The hard rock life: Why "black rock" is a common namesake in Germantown

In Germantown we have a number of things named “Black Rock” -- Black Rock Road, Black Rock Mill, and Black Rock Center for the Arts, for instance.

But what exactly is black rock and why is it specific to this area?

It turns out that black rock has a connection not only to local geology, is tied to Native Americans, the first European settlers and the railroad here in Germantown.

Black rock is a metamorphic rock called schist. This means that is made from compressed sedimentary rock, in this case Seneca red sandstone. It is dark in color, but actually has many different hues variegated through the layers. These different hues come from minerals within the lagoon sediment from which it was formed. The darkness comes from the minerals chlorite and biotite. The sediments were compressed deep in the earth for millions of years until they changed form. The resulting metamorphic rock layer was suddenly uplifted to the surface by an earthquake and split across in stress fractures, making the vertical walls and jagged edges that we see today.

In this area of Maryland, between the piedmont and coastal plain, the Marburg Formation of black rock stretches diagonally from near the Potomac River above Great Falls up to Pennsylvania. It was created in the pre-Cambrian era 800 million years ago. Outcroppings of the rock can be found at several points along Seneca Creek, north of Great Falls and west of Brookville along Brookeville Road and north of Brookeville along the Patuxent River. The metamorphic rock outcroppings are much harder and higher than the surrounding land and can be easily spotted. Creeks and rivers must run around the rocks and as the water erodes the soil around the rock more and more of the rock is exposed.

Some places where the rock juts out over the waterway were used as rock shelters by the Native Americans.  There are two Indian rock shelters in Germantown – one across from Black Rock Mill and one below Watkins Mill High School. Both of these are on Seneca Creek which was a major water-road for the Indians. These rock shelters date back to the early Woodland period, but were also used by hunting parties of Piscataway in the 17th century. The Piscataway had a large village at the mouth of Rock Creek, where Georgetown is today.

Black Rock is one of the hardest rocks found in Montgomery County and as such made a fine building stone, except for the fact that it breaks into uneven layers and cannot be easily cut into true-line rectangular blocks like Seneca Red Sandstone. Because of this, it is a good stone for building foundations, chimneys and low walls, but usually not for entire buildings. A good stonemason using more modern tools could shape the Black Rock into even rectangles, so in the 19th century it can be found in the high stone walls of barns, mills and house if the more precisely cut sandstone is used at the quoins (corners) to stabilize the walls.  Black Rock Mill is one of these rare buildings and local stone was quarried across from the mill.

In Germantown black rock can be found in foundations of most of the older homes and even in some chimney bases.  Black rock was so hard and durable that it was used to construct culverts, bridges and viaducts for the Baltimore & Ohio Railroad when it was constructed through Montgomery County 1871-73. The railroad overpass at Game Preserve Road is an example of this.

Many more examples can be found around the area if you know what to look for.