Gneiss -
The Old Man of the Rocks

Gneiss is a metamorphic rock that is found in Maryland, chiefly in the Piedmont section. Our Piedmont gneiss is the oldest rock in the whole state. At one time, gneiss was igneous in nature, forming the rock granite.

This gneiss was subjected for a long time to intense heat and pressure. For millions of years, it was buried under younger rocks that later were worn away in some areas. It was so changed that now it is hard, and its minerals are arranged in light and dark bands of varying thickness. These bands are typical of gneiss. Its minerals are chiefly quartz, feldspar, hornblende, and mica.

Since the late 1700's, gneiss has been quarried in and around Baltimore. One of the most recent quarry sites is found in our county near Havre de Grace. Along the banks of the Susquehanna River, the Arundel Corporation quarries large blocks of gneiss for various uses. Gneiss is used as a building stone, but it has also been used in making paving blocks, curbing, and crushed for roads. Additionally, it is Harford County gneiss that was used to build the long jetties that we see in Ocean City and Assateague, Maryland.