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Part 2

The Geology of Button Bay State Park · Harry W. Dodge — chapter 2 of 3 · ~1,921 words · public domain

Shells of pelecypods 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?

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.

THE OLDER ROCKS

The rocks which underlie Button Bay State Park can be seen along the small creek which is located just south of the Park. It is suggested that the visitor walk southward (to the right if approaching the road from the lake front) along the main park road until the first culvert beneath the road is reached. Looking down the creekbed toward the lake, one can readily see the older rocks (see A, Figure 11). Fossils are found in these tilted rocks which tilt or dip toward the northeast and “strike” northwestward. Fossils date the rocks underlying Button Bay State Park as Middle Ordovician. The most abundant fossil is a trilobite, Triarthrus, but, even this ancient arthropod is not easily found in these limestones and limy shales. A sketch of Triarthrus (that portion found fossilized) appears in Plate 1. These rocks containing Triarthrus beckii belong to the Stony Point formation and are of late Sherman Fall or Denmarkian age.

One of the jobs of the geologist is to reconstruct the paleogeography (ancient geography) of a region. The different kinds of rocks present and their distribution patterns, together with the types of fossil plants and animals found, tell the geologist of past lands and seas, warm and cold climates. The rocks of the Stony Point formation (the rocks which underlie most of the Park) tell of warm marine waters, a past sea, bordered by relatively low land areas. The fossils contained in the Stony Point formation attest to the presence of relatively shallow marine waters. The fact that these Ordovician rocks are tilted and broken by faults proves that major earth movements took place sometime after their lithification.

1A, B, C; Maclurites magnus Lesueur (X 0.5). Lower, upper and side views. Crown Point limestone. GASTROPOD (snail). 2; Triarthrus beckii Green (X 3). Top view of central part of head region. Stony Point shale. TRILOBITE. 3; Saxicava (X 3). Pleistocene marine clays. PELECYPOD. 4; Macoma (X 3). Pleistocene marine clays. PELECYPOD. 5A, B; Rhinidictya (X 9 and X 1). Orwell limestone. BRYOZOAN. 6A, B, C: Cryptolithus tesselatus Green (X 2). Top, side and front views. Glens Falls limestone. TRILOBITE. 7; Rafinesquina. Internal view. Orwell and Glens Falls limestones. BRACHIOPOD.

(fault) CROWN POINT LIMESTONE CHAZY (VALCOUR?) (Clay covered in most areas) ORWELL LIMESTONE (clay covered) GLENS FALLS LIMESTONE (clay covered in most areas) (fault) STONY POINT FORMATION (clay covered in most areas) GEOLOGY NORTH OF PARK BOUNDARY ADAPTED FROM CHARLES W. WELBY—1961

From the north boundary of the Park and along the Lake Champlain shoreline older and older Ordovician rocks are encountered. The rocks (see geologic map, Fig. 11), primarily tilted marine limestones, display a variety of fossils, some of which are illustrated in Plate 1. Figure 12 is a view of the west side of Button Island showing an 18-inch thick reefy zone in the Orwell limestone. The reefy zone is composed largely of tumbled heads of colonial corals and stromatoporoids. A selected list of reference books and articles, some of which contain plates picturing Ordovician fossils, is found at the end of this section.

Ordovician and older rocks were lifted, folded and faulted during the Taconic Disturbance. Dramatic evidence for this period of crustal instability is seen in the Champlain Thrust, a major series of faults which can be seen east of Button Bay State Park. The evidence for such a fault system, which is more fully discussed in another pamphlet, is readily seen on Mount Philo where older Cambrian rocks have overridden (been thrust over) younger Ordovician ones.

As time passed in the Button Bay region younger and younger sediments were deposited over the top of folded and eroded Ordovician rocks. These sediments became rock and in turn slowly broke into fragments which were transported to other areas where they were deposited again as sediments. The glaciers passed over, leaving their rock debris behind as they wasted northward. Marine clays were slowly deposited from the waters of the Champlain Sea. Today only the marine clays resting on the beveled edges of Middle Ordovician rocks can be seen in the Park.

The history of the earth is open for all to read. Button Bay State Park can tell us only the history which is recorded in its clays and underlying rocks. As we have seen, there are certain giant geologic time gaps in the Park area, but many of these do not exist in other places and in other Parks. If this pamphlet has stimulated an interest in filling in these time gaps through your study of rocks in other places, then it has fulfilled its purpose. The present interest in Space demands a good long look at the Planet Earth. Good Hunting!