Mail ill the Ice Age. — Uphani. 139 
and at the end of the tunnel, consists for its upper two-thirds 
of the very fine sihceous and calcareous yellowish gray silt 
called loess, containing no rock fragments nor layers of gravel 
and sand, excepting a thin layer of fine gravel, with limestone 
and shale pebbles up to a half inch in diameter, which was noted 
by Mr. Butts near the roof of the tunnel, having a thickness 
of about four inches and an observed extent of some 30 feet. 
Soon after the skeleton was imbedded in the stony debris, or 
lay exposed on its surface, the geologic conditions that appear 
to have long prevailed were somewhat suddenly changed, and 
there ensued a more rapid deposition of the very fine water- 
laid loess, deeply enveloping the bones before they had time 
to be generally removed by decay under the influences of the 
weather and infiltrating air and Avater. From the horizon of 
the skeleton, the loess extends up to the surface, a vertical 
thickness of 20 feet, and continues in a gently rising slope 
to a slight terrace on which Air. Concannon's house stands. 
With similar irregularly eroded slopes, the loess continues up- 
ward to the general elevation of aboitt 200 feet above the river 
within a distance of a fourth of a mile to a half mile south- 
ward and westward, attaining there a general level which was 
probably the surface of the river's flood plain at the maximum 
stage of the loess deposition. This plain appears to have 
been built up by gradual deposition from the broad river floods 
during many years and centuries, and to have stretched 
then over the present valley and bottomland of the Mis- 
souri, in this vicinity two to four miles wide, from Avhich area 
it has been since removed by the river erosion. The great val- 
ley, as to its inclosing rock outcrops, is of preglacial age; it 
was not much changed by glacial erosion and deposition of the 
boulder drift ; but it was deeply filled by the loess, in which the 
valley was afterwards re-excavated. 
Professor Williston noted a di;-tinct darker layer of loess, 
mostly about two inches thick but in part merely a threadlike 
line, traceable continuously through all the 72 feet of the west 
wall of the tunnel, running about 3 to 4 feet above the lime- 
stone floor, and one foot or a little more above the base of the 
loess. Pegs driven by our party at the line of this stratum 
along all its extent were seen to be in a straight jilane, which 
by a hand level was found to have a descent of 7 or 8 inches 
