Geological Formation of St. John
Drifting westward along with the North, Central and South American continents are the North American, Caribbean and South American Plates, which are the corresponding underwater landmasses.
All these plates are generally drifting to the west, however the Caribbean Plate is drifting at a slower rate than the denser American Plates. These differences in motion and density are forcing the American Plates to squeeze underneath the Caribbean Plate creating friction and large amounts of energy in the form of heat. From time to time this energy is released violently in the form of volcanoes and earthquakes.
Fifty to one hundred million years ago the first layers of rock which were to become the island of St. John were laid down on the ocean floor three miles beneath the surface as these friction-generated volcanoes erupted, spewing molten lava into the sea. Under the extreme pressure of three miles of water, the lava formed large solid layers of rock. If the volcano had erupted in shallower water, the hot lava would have exploded upon encountering the cool water resulting in a violent scattering of smaller rock fragments.
Geologists call this layer of rock the Water Island flow after Water Island near Charlotte Amalie in St. Thomas which is made primarily of rock from this period. These rocks, which are common on East End and the southeast portion of St. John, are the oldest rock formations on the island.
The best place on St. John to observe these giant ancient boulders is at Ram Head.
Evidence supporting this theory was gained when geologists, using diamond tipped drills, bored into the rock at Ram Head. They drilled down over one half a mile before breaking through the last of the rock. The new substance brought up by the drill was examined and shown to be the same material that makes up the ocean floor, showing that no other rock was there before it.
The movement of the continental plates continued with the Caribbean Plate being pushed upward by the advancing American plate beneath it. The gradually cooling Water Island rock was brought closer and closer to the surface of the ocean.
The area between St. Thomas and St. John, now called Pillsbury Sound, was the center of the next period of active volcanoes. This time the volcanoes erupted at a shallower depth under water and, later, above the surface of the ocean. This was a violent and explosive era. Molten lava, volcanic ash and fragmented boulders and smaller rocks of the Water Island period were thrown up on the developing hillsides of the newly formed island. The rocks produced by these volcanoes are called the Louisenhoj (Louise-en-hoy) formation.
These rocks make up the northwestern and northern sections of the island, including Cruz Bay, Caneel, Hawksnest, Trunk, Cinnamon, Maho and Francis Bays, which were the closest to the volcano center at Pillsbury Sound.
The well known Easter Rock, found just off the North Shore road just west of Peace Hill, is a Water Island rock which was broken off and cast up on the hillside; a good example of the violence and power of the Louisenhoj era. Also evident from those times are the dark blue cobblestones that make up some of St. John's beaches. The local name of this type of very hard rock derives from the cobbles found on these beaches which has been corrupted from "blue beach" to "blue bitch", possibly because of the difficulty incurred in excavating areas were this rock must be removed.
The outer layers of St. John's mountain slopes gradually eroded from the effects of the elements depositing small, fine-grained rocks on the shallow areas adjacent to the slopes. St. John was again submerged beneath the sea. During a period of lessened volcanic activity, limestone and other carbonate materials from coral and associated sea life were slowly deposited on the older rocks. Geologists call this dark colored limestone the Outer Brass formation. In St. John this formation can be seen in the area of Mary's Creek in Leinster Bay.
Later on earthquakes caused bits of rock, sediment and other debris from the Outer Brass and Louisenhoj period to settle on top of the Outer Brass layer. All this occurred under shallow seas. This layer of material, called the Tutu formation, is also found in the vicinity of Leinster Bay.
More volcanic eruption produced another mass of liquid rock that cooled slowly and formed the large boulders that line the coast at the bottom of the cliffs and steep hills on the seaward or north shore of Mary's Point (The Narrows). Geologists call these boulders diorite. These were formed about 60 million years ago and this completed the formation of the different classes of rock which make up the core of St. John.
Subsequent earthquakes formed the major St. John valleys such as Coral Bay, Reef Bay and Fish Bay, while the continued movement of the continental plates caused the island to rise higher.
During the Ice Age much of the oceans water was frozen in the form of giant glaciers. As a result the sea level dropped considerably, perhaps as much as 200 to 300 feet. The temperature of the water around St. John became too cold to favor the growth of coral reefs at this time. Thus, unprotected by reef, the island's coast was eroded by waves. Debris from the weathering and erosion of the mountain sides and valleys joined with the water eroded coastal sections and contributed to the formation of the Puerto Rico Shelf that extends some miles around Puerto Rico and the Virgin Islands. It is probable that the islands of the Puerto Rico Shelf, including Puerto Rico, Vieques, Culebra and the northern Virgin Islands, were all connected above sea level at the extreme of the ice age when water levels were at their lowest.
When temperatures rose again, the ice began to melt. The sea level increased and the water temperature rose. This was ideal for the formation of coral reefs that began to flourish all around St. John.
This gradual rise in sea level explains the proliferation of small bays and the lack of coastal plains. Erosion of the upper hillsides would normally accumulate at the bottom and cause coastal plans to develop. The rising sea, however, kept up with this process and flooded into the valleys creating the series of bays and headlands characteristic of the island of St. John.