Problems, Middle Mississippi River Region, Pleistocene Time 203 
may be mentioned Wright,? Wheeler,’ and Todd.2 The climax 
of the work on the drift in this region during the nineteenth 
century came with the publication in 1899 of Leverett’s'® ex- 
tensive monograph on the IlIlinoian ice lobe. His work clearly 
established the location of the former border of the ice of the 
Ilinoian Age in Tlinois. Drushell,!" in 1908, and again in 1911, 
called attention to numerous exposures of drift in St. Louis, 
including an excellent exposure at Chain-of-Rocks, St. Louis. 
Radebaugh'? made a study of the drift in the northern part 
of St. Louis and St. Louis County. Many others have examined 
the drifts in the Middle Mississippi River Region, especially in 
the vicinity of St. Louis, where the evidence is rather abundant 
and the situation critical. Practically all that remained for the 
present writer was to re-examine previously described exposures 
and observe some new cuts that were not made at the time of 
the earlier workers. 
Pebble counts have been made of the drift at critical spots. 
Samples of drift were collected and studied in the laboratory. 
These were taken in the field by cleaning the surface of the 
drift from possible contamination, using, insofar as possible, 
a place where a section of the drift was exposed that would 
permit vertical sampling. Samples, weighing from ten to twenty 
pounds, were collected from the cleaned surface of the drift, 
Particular care being taken not to select pebbles simply because 
they had a conspicuous appearance. Everything was taken from 
the surface for a depth of as many inches as was necessary 
to collect a sample containing not less than 400 pebbles. 
These samples were then analyzed in the laboratory by a 
method similar to the one suggested by MacClintock.’s The 
Samples collected in the field were screened. All pebbles that 
would not pass through a half-inch screen were retained for 
analysis! In addition to this, all the pebbles retained by the 
quarter-inch screen were set aside and a sufficient number of 
these added to those retained by the half-inch screen to make 
a total number of pebbles not less than 400. Coning and quarter- 
ing was resorted to in order to select the proper number of 
