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.