Most of the metamorphosed sedimentary rocks of the area are named the Mather Gorge Formation, after the rocks exposed there. They lack fossils but are interpreted to be about 600 millions years old. These rocks were originally deposited as sand, silt, and mud in an ancient sea, called the Iapetus Ocean, which existed before the Atlantic Ocean. Large blocks and fragments eroded from an older igneous rock (map unit CZu) were mixed with the sediments. Igneous rocks–—hot, molten rocks deep below the Earth's surface–— were injected up into the sedimentary rocks at various times in the geologic past. These igneous rocks, and their times of emplacement, include amphibolite (about 540 million years ago), granodiorite and pegmatite (about 470 million years ago), and lamprophyre (about 360 million years ago). 

The sedimentary rocks were changed by heat and pressure (metamorphosed) and deformed during several collisions of the Earth's continental plates that resulted in the formation of the Appalachian Mountains. Metamorphism changed the sedimentary and igneous rocks into schist and gneiss (map unit CZms), and migmatite (map unit CZmm). Hot solutions of silica were injected as veins of white quartz into the rocks, and crystals of muscovite (white mica), biotite (black mica), garnet, staurolite, and kyanite grew as the rocks cooled. Some of the vein quartz nearby in Maryland was extensively prospected and mined for gold from 1861 to 1951; note the abandoned prospect sites on the map. The metamorphosed rocks are tightly folded, they are well layered with mica-rich foliation planes, and locally they contain close-spaced fractures.

Metagraywacke and schist partially melted to form migmatite

Vein quartz intrudes all rocks of the Mather Gorge Formation

Folded beds of metagraywacke and schist

Amphibole (dark) and plagioclase (light) crystals in amphibolite 

Pegmatite body (white) intruded migmatite

Pink feldspar crystal in pegmatite

Bear Island Granodiorite (light) intruded schist and was later folded

Close-up of granodiorite shows biotite (dark) and muscovite (light) crystals

Folded metagraywacke and schist

Migmatitic schist

Garnet schist with vein quartz

Amphibolite (dark), granodiorite and pegmatite (light), and migmatitic metagraywacke
and schist (photo by Dave Usher, USGS)

Gold mines and gold specimen at Great Falls, Md. (Reed and Reed, 1969)

Lamprophyre dikes intruded along fractures in metagraywacke about 360 million years ago (photo by Dave Usher, USGS)

Metagraywacke and schist was recumbently folded (F1), folded upright (F2), and then intruded by pegmatite and granodiorite (Ob) (photo by Dave Usher, USGS)

Diamictite of Sykesville Formation contains fragments of the Mather Gorge Formation

Migmatite, or partially melted metagraywacke and schist