It . is less notice- 
able further southwest up the valleys of North, Middle and South 
rivers. Valley Junction southwest of Des Moines is built upon 
it, the low plain southwest of the fair grounds in Des Moines 
is a continuation of it, and the railroad stations at Avon and 
Levey are built upon it. The topographic maps of this portion 
of the state: the Des Moines, Milo, Knoxville and Pella sheets, 
give as good evidence of the terrace as it seems possible to repre- 
sent with a contour interval of twenty feet; and farmers all 
through the area recognize the terrace by the location of houses 
and buildings upon it, and not upon the lower ground of the 
first bottom, or fiood plain. Along the Mississippi river a simi 
