My Favorite Places – Lake Elkhorn Outcrop Offers a Journey to the Center of the Earth

Rock outcroppings offer us a peek at the normally hidden, inner world of our planet. In the Maryland Piedmont good outcrops are few and far between. In Howard County they occur along the major rivers that border the county. Good exposures to the inner workings of the earth can also be found in the quarries at Savage (gabbro), Marriottsville (marble), and Old Ellicott City (granite). Collectively these outcrops provide us with a record of a long history of plate tectonic movements driven by major thermal convection cells deep in the mantle of the earth.

 

The black gabbro outcrops along the eastern edge of our county are remnants of oceanic crust from the proto-Atlantic ocean that formed as the African Plate moved away from the North American Plate some 600 million years ago. The marble that underlies some of the middle parts of the county originated as limestone, along with other sedimentary rocks, on the eastern shores of the North American Plate during this rather quiescent period of time. It is composed largely of the remains of oceanic life that died and settled on the ocean floor.

Geologic Map of HC

Geologic Map of Howard County, MD

The granite found in Old Ellicott City, Woodstock, and Guildford quarries was the result of a later collision when the African Plate came roaring back – at a rate of a few inches per year – and collided once again with the North American Plate to form Pangea. During the collision, the sediments that had eroded from the continents and which had been accumulating for hundreds of millions of years were squeezed together and heated up. Some of these were pushed to great depths and melted into granitic magma and others were pushed to dizzying heights into a mountain belt, maybe as large as the Himalayas. Howard County is underlain by these metamorphic and igneous rocks that reveal the roots of these old mountains – they were right here but have been eroded away and washed into the current Atlantic Ocean.

 

plate model for AppalachiansAt the east end of Lake Elkhorn, on the south side of the lake, is a large exposure of these rocks. Note that the once horizontally layered metamorphosed sediments are tilted up to the west. They were uplifted, folded, and accreted onto the North American plate as the continents collided. These gray metamorphic rocks have been intruded by a fine-grained, pink granite (made up of quartz, feldspar and mica). If you look closely you can see how both the metamorphic rocks and the pink granite have later been intruded by a coarse grained granitic pegmatite. This period of continental mountain building climaxed 350 million years ago but the story is preserved and can be read in these rocks outcropping on Lake Elkhorn.

Take-a-way: Everywhere you look there is evidence of the past and evidence of the balance of how all things work together to form the earth as we know it.

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