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?
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. 7]B).
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.