The best location in the park for a good look at the clays which cover
the entire Park area is along the lake beach (see Figs. 4 and 8). The
bank averages 23 feet in height measured from the beach to the top of
the bank. The width of the beach varies seasonally and during storms as
the water level of Lake Champlain fluctuates (see Figs. 4 and 8). A walk
along the beach will reveal some interesting facts about the composition
of the beach and the adjacent clay banks.
One cannot help observing the scarcity of _sand_[10] on the beach. The
only adjacent material to build this beach is clay, which here contains
a small quantity of sand and some concretions containing lime. The
sickle shape of Button Bay keeps most foreign sand from the beach. The
small patches of sand present soon give way to _blue clay_[11] which, in
many places, forms the base of the beach. This blue clay can also be
seen in the base of the bank in the southern portion of the Park, where
it underlies and is therefore older than the _brown clay_.[12]
Actually very little of the blue clay is exposed in the Park. From other
locations along the shore of Lake Champlain and in the more inland areas
of the Champlain Lowlands geologists have found many clues to help our
understanding of its history.
[Illustration: Figure 9A. View of the Brown Clay at the base of the
clay-cliff, Button Bay State Park. For scale, note the small
ballpoint pen resting on top of ledge (left of center). A close-up
view of this ledge (pen in same location) is seen in Figure 10.]
[Illustration: Figure 9B. Close-up view showing general
characteristics of the Brown Clay. Notice the vertical cracks in the
clay. These cracks or joints are very common in the clays of this
State Park.]
In August 1849 the incomplete skeleton of a _whale_[13] was found in a
railroad cut being excavated for the Rutland and Burlington Railroad.
This cut was located approximately 12 miles south of Burlington and a
mile east of Lake Champlain. The significant fact about this “find” is
that these bones were found in the same type of blue clay that occurs at
the beach in Button Bay State Park. Clam and oyster shells (Pelecypods)
were found in association with the whale bones. These _pelecypod
shells_[14] and the whale bones show that these animals lived and died
in cold marine waters. Lake Champlain today is a fresh-water lake and
the fish and other organisms in it are those of fresh, not salty water.
Yet the blue clays (and we are soon to learn, the brown clays) hold
evidence proving the existence of sea water in an area where a
fresh-water lake is found today. Can this be explained?
The brown clay forms most of the Park’s clay banks (see Figs. 9A and 9B
). It can be examined easily by the visitor. The bank is composed of
clay and variable amounts of _fine sand_[15] in thin lens-shaped bodies.
The hard dry outer clay, if rain has not just fallen, of the upper 15
feet of the bank appears light brown or tan-white and commonly contains
interspersed white and blue streaks. In the section of the bank which
was the object of detailed study, two conspicuous _convoluted beds_[16]
were found, one 5 feet above the beach, the other nearly 2 feet above
the first.
Shells of _pelecypods_[17] occur at two and perhaps three levels within
the bank. They can be seen easily in many slumped clay bodies along the
beach (see Fig. 10, in which the shells are mainly _Macoma_ and
_Saxicava_). These shells once belonged to living animals whose very
close relatives and identical younger generations are found living today
in cold Arctic waters. Brown clays were deposited in a marine Champlain
Sea. How did these marine waters get into the Champlain Valley?
[Illustration: Figure 10. Close-up view showing the pelecypod (clam)
shells which occur within the Brown Clays. Note small Ballpoint pen
for scale. These shells tell us of the marine origin for these
clays. The pelecypods here are _Saxicava_ and _Macoma_.]
Remember from the previous discussion of Lake Vermont that as the
Champlain ice lobe retreated into Canada, fresh water from Lake Vermont
began to seep through the ice lobe into the St. Lawrence Valley. This
seepage caused a gradual lowering of the Lake Vermont water-level until
the lake reached a stage lower than the marine waters of the St.
Lawrence Sea. Soon the sea water began to flood Lake Vermont and true
marine conditions were reached. These marine waters reached as far south
as Whitehall, New York, which is located near the southern end of
present-day Lake Champlain.
The maximum area covered by the Champlain Sea did not equal the greatest
size attained by the fresh waters of Lake Vermont. For instance, the sea
did not extend as far east as the Green Mountains except in the very
northernmost region of Vermont. In the vicinity of Button Bay State Park
these waters extended less than a mile east of Vergennes, and the towns
of Middlebury, Hinesburg and Charlotte would have been entirely above
water (see map, Fig. 7B).
It has been postulated that marine waters extended throughout the length
of the Champlain Valley and into the Hudson Valley, forming a continuous
strip of marine water from the Gulf of St. Lawrence to the Atlantic
Ocean at the mouth of the Hudson River. If this were true, New England
would have been an island only a few years ago (geologically speaking).
The weight of evidence available today does not support the prior
existence of this connecting ribbon of sea water. Today Lake Champlain
contains fresh water again, and the marine conditions have retreated
back into the Gulf of St. Lawrence. What happened to permit a return to
fresh water conditions?
The lakes and lake-stages discussed are known from the presence of
various shoreline features, most of which are now “high and dry.” Each
lake or lake-stage has its own set of lake-level features. An
interesting fact emerges from a study of any one former lake. The
present elevation above sea level of delimiting lake-level features is
_not_ the same throughout. To explain this more fully, the present
elevation of those features formed when the Champlain Sea was at its
maximum extent will be examined.
At Shelburne Falls the present elevation of these features is 300 feet,
at St. Albans 440 feet, and at Roxton, Quebec, 552 feet. It will be
noted that proceeding from south (Shelburne Falls) to north (Roxton) a
difference of over 250 feet is found between features formed at an
identical time in the past. The water in any lake is practically level.
It therefore follows that the present 300, 440 and 552 foot elevations
of the shoreline features are the result of subsequent tilting of the
earth’s surface. Perhaps the following reference to the water level of
present Lake Champlain will help in your understanding of this tilting.
The present-day average level of the water in Lake Champlain is about 92
feet above the level of the Atlantic Ocean. This water-level elevation
is constant throughout the extent of Lake Champlain and _is not_, say,
92 feet at Button Bay and 192 feet at Burlington. Again, the only way
that features demonstrating past lake margins or levels could, for any
one lake or lake-stage, now exist at different elevations would be if
some earth movements took place after the formation of the features,
resulting in a change from their original elevations.
The earth’s crust has been tilted, higher in the north than in the
south, during recent times. This tilting provides the clue to the
formation of present-day Lake Champlain. Major tilting took place after
the maximum marine invasion. Tilting continued, at a decreasing rate,
perhaps, to the present day. This tilting, with greater relative rise in
the northwest, eventually reached a point where marine waters were
excluded from the Champlain Basin.
Fresh water slowly diluted the salty marine water. The lake gathered
more and more fresh water through rain and melting snow, and a new
outlet formed in the north, in the approximate location of the present
Richelieu River. The area of Lake Champlain slowly increased to its
present size, and the clays which once formed lake and sea bottom became
the “hard earth” of the Champlain Lowlands. Continued tilting caused
flooding of the streams, especially in the southern portion of Lake
Champlain, such as the now swampy Otter Creek and its tributary Dead
Creek (see map, Fig. 2). This concludes the story of the Park clays.
From the rocks which crop out within a short walking distance of Button
Bay State Park a much older segment of geologic history can be studied.