48 
IOWA ACADEMY OP SCIENCES. 
past summer. For a more detailed description, reference may- 
be made to the complete report in the next volume of the 
“Iowa Geological Survey.” 
The county is thoroughly drained, the uplands well dis- 
sected by ravines that have left no swamps. The streams have 
well established grades over loess and drift. Only at the 
heads of smaller ravines is present erosion still in progress. 
Such a drift topography is again approaching maturity. 
Above the flood plains of the streams a line of low, rounded 
knolls rises about six feet. These constitute a river terrace in 
the normal development and mark the highest limit of spring 
fl3ods. About fifty feet above the bed of Middle river the 
remains of a second terrace are found in various places along 
the stream. At various points terrace-like places appear 
along the hillsides. Some of these are undoubtedly dependent 
on the resistant character of underlying strata. As a whole 
they bear so little relation one to another and to the river bed, 
that they are judged not to be terraces dependent on former 
stages of water in the stream, but of local character dependent 
on the differential weathering of the hillsides. 
The geological formations of the county are given in the fol- 
lowing table: 
CLASSIFICATION OP FORMATIONS IN MADISON COUNTY. 
GROUP. 
SYSTEM. 
SERIES 
STAGE. 
SUBSTAGE. 
Cenozoic. 
Pleistocene. 
Recent. 
Alluvium. 
Glacial. 
Iowan. 
Loess. 
Kansan. 
Drift. 
Paleozoic. 
Carboniferous 
Upper. 
Missourian. 
Represented by 
the Winter set 
limestone. 
Des Moines. 
Alluvial deposit is to be found iu the broad river valleys. It 
generally lies on loess extending down into the river bottoms. 
