It is composed of an igneous intrusion of syenite, and became an island when canal locks were built so boats could avoid the 40-foot (12 m) falls.

[1] Efforts by the local community to turn Moss Island into a New York State Park were begun in 2008.

[3] It is bordered by the Mohawk River on the north and the New York State Barge Canal to the south.

Syenite is an igneous rock, similar to granite, composed of crystals of quartz (SiO2), alkali feldspar (a continuous mixture from KAlSi3O8 to NaAlSi3O8), and plagioclase feldspar (a mixture from CaAl2Si2O8 to NaAlSi3O8), with small quantities of muscovite (common, white or potash mica), biotitic (black mica, an iron rich version of muscovite), and hornblende (a black to dark green crystal mixture of calcium-iron-magnesium silicate, aluminum-iron-magnesium silicate and iron-magnesium silicate).

After forming as an igneous rock, the syenite was subject to massive heat and pressure, which caused the minerals to rearrange, but not melt.

Goat Crack (5.2), a 45-foot (14 m) slab, is a classic Little Falls route where many trad climbers do their first leads.

The island has had several famous climbers climb here including Lynn Hill.