The Black Rock Forest is located at the intersection of two major geological features: the Highlands Physiographic Province (also know as the New York-New Jersey Highlands) and the Hudson River Basin.
The Forest is situated within the central, highest portion of the Hudson River Highlands (Spy Rock, in the Forest, is the highest point in the Highlands west of the river). The Highlands are underlain by ancient Precambrian granites and gneisses more than one billion years old that have been extensively folded, faulted, and metamorphosed. These are the basement rocks of the entire Reading Prong Province, which stretches from eastern Pennsylvania for nearly 200 miles to the northeast to western Connecticut. These low but rugged mountains have been resistant to human use and exploitation compared to surrounding areas, and thus they retain a greater portion of their natural biological diversity.
The Highlands drop off dramatically for 1000 feet or more on all sides. Immediately to the south are the Newark Basin Lowlands, composed of much younger and more easily eroded sedimentary rocks. Between them and the ocean is the Atlantic Coastal Plain, including extensive surface deposits of glacial debris on Staten and Long Islands. Immediately to the north of the Highlands is the Great Valley, underlain by Paleozoic sedimentary rocks and comprising the first Valley of the Ridge and Valley Province. Further to the north are more substantial mountain ranges, the Catskills and Adirondacks.
The most dramatic drop-offs in the Highlands, however, occur where the Hudson River has cleft the range down below sea level, a little more than one mile east of the Black Rock Forest. At the northern entrance to this Hudson River Gorge, between Storm King Mountain and Breakneck Ridge, the mountains rise more than 1300 feet straight up from the river on both sides. The Hudson River itself is actually an arm of the sea with measurable salinity as far upriver as Black Rock or Newburgh Bay, with up-river and down-river tides alternating twice each day.
Black Rock Forest contains some of the oldest rock in New York State. The gneiss bedrock is a metamorphic rock from the Precambrian age; it ranges from 1.1 to 1.3 billion years old. There are many different types of gneiss at Black Rock Forest. Gneiss is composed of the minerals feldspar, quartz, amphibole, pyroxene, and mica, and some gneisses contain magnetite. The feldspar and quartz are both lighter-colored minerals and are seen as white or gray bands in the rock. Hornblende, pyroxene, and some of the other minerals are dark and can be seen as black bands in the gneiss.
A series of three major geologic events uplifted these rocks to create mountains in New York, and the mountains were subject to weathering and erosion after each mountain-building event. The first, 460-440 million years ago during the Ordovician period, was caused by a collision with an arc-shaped set of volcanic islands located east of North America (the Taconian Orogeny). It was this event that is responsible for the creation of the Highlands. The second event, during the Mississippian and Devonian periods (375 to 335 million years ago), was caused by a collision with a continent located east of North America (the Acadian Orogeny). The final event, about 250 million years ago during the Permian period (the Alleghanian Orogeny), was caused by a collision with West Africa and created the super continent Pangaea. About 200 million years ago, Pangaea began to split apart and form the continents we know today.
The Catskill and the Adirondack Mountains, both to our north, were also formed through these large events. The Hudson Highlands are considered stable bedrock because they have remained intact for such a long period of time.
The mountains were, however, subject to erosion and glaciation in more recent times. The last glaciation ended about 16,000 years ago and is responsible for much of the local topography. The ice sheet stretched from Canada down to Long Island, and scraped out valleys and mountains as it moved. Some of the rocks and ground material it left behind as it retreated now cover the bedrock over much of the Forest, and the material is known as till.
The Hudson River watershed, which includes areas from northern New York and the Adirondacks down to the Atlantic, has also played a role in the formation of Black Rock Forest. The river and its tributaries are constantly carving away at the land to create the landscape we see today.
Black Rock Forest is named for the mineral magnetite that can be found throughout the Forest. This iron ore, characterized by a low melting point, may be extracted from rocks by heating them in a furnace. The liquid iron can be poured into molds and was used to make guns, cannons, and tools of the Revolutionary War period. Much of this mineral was found south of the Forest near Bear Mountain and Harriman State Parks. Although the mineral is found in this area, it was not exploited or mined extensively as other higher-quality iron ore was found in Pennsylvania and New Jersey.
One extremely interesting attribute of magnetite is that, just as the name implies, it is magnetic. When magnetite is held near a compass, the compass needle swings away from true north towards the closer magnetic field of the rock. It is believed that a magnetite deposit may have been partly responsible for the two plane wrecks on Hill of Pines, which are within 200 yards of one another.