GLACIAL VARVED CLAY CONCRETIONS—BASSLER 
distribution of the glacial clay con- 
cretions of the northern United 
States, one must turn back to about 
the year 23000 B. C., as reckoned by 
the geologist. At that time the last 
of the four great ice sheets, marking 
the Nebraskan, Kansan, II]linoisan, and 
Wisconsin stages of glacial time which 
in turn had passed over northern 
North America, had reached its 
southernmost limit, roughly indicated 
today by Long Island and the Ohio 
and Missouri Rivers. The broken 
rocks, boulders, mud, and sand pushed 
up in front of the ice formed the ter- 
minal moraine, the conspicuous ridge 
today marking this southern border. 
The climate, slowly becoming milder, 
caused increased melting and a grad- 
ual withdrawal northward of the 
glacial front. The mixture of sand, 
clay, gravel, and boulders ground up 
under the ice cap, now becoming ex- 
posed, formed the surface deposit 
known as till or boulder clay. 
Water flowing from beneath the ice 
sheet removed the finer clay material 
spreading it out on the plains to the 
south, where evaporation soon dried 
it ready for distribution by the winds 
as dust. The result was the loess, a 
rock term originally applied to the 
fine, yellowish-gray loam of the Rhine 
Valley but now recognized in many 
other parts of the world. Everywhere 
the loess is characterized by its fertility 
and by the vertical exposure of the 
outcrops when cut into because roots 
of vegetation hold the deposits to- 
gether. The loess of China, the most 
extensive deposits in the world, orig- 
inated in the dust carried by the winds 
from the Gobi Desert eastward and 
deposited as soils of such high fertility 
that thousands of years of cultivation 
have failed to exhaust them. In north- 
ern Europe similar deposits have 
accumulated in which special forms of 
doll-like concretions, known in Ger- 
many as loesspiippchen, loesskindel, 
loessmannchen, etc., were developed. 
Similar concretionary bodies charac- 
terize the loess in the United States 
and China. 
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271 
Returning to North America, several 
lobes of the ice-cap front now slowly 
melted, leaving bodies of water in 
their wake which ultimately became 
the Great Lakes of today. South and 
east of this lobed area, smaller tempo- 
rary fresh and brackish bodies of 
standing water formed from time to 
time owing to the damming of the 
melting glacial waters against the ter- 
minal moraine and other obstructions. 
In these lakes and ponds another type 
of glacial deposit, laminated clays, 
formed under the following condi- 
tions, giving rise in time to the par- 
ticular kind of clay concretions de- 
scribed in this paper. In the summer, 
the time of active melting, abundant 
waters emerging from the glacier 
carried sand, silt, and mud particles, 
usually light-colored, to these lake 
bottoms where they settled in a single 
layer. With the coming of winter, 
when melting ceased, there was no 
further deposition except a thin layer 
of minute clay particles normally 
dark-colored, which had been held in 
suspension in the lake water through- 
out the previous summer by breezes 
and currents. Thus, as described by 
the Swedish geologist DeGeer, every 
year was marked by two distinct layers, 
a summer one of light-colored, coarse 
material, and a winter one of finer 
consistency and darker color. ‘These 
two together, often less than a centi- 
meter in thickness, form a_ distinct 
band, technically called a varve, from 
the Swedish word meaning “‘layer.” 
Since each varve registers a single 
year’s accumulation, the entire num- 
ber developed in all these lake deposits, 
if duplication is avoided, will approxi- 
mate the sum total of years occupied 
by the latest retreat of the ice cap. In 
other words, varves, like the growth 
rings in trees, give a record of geolog- 
ical time in actual years. Hitherto 
this figure, the time required for the 
last glacial retreat, had been judged 
by the amount of recession by water- 
falls, like the classic example of the 
Falls of St. Anthony in the Mississippi 
River and also Niagara Falls, but varve 
