The earthcache is located within Salisbury Beach State Reservation. The beach is open from dawn to dusk; night caching is not allowed. The Reservation is a Massachusetts DCR facility so there is a day use entrance fee between Memorial Day weekend and Labor Day and on weekends between Labor Day and Columbus Day. Dogs aren't allowed between May and September.
This is an earthcache so there isn’t a container to find but instead you discover something about the geology of the area. To log this cache as a found, you will visit a jetty and then provide answers to a few questions.
What is a jetty?
A jetty is a long, narrow structure that protects a coastline from the currents and tides. Jetties are usually made of wood, earth, stone, or concrete. They stretch from the shore into the water. A jetty serves a multitude of purposes such as protecting boats as they enter the harbor and as a way to prevent beach erosion.
Wind-driven waves naturally move sand up and down the coast. Jetties are meant to stabilize the channel of the inlet for safe navigation and deter the natural flow of sediment into the inlet. They do this by slowing the down-coast sand movement created by littoral drift. Littoral drift is the term used for the transport of sediments, mostly sand, along the shore due to the action of the breaking waves and the longshore current. Nearshore currents, for the most part, travel from north to south. This has to do with the angle that the waves hit the beach. These nearshore currents carry sand and other sediments down the coast.
As the currents run into the jetty walls, it loses energy and drops the sand that it was transporting down the coast. On the updrift side of the jetty we get an accumulation of sand. When the current passes by the inlet, it increases in energy and is able to pick up sand to transport. This process causes erosion on the downdrift side of the jetty. One can see evidence of the effect of the jetty by observing that more sand accumulates on the upcoast side of the jetty.
Newburyport harbor jetties
The jetty you see at ground zero along the northern shore of the Merrimack River, provides protection to the navigation channel into and out of the Merrimack River and obstructs the flow of sand to the south towards Plum Island.
This area looked very different in the early 1800s before the construction of the jetties. At that time, the Plum Island Basin and all the land between it and the ocean were nonexistent. In December of 1839, three destructive storms served as catalysts for dramatic changes that would follow in the next few years. These storms cut a channel through a sizeable portion of Salisbury Beach at the mouth of the Merrimack, thereby creating an island in the middle of the river. For a brief period there were two channels into the harbor. Eventually the mouth of the south channel, which had previously been the single point of entry, filled with silt and sand. As this reef expanded northward, it formed the cove we now call the Basin. This new, slender arm of land, which continued to increase in area on Plum Island, was called New Point and retained that designation for many years.
The three maps below, based on surveys by the Army Corps of Engineers in 1827, 1851 and 1883, show the changing contours at the mouth of the Merrimack River in relation to the present-day jetties indicated by dotted lines.
The construction of the jetties at the mouth of the river commenced in 1881 and were completed in 1900. The purpose of the jetties was to increase the depth of the water at the river’s mouth and to prevent the accumulation of sand on the bar. The plans called for two stone jetties to be built, each fifty feet wide at the base and fifteen feet wide at the top. Both jetties were to be at least four feet above the surface of the water at high tide. The north jetty was to be extended nearly one mile in a southeasterly direction from Salisbury Beach, and the south jetty was to run in a northeasterly direction from Plum Island to within a thousand feet of the north jetty. The jetties are made of stone which was brought to the mouth of the river in barges, most of it from Rockport. Over the years, repairs were made to both jetties to extend the north jetty and to overcome settling of the rocks.
Earthcache questions
To get credit for this earthcache, you will need to send to me the answers to the following questions by email or Message Center.
- You are currently standing at the base of a jetty. Based on what you see with the build-up of sand and signs of erosion, which direction do you think the longshore current is running? Explain why.
- Based on what you have read and what you see here, do you think the jetty is an effective tool at stopping the deposit of sand into the mouth of the harbor? Why or why not?
- Longshore currents remove sand from the beach, but it's not the only source of erosion. Based on your observations, what is another prominent form of erosion?
- What compass direction are the waves coming from?
- Which side of the jetty do you see the highest wave action? Which side do you see the lowest? Explain why this might be.