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Sligo's Amazing Geology

Sunday, March 16, 2008

Smithsonian geologist Janet Douglas, a long-time Takoma Park resident, led 21 adults on a walk through the 500-million-year history of Sligo Creek's geologic evolution, focusing on the area between Maple Avenue and the Flower Avenue playground.

We started at a wide, flat rock emerging from the lawn behind the bus stop at the corner of Maple and Sligo Creek Parkway. This rock's grayish color and lumpy texture says a lot about its makeup and origins that we learned along the way.

Janet explained that the bedrock now visible in the Park shows evidence of metamorphic processes that require levels of pressure and heat (at least 700° F) only possible beneath mountains 7-10 miles high above the current surface. This is one reason why geologists describe our area as having once been dominated by Himalayan-sized mountains. Erosion over hundreds of millions of years has uncovered these rocks once so deeply buried.

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.