The surfaces of many rocks in Sligo are obscured by moss, lichens, and chemical weathering. A freshly broken rock shows its true salt-and-pepper texture. Each bit of color is a crystal grain of muscovite (white or shiny mica), biotite (dark), or feldspar (white or pinkish), the basic constituent minerals of our Sligo rocks. Janet said that these types of minerals, and the size of the grains, indicate a medium grade sandstone that cooled slowly enough from the high pressure and temperature of metamorphism for visible crystals to grow, but not slowly enough for very large crystals to form.
Geologists believe this rock material originated as muddy sediments at the bottom of the ancient Iapetus Ocean about 450 million years ago. This ocean separated the proto-North American continent from an approaching volcanic island arch that eventually collided with our continent during the Taconian Orogeny between 460 and 440 million years ago.
Embedded within this salt-and-pepper background is a profusion of rock chunks of various sizes called clasts or olistoliths. Geologists infer these fell into the muddy ocean bottom during landslides off the volcanic island chain before the muddy sediments became rock. The lighter-colored objects (mostly schist and metamorphosed quartz-sandstone) are made of quartz and other resistant felsic minerals and may thus protrude from the surface of the rock. The less common darker objects are mostly amphibolite, a metamorphosed igneous rock made of less-resistant mafic minerals (high in magnesium and iron) and often recessed into the rock face