THE MIAMI SERIES OF SOILS. 13 
dently water washed as not to be included within the soils of the 
Miami series. 
Another form of glacial deposits of less extent consists of the 
long, low, rounded hills, frequently elliptical in shape and made up 
chiefly of unstratified glacial material, which are known as drum- 
lins. These hills usually occur in groups. The longer axes of the 
different ridges are as a rule approximately parallel, and the result- 
ing topography is fluted and ridged with greater or less regularity. 
It is thought that these glacial forms were produced beneath the 
ice and that the direction of the longer axes marks in a general way 
the direction of ice flow. These hills are mainly covered by the 
unstratified glacial till, so that they are occupied generally by the 
same soils as the intervening till plains and the associated morainal 
ridges. 
Along the southern and western margin of the region occupied 
by the soils of the Miami series and to some distance within its 
outer border the surface of the moraines and till plains alike is 
covered by a thin layer of distinctly silty, rather homogeneous and 
stone-free material. It is probable that this material originally was 
carried beyond the ice border in the form of fine sediment washed 
out by the water from the melting ice. It is also thought that it 
owes its present position over the uplands to the long-continued 
action of the wind, which, sweeping over silt-covered plains, car- 
ried large quantities of this fine earth over the upland, depositing 
it as a surface covering of varying depths over the moraines and till 
plains. Where this silty mantle, known as loess, attains a thickness 
of more than 3 feet, it gives rise to distinct soil types not included 
within the Miami series. In many cases it forms only a thin surface 
covering, as in large tracts in Indiana, Ohio, and Wisconsin, and in 
these localities the glacial till forms the deeper subsoil, the resulting 
soil type being classed as the Miami silt loam. 
It is a common characteristic of all the materials which were min- 
gled to form the moraines, the till plains, the drumlins, kames, and 
other forms of glacial drift, and which give rise to the soils of the 
Miami series, that the earthy mass and the included gravel and stone 
were derived from various sources along the path of the glacial inva- 
sion. It is probable that at the time of the latest stage of the Wiscon- 
sin glacial advance there were exposed extensive areas of the older 
drift sheets which mantled a large part of the region now covered by 
these later deposits. Numerous well borings and many exposures of 
the older drift in deeply cut stream channels show that it still exists 
and that the newer drift rests upon its surface throughout a great 
part of the general region of the Miami soils. Since it presented a 
soft, unconsolidated surface to the erosive action of the readvancing 
