212 
IOWA ACADEMY OF SCIENCES. 
the deposit measures forty feet or more. The base is not more 
than thirty-five or forty feet above low water. The ridge is 
made up almost entirely of erratics. Some of them are quite 
large; one, which was particularly noticed, had one surface 
exposed which was fully six by ten feet. There are a few lime- 
stone boulders of local origin, but they are generally small and 
form a very little part of the whole. The ridge occupies the 
front of a broad depression in the high bluff, which, with this 
exception, borders the Mississippi from Montrose to Keokuk. 
It is apparently undisturbed, excepting at the south end, where 
Lamlee creek has cut its channel through and down into the 
limestone beneath. Surely no subsequent ice sheet ever moved 
over it. This boulder ridge marks the termination of a small 
lobe of the attenuated margin of the great Iowa ice sheet, just 
before its final retreat. The position of the ice lobe was locally 
determined by the depression already mentioned. 
There is also a boulder bed in the southern part of the city 
of Keokuk. It is oh the very margin of the bluff and lies on 
the very-much-disturbed surface of the St. Louis limestone. 
From its position it is not easy to tell from which direction its 
material came — possibly from both east and west. This Keokuk 
boulder bed is not brought up as evidence in the present discus- 
sion, but simply as a fact of concurrent interest. 
The Sandusky boulder ridge and the glacial scratches are 
probably the dying records of the great ice sheet, when its 
attenuated margin would be much cut up, and the movements 
greatly influenced by the local topography. And to me the evi- 
dence at present seems clear that these last movements were 
from the west and northwest. Future developments may change 
these views. When some one has taken up the subject of errat- 
ics and made it a minute study, so as to positively identify them 
and locate their origin, then the question of ice flows in this 
locality will be a comparatively easy one. 
