272 
counting furnishes accurate 
results. 
Had a single lake existed throughout 
this period the total number of its 
varves would have indicated the exact 
time figure, but instead there were 
many such lakes with some overlap- 
ping each other and lasting quite 
different lengths of time. However, 
by matching the number and thick- 
nesses of the successive bands in each 
lake basin one after another, the 
varves can be correlated from lake to 
lake and the records be pieced to- 
gether to give a fairly complete time 
measurement. Such a study of these 
laminated varved clays indicates that 
the retreat of the ice from New Eng- 
land to its present position required 
about 25,000 years or, in other words, 
this area was covered by glaciers prior 
to that time. For example, the single 
ancient Hackensack Lake, one of 
several glacial lakes developed in the 
vicinity of New York City, shows 2,500 
varves in its present-day clay expo- 
sures, thus giving a clue not only to the 
age of the lake but also the rate of 
retreat of the glacier, namely, about 
100 feet per year. Varved clays, 
however, are not restricted to North 
America. They are common in gla- 
ciated areas in such widely scattered 
regions as Europe, Cape Colony, and 
Australia, where, as in New England, 
they bear the characteristic clay- 
stones. In Scandinavia similar con- 
cretions are known under the special 
names of marlekor or fairy stones, and 
Imatra stones. 
Postglacial uplift elevated these 
varved clay lake basins and drained 
the lands, after which stream erosion 
in the course of succeeding years 
exposed the strata. Since then also 
subsurface waters percolating through 
the clay beds have been active in 
depositing their lime content, forming 
their present-day occupants—the clay- 
stone concretions. 
The explanation for this concretion 
formation, or “concreting” mentioned 
on a previous page, is based upon the 
fact that molecules of certain substances 
more 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
held in solution tend to flock together 
or segregate around some nucleus. In 
crystallization, should a tiny particle 
of calcium carbonate be precipitated 
from solution, it will act as a center of 
attraction and draw more of the same 
material to itself until a symmetrical 
crystal is formed. If, however, a for- 
eign substance such as clay be present 
which cannot be pushed aside by the 
calcium carbonate particles in their 
endeavor to crystallize, then the min- 
eral is deposited between and around 
the clay particles. The result is a 
claystone concretion modified in shape 
according to the nature of its environ- 
ment. The lime in these glacial clays 
of New England originated through 
the decomposition of old lime-bearing 
feldspathic rocks brought down by the 
glaciers from higher latitudes. This 
lime passed into solution upon contact 
with percolating surface waters 
charged with carbon dioxide derived 
from decomposed organic material. 
Then the subtle force, be it chemical 
attraction, affinity or otherwise, that 
draws the particles of inorganic sub- 
stances together, came into play and 
concretion growth was inaugurated. 
Prof. Edward Hitchcock in his 
“Geology of Massachusetts” (1841), 
after first noting that the subject was 
very imperfectly understood, published 
important remarks upon the formation 
of the Connecticut Valley concretions. 
He explained their origin as an at- 
tempt, as indicated above, at crystal- 
lization of calcite in clay in which 
lateral accretion predominates be- 
cause of the easier movement in the 
clay particles among the bedding 
planes. His studies led him to the 
supposition that concretions passed 
through a preliminary or soft stage 
before hardening, that they had a 
tendency to segregate or concrete 
toward the center from all sides 
equally, unless there was a deficiency 
of material on one side in which case 
their form varied correspondingly. 
He divided these claystones according 
to their predominating forms into 
spheres, oblate spheroid, prolate spher- 
