Correspondence. 259 
Feet. 
Boulder clay (sub-Aftonian), a blue black clay non weath- 
ered at top and coming into sharp contact with the 
ferruginated gravels, containing mainly small pebbles, 
predominantly of vein quartz but with a fair propor- 
tion of granite. Many if not most of the pebbles fresh 
and hard 40 
Ked and blue shales of Missourian stage 20 
The peculiar physical character of the lower bowlder clay is striking. 
It is dense, and breaks usually in flakes rather than joint blocks. It 
is of a strikingly dark color. There are few joint cracks and these' 
show no special signs of weathering. The sharpness of the contact 
between the gravels and the bowlder clay with the presence of many 
hard pebbles indicates apparently one of two things (i) either this 
lower clay was not exposed to surface action before the gravels were 
laid down or (2) it was so vigorously eroded immediately before the 
deposition of the gravels as to cut away all evidence of former sur- 
face exposure. The balance of probabilities between the two will be 
discussed later. 
Thayer Section. The Thayer section is of interest since it seems 
that here the evidence of two drifts was first detected. The section 
as now shown varies a little from point to point in the pit, but a repre- 
sentative exposure shows the following beds: 
Feet. Inches. 
9. Black soil 6 
8. Reddish gravelly clay (ferret to) 1 
7. Yellow bowlder clay becoming gravelly below 
and containing Quartzyte, greenstone and 
granite ; flattened and striated pebbles with 
lime concretions 10-20 
6. Fine sand 1 6 
5. Drab to blue pebbly clay with sticks and bits 
of undetermined wood 4 
4. Fine sand 3 
8. Drab pebbly clay as above 12 
2. Fine sand 2 
1. Gravel as seen befor(% stratified and cross-bedd- 
ed ; pebbles mainly less than 1 V4 inch in 
diameter but with some large bowlders. 
Material of the usual Kansan facies, much 
weathered and highly colored 15-20 
Summarizing the above we have the loess and yellow and blue clay 
phases of the Kansan with the underlying gravels. The blue clay phase 
of the Kansan is unusual in the presence of interstratified beds of fine 
sand and the abundance of woody material. It is dark and might read- 
ily be taken for a buried soil, but it is believed that this is not the true 
interpretation. The exposure does not now show the beds as seen by 
Messrs. McGee and Chamberlin. The same horizon as exposed some 
feet eastward shows merely the blue black pebble clay as mentioned 
above. The presence of so much wood in the bowlder clay is difficult 
to explain unless it be regarded as basal, and the beds as now exposed 
have no thoroughly satisfactory explanation on either hypothesis. Re- 
garding the clay, however, merely as the bhie clay phase of the Kansan 
