Ulrich—Paleozoic Systems in Wisconsin. 
85 
The Trempealeau Formation and Its Subdivisions. 
The Trempealeau formation is well displayed and in fairly typi¬ 
cal composition in Trempealeau Bluff on Mississippi River. Other 
good sections may be seen (1) at Norwalk, (2) in and below Beans 
quarry, 2 miles northwest of Tunnel City, (3) in the bluff south¬ 
east of Mazomanie, (4) at Hillside just south of Wisconsin River 
from Spring Green, (5) in the bluffs on both sides of St. Croix 
River at Osceola, and many other places in Wisconsin, Most of 
the beds are exposed also at Lansing, in Iowa, and at Winona, 
Redwing, and Stillwater, in Minnesota. 
There is considerable variation in the character and sequence of 
the component beds of the formation from place to place. Thus 
in eastern Wisconsin the upper (Norwalk) member is commonly 
and perhaps always absent. But the yellow calcareous shaly Lodi 
member is generally present and may also be called the most char¬ 
acteristic part of the formation. The same might be said of the 
next underlying St. Lawrence limestone or dolomite member, but 
in the middle and northern parts of the State this member is en¬ 
tirely wanting in many places, or it is so much altered by addition 
of relatively coarse quartz sand that the recognition of its zone 
is rendered difficult and uncertain. 
In calling the Lodi member a shale it is to be understood that 
as defined by me the word shale applies generically to a rock of 
very fine grain, thinly laminated construction and containing some 
clay but not necessarily in preponderant quantity. All of these 
Cambrian shales consist mainly of finely divided siliceous matter. 
Subdivisions of the Trempealeau Formation. 
In its fullest development the Trempealeau formation is divisible 
into four lithologically and faunally distinct members. Locally 
one or two of these subdivisions might be mapped separately, but 
as a rule the topographic conditions are such that only the lower 
magnesian limestone member lends itself readily to such separate 
treatment. Names for at least three of these members are desir¬ 
able if only for purposes of discussion. They differ notably in 
geographic distribution, and such facts are neither easily nor 
clearly to be brought out in descriptions of local stratigraphy 
without definite names for each of the several members. 
