1050 Surface Geology of Burlington. 
to deviate slightly from actuality, and project the individual con- 
tours distinctly. 
The drift over the region cartographically represented in Plate 
XXIII, exhibits only the “ Lower Till”—the southern boundary of 
the “ Upper Till,” or the till of the second glacial epoch, being con- 
siderably to the northwestward. Over this portion of the state the 
drift is usually more or less modified superficially, The boulders 
contained are for the most part comparatively few and of small size; 
they are seldom more than five or six feet in diameter, though a 
few miles from Burlington one is to be seen, the diametric measure- 
ment of which is more than fifteen feet. The preglacial surface 
expression of the region under consideration has manifestly not been 
completely obliterated by glaciation and the concomitant depositions, 
and the present topographic features are consequently in greater or 
less degree dependent upon the subjacent stratigraphic rocks which 
make up the greater portion of the altitude of the bluffs on either 
side of the Mississippi river at this point. The extreme attenuation 
of the till over the more elevated areas, and the deep accumulations 
of drift materials over the less elevated places, is evidenced by 
numerous exposures. In the valleys of Flint, Hawkeye and other 
smaller creeks, the depositions of the till attain a maximum thick- 
ness in some places of seventy or eighty feet. 
The city of Burlington is built upon four “hills,” all of which 
_ rise to a height of nearly two hundred feet above low water ' in the 
- Mississippi river at that place. Perhaps five-sixths of the altitude 
_ is formed of Burlington limestone and Kinderhook shales, which 
along the Mississippi river at Prospect and North hills, and also 
some parts bordering Flint creek, rise from the water’s edge in high 
mural escarpments. 
North of Hawkeye creek is a nearly insulated plateau, all sides 
of which are scalloped by steep-sided ravines, very deep toward the 
lower extremities, but interiorly becoming quickly lessened in depth, 
"1 This is the basis of all elevations given in the accompanying Map, 
and is assumed to be 510.77 feet above the sea-level. It was determined 
from a line of precise levels recently run by the Mississippi River Com- 
mission up the Mississippi river from the Gulf of Mexico, which gives 
the elevation of the U. S. P. B. M. 14, on the north end of the east abut- 
ment of the C. B. and Q. R. R. bridge over the Mississippi river at Bur- 
lington as 171.4352 meters. : 
