Fig. 12a. Over a typical Grand Isle split-rail fence at Sand Bar State Park.

Fig. 13. A portion of the beach at Sand Bar State Park looking east. The escarpment in the left distance marks the trace of the Champlain thrust fault.

THE GEOLOGY OF SAND BAR STATE PARK

INTRODUCTION

Sand Bar State Park is located in northwestern Vermont on U.S. Route 2, approximately 14 miles north of Burlington and near the east approach to Sand Bar Bridge which leads to South Hero Island in Lake Champlain (see map, Fig. 1). Tenting, picnicking and swimming are the Parks main attractions (Fig. 12a). The swimming beach is on the north side of U.S. Route 2 and fronts on Lake Champlain. Its shallowness makes the beach safe for children (Fig. 13). The tenting facilities are located on the south side of U.S. Route 2 on a south-facing shoreline.

THE GEOLOGY OF THE PARK

The geologic history of Sand Bar State Park is recent, geologically speaking, especially when compared with that of the other Parks treated in this pamphlet. The sediments of the park are blue and brown clay which were deposited throughout the Champlain Valley less than 10,000 years ago. This clay, which can be seen in many places along the bathing beach, was deposited from marine waters which flooded the Champlain Valley just prior to the formation of present-day Lake Champlain. No bedrock crops out in Sand Bar State Park.

The blue clay is covered with deposits brought downstream by the Lamoille River during very recent times and deposited as a delta[16] into Lake Champlain. This delta has shifted its distributary channels frequently and continues to grow southwestwardly into Lake Champlain. Much of the finer material (sand) brought into Lake Champlain by the Lamoille River has been shifted and concentrated by lake currents into ridges or bars; one sand bar stretches to South Hero Island and forms the foundation for the causeway named Sand Bar Bridge. Prior to the building of Sand Bar Bridge (causeway was started in 1849, opened to travel on December 5, 1850), this sand bar was fordable and was used as a link between South Hero Island and the mainland.

Most of the sand now found north of the Park bathing beach and which is responsible for the extensive “shallows” in the swimming area, was supplied by the now abandoned northern channel of the Lamoille River. It is interesting to note that most of the sand now seen on the bathing beach has been imported from nearby areas of Vermont. Since the northern distributary channel of the Lamoille River is no longer supplying sand, and sand from the active southern channel cannot work its way northward because of the Sand Bar Bridge causeway, there is a lack of sand for the beach.

The extensive swamp areas near the east end of Sand Bar Bridge are a wildlife sanctuary. The north-trending prominent escarpment east of the Park marks the trace of the Champlain thrust fault (Fig. 13). In a quarry at the east end of Sand Bar Bridge may be seen the fault contact between the younger, Middle Ordovician, Stony Point Formation, and the older, Lower Cambrian, Dunham Dolomite.

SUGGESTED READING

Erwin, R. B., 1957, The Geology of the Limestone of Isle La Motte and South Hero Island, Vermont, Vermont Geological Survey, Bull. 9.

Stone, S. W. and Dennis, J. G., 1964, The Geology of the Milton Quadrangle, Vermont, Vermont Geological Survey Bull. 26.

Additional reports on the geology of Vermont state parks distributed by the Vermont State Library, Montpelier, Vermont 05602.

The Geology of Groton State Forest, by Robert A. Christman, 1956
The Geology of Mt. Mansfield State Forest, by Robert A. Christman, 1956
The Geology of the Calvin Coolidge State Forest Park, by Harry W. Dodge, Jr., 1959
Geology of Button Bay State Park, by Harry W. Dodge, Jr., 1962
The Geology of Darling State Park, by Harry W. Dodge, Jr., 1967

FOOTNOTES

[1]A “strike” measurement is expressed as so many degrees east or west of north or south. For a diagram illustrating the dip and strike of a rock layer see Figure 3.
[2]This is one of the three major rock groups or families. The first consists of igneous rocks, including granite, syenite, and basalt, which were formed by solidification of molten rock-material. The igneous rocks are ancestors of the other two rock families; they form over 90 percent of the outer 10 miles of the Earth’s crust. The second family, consisting of sedimentary or layered rocks including shale, sandstone and limestone, is composed of pieces and grains and other materials from all the families of rocks. In addition, sedimentary rocks are formed also from lime secreted by marine plants and animals or chemically precipitated from sea water, or by the accumulations of shells. The third family, metamorphic rocks, including gneiss, schist, slate and marble, were igneous or sedimentary rocks that have been subjected to heat and pressure in the presence of mineral-forming solutions. Metamorphic rocks generally look different from the rocks from which they formed, because the original minerals of the rock have been changed and reoriented.
[3]The actual remains are usually not preserved in their original state but are represented by molds and casts. Picture an ancient sea. The sea bottom mud slowly hardens around a shell. Water then seeps through the hardened mud and dissolves the shell leaving an open space where the shell once was. This open space is a mold. If the mold is filled a copy of the original shell is formed. This is called a cast.
[4]The relative rather than the absolute age of the rocks can be determined from a study of their fossil content. These fossils are compared with collections from various places in the world where the standard geologic time scale assigns them a place (see Fig. 4). The Park rocks were deposited during the Ordovician Period. How is a standard geologic time scale put together? Several geologists first worked out the sequences of rocks according to the Law of Superposition in Great Britain and neighboring parts of Europe. When systematic collections of fossils were made from these layers and arranged according to age it was found that certain fossils occurring in rocks in distant areas were identical and occupied the same relative age position. These fossils were considered to be of the same relative age. Fossils found in the Park can be compared with these reference fossils and a relative geologic age can be assigned to them. Absolute ages can be determined in some cases by the use of rates of decay of radioactive elements and in general these ages agree with the relative ages derived through the use of fossils.
[5]The capitalized syllable is the accented syllable.
[6]An index fossil is used to date the rocks in which it occurs. A good index fossil must be abundant, widespread and easily recognized. Its vertical range is restricted to a small number of rock layers, therefore the geological span of life of a good index fossil is usually short.
[7]Chitin is a colorless horny substance similar to the material which makes up fingernails.
[8]The black color is due to an abundance of finely divided organic (plant and animal) material within the rock.
[9]A quartzite is either a metamorphic or sedimentary rock consisting of fragments of the mineral quartz (SiO₂) which are cemented together by silica (quartz). The combination of quartz fragments held together by quartz cement creates a very hard rock which oftentimes will break across the fragments rather than around them. The quartzites of the Park area are primarily of a sedimentary origin. For a description of the three major rock groups, of which the sedimentary and metamorphic groups are two, see footnote, D.A.R. State Park, page 6. A dolostone is a sedimentary rock composed of fragmental, concretionary, or precipitated dolomite (a mineral of chemical composition, CaMg(CO₃)₂) of organic or inorganic origin.
[10]The black color is due to the inclusion of finely disseminated carbonaceous material (animal and plant remains) within the rock.
[11]This splitting or cleavage was produced after the layers had hardened into rock. The cleavage planes were produced when the rocks were subjected to pressures too great to withstand. In some places these cleavage planes do not parallel the layers.
[12]According to the basic geologic law, the Law of Superposition, younger rocks (those deposited last) are always found resting on older rocks (those deposited before the younger). The only time that this is not true is when either breaks (faults) or folds in the earth’s crust place the layers in an inverted order, as in the case here cited.
[13]The fault plane of a high-angle fault forms a large angle (generally from 30 to 90 degrees) at its intersection with an imaginary horizontal plane. The plane of a thrust fault, or low-angle fault, forms a small angle (generally less than 30 degrees) at its intersection with an imaginary horizontal plane.
[14]This is the Burlington till (Stewart, 1961) and was deposited from the Burlington Ice Lobe during its period of wasting. The till is a hodge-podge mixture of clay, sand and pebbles and is usually brown in color.
[15]A kame is a mound or ridge of poorly sorted (sometimes well-sorted, that is, made up of all the same sized particles) water deposited materials. Most kames are ice-contact features; that is to say, the materials which make up the kame were deposited in contact with a glacial ice surface. The Mt. Philo kame may be the filling of an ice-free area during the final melting of the glacial ice.
[16]Delta is the name of the fourth letter of the Greek alphabet, the capital form of which is an equilateral triangle. The triangular-shaped tract of land formed by the deposit of river sediment at river mouths is named for the triangular shape of the capital Greek letter delta.

Transcriber’s Notes