THE MIAMI SERIES OP 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 erjosive action of the readvancing 



