IOWA ACADEMY OP SCIENCES. 
55 
ridge proved most interesting to me, and I shall therefore 
attempt its description somewhat in detail. 
Beneath the eighteen inches or so of black soil at the sur- 
face, covering the top and slopes of the hill, is a yellow clay 
with a liberal admixture of sand, gravel, pebbles and bculders. 
Many of the boulders show striated and polished surfaces. 
Numerous small, angular fragments of limestone are every- 
where present. In one of these was a number of specimens of 
Nucula levata, a lamellibranch which is found in the Maquoketa 
shales. There are great variations in the composition of this 
bed, but they occur in the form of irregular, curling drifts 
rather than of definite strata. This lack of any definite plan of 
structure combines with the great variety of materials found to 
give the yellow clay the heterogeneous look of a dumping 
ground. 
At an average depth of about eight feet below the surface 
the yellow clay shades almost imperceptibly into a blue, which 
is so tenacious and compact as to require the use of the pick 
instead of the shovel in digging it. It offers an effectual bar- 
rier to water, which readily penetrates the loose, sandy clay 
above. It is everywhere broken up into polyhedral, usually 
cubical, fragments, whose angles project conspicuously in the 
face of the exposed section. This tough blue clay fills a trough 
under it, and rises in a broad curve above, determining the 
form of the hill; hence, it varies much in thickness. Below 
the highest point of the hill it is fully eighteen feet thick; 
three hundred feet either side, about one-fourth as much. Its 
structure is fairly uniform throughout. Boulders are very few 
aud much decayed. Limestone fragments are found, as in the 
bed of yellow clay above, but there are also small fragments of 
wood and peat sparsely scattered through the whole bed, sev- 
eral fragments of both being found within eight feet of the 
surface of the ridge. 
Next below this lenticular bed of clay is a bed of grayish- 
blue clay which has a nearly uniform thickness of about four 
feet. This bed curves downward at the center, its lowest point 
being about under the crest of the ridge. While the face of 
the section was fresh and unaffected by exposure, no distinction 
was noticed between this and the lenticular layer of clay above, 
but after repeated visits, the last one after the clay had been 
washed by the heavy rains and repeatedly frozen by night and 
thawed by day, a dim yet definite line of demarkation was visible. 
