7s rin American Geologist. August, 1894 
Adjoining the west end of University hill, beyond an inter- 
vening depression of 30 feet below their summits, rises the 
Observatory hill, 101 feet above lake Mendota, about 1.300 
feet long and two-thirds as wide, trending from east to west, 
but with its western extremity slightly deflected southwest- 
ward. The observatory is 57 rods west of the University 
main building, and 39 rods from lake Mendota. A contour 
line 70 feet above the lake, at the level of the sag between 
the two hill tops, encircles an upper area of the Observatory 
hill 450 feet wide and 1,100 feet long, having nearl} T the ratio 
of 2:5. 
An excavation for a cess-pool at the top of this hill close 
to Prof. Comstock's house, reaching a depth of 21 feet, found 
the boulder-clay 7 feet thick, and all its lower portion was in 
sand and gravel, the coarsest layers containing pebbles up to 
six inches in diameter. Here and there a few boulders, up to 
two feet in diameter, were encountered in the stratified drift. 
Three to six rods southwest of the observatory, an excava- 
tion in the southern slope to a depth of 20 or 30 feet was 
worked many years ago to supply sand and gravel for masons' 
use and road repairing. Much gravel and many boulders of 
small and large size are embedded in the superficial till of this 
hill, as seen hy me on its surface and in excavations to the 
depth of ten feet for the foundations of the Agricultural 
( ollege green-house, near the dairy house on its western slope. 
On the very top, only about four rods west of the observatory, 
a boulder of Arcfnean gneiss, ten feet long, lies half or more 
embedded in the till. Prof. R. D. Irving stated the depth of 
the drift under the top of this hill to the bed rock to be 122 
feet.* Under its western part, at the dairy house, according 
to Prof. F. H. King, a well 48 feet above lake Mendota went 
six feet in till, and all its remaining depth, to a total of 84 
feet, in sand and gravel, not reaching the bed rock. 
Si bglacial deposition of the nucleal beds. 
In Madison, as in Winthrop and Scituate, Mass., I regard 
the nucleal sand and gravel of these drumlins as a subglacial 
deposit, brought to its present place by streams flowing from 
the melting surface of the ice-sheet during its final recession. 
The sand and gravel were gathered, as I think, from englacial 
^Geologj "I' Wisconsin, vol. ii, p. 625. 
