The Geology of D.A.R. State Park, Mt. Philo State Forest Park, Sand Bar State Park

by Harry W. Dodge

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.