INTRODUCTION. 



Soil is to be defined as the superficial wealliered layer of the solin 

 earth-crust, the last including not only the solid rocks but also unconsol- 

 idated formations such as loess, dune-sands, and glacial deposits. 

 Soils are variously defined, according- to whether the particular writer 

 is viewing them as the hal)it.it of plants — the botanical viewpoint, or as 

 simply the superficial geological formation — the geological viewpoint. 

 Accordingly the depth of the superficial layer included under the gen- 

 eral designation "soil" varies from writer to writer, the geologist often 

 taking for this a higher value than the botanist. The latter will con- 

 sider it to extend at least as deep as plant roots penetrate, and the form- 

 er as deep as the composition of the parent rock may have been affected 

 by percolating water, aided more or less by organisms or their 

 products. 



As the common farm crops in southern .Minnesota derive not 

 only their mineral constituents but also their supply of water chiefly 

 from the first three feet and almost exclusively from within the fir.st 

 five feet, we scarcely need to consider the portion below the fifth foot. 

 As most of the crops draw more or less upon the third foot for these, 

 and hence the whole of the first three foot section is to be considered 

 as well within the root zone so that a study of the soil should include 

 sections to at least this depth. 



The great majority of Minnesota soils are glacial in origin and. 

 for the most part, developed upon the vinassorted glacial till left behind 

 by five separate and distinct ice sheets. Under these circumstances 

 one of the first questions occurring to a soil investigator in this state is 

 as to what relation, if any, exists between the character of the soils 

 upon the various glaciations and the age of the latter. The object 

 of the investigation reported in this thesis was to answer the question in 

 part. 



Soil Formation. 



The surface three feet or so of a freshly exposed lake-bed, 

 recently formed sand-dune, till-plain or terminal moraine just exposed 

 by a retreating glacier, is not to be considered as soil but as the parent 

 rock upon which soils may be developed by the combined agencies of 

 living organisms and the products resulting on the death of these, and 

 of percolating or descending waters, these carrying carbonic acid, solu- 

 ble salts and organic compounds. 



The form of alteration in the parent rock that will be induced by 

 the soil- forming agencies will de])end upon the vegetative covering, the 

 temperature, the precipitation, the rate of evaporation, and the level 



