Geology


Black Point Geology

The bedrock in the area of Black Point is the Narragansett Pier/Westerly Granite which is Permian in age (275 million years old). 

The granite formed in a magma chamber that was 15-20 km below the surface.  The minerals in the granite include potassium feldspar, white plagioclase, smoky quartz, garnet, muscovite, biotite and magnetite.

You can see a variety of textures (grain sizes) in the minerals.   The texture was controlled by the volatile content in the magma.  Water-rich magmas have rapid crystal growth and large crystals whereas drier magmas have fine-grain crystallization.  You can come up with a chronological sequence by looking at the textures.  In some areas of Black Point medium-grained granite is cut by a younger coarse-grained pegmatite (see Figure 1 below).

Figure 1 (Photo by Crystal Gryszkiewicz)

There cracks in the rocks (see Figure 2) that are either joints (a fracture where there is no relative displacement of the rocks on either side) or faults (where there is relative displacement along the fracture).  The joints are due to unroofing.  When the material above the magma chamber (the Rhode Island Formation) was removed, the rock expanded and cracked.

Figure 2 (Photo by Jacob Schlapfer)

You can see evidence of differential weathering at the bedrock surface (see Figure 1).  Salt wedging occurs in coastal zones where seawater seeps into cracks then evaporates.  The salt crystals grow within the cracks and apply pressure on the surrounding rock.

During the Late Wisconsin glaciation, which culminated 26,000 years ago, glaciers advanced from the north to the south and picked up, transported and deposited glacial material in Rhode Island.  Large boulders, called erratics, can be found in the Black Point area (see Figure 3).

Figure 3 (Photo by Phil Dusky)

-Information from O. Don Hermes and Beth Laliberte, URI Geosciences Department