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Where did the rocks in Rock Creek come from?

The huge boulders and large outcroppings of granite and other bedrock within Rock Creek Park were once part of mountains formed when volcanic islands slammed into the North American coastline about 450 million years ago. Over time, the mountains eroded, leaving behind the rolling hills of the Piedmont.

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Through heat, pressure and faulting within the rock, the stone itself fractured and deformed. Today some of the exposed rock still reveals the effects of these geologic pressures in kinks and folds, veins of quartz and other minerals that filled in the spaces, reddish-brown dots of embedded garnet, and sites where blocks of rock are offset along faults.

The action of rain, gravity and flowing water, along with repeated freezing and thawing, began weathering the rock at its weakest points along cracks and folds. Some of the rock was more resistant to erosion, making it impossible for the streams to carve out broad, flat channels. Instead, the resulting creek beds twist and turn.

Plains, Rapids and Terraces

The flatter landscape beginning just east of the Park is part of the younger Coastal Plain, made up of sandstone and other sedimentary rock deposited less than 100 million years ago and prone to more uniform erosion. At the boundary where the two formations intersect, waterways spill out from the Piedmont in rapids and waterfalls.

The intersection of the Piedmont and Coastal Plain coincides with a large system of faults running down the center of the Park, called the Rock Creek Shear Zone. The wildest parts of Rock Creek—
between Boulder Bridge and Military Road—show ample evidence of the Shear Zone and the transition from Piedmont to Plain, as whitewater splashes among countless boulders severed and smoothed from the remains of ancient mountains.

Washington's geology continued to shift. Over the past few million years, periodic drops in sea levels and local uplifts caused the streams and rivers to cut deeper channels into the landscape. With each round of change, the previous flat river bottom was left high and dry. The result was a series of broad terraces across the city, rising like steps from the Potomac. One place to appreciate these plateaus is Meridian Hill Park, which is bordered north and south by terraces that today give a fine view of old DC.

(Geo-)Logical Conclusions

Geology has created opportunity. Colonial settlers took advantage of the rushing water by erecting mills along the banks of Rock Creek. Water percolating through terraces and coursing through fissures formed springs that were once an important part of the Washington water supply. Some of the rock outcrops were utilized by ancient Native Americans who shaped quartzite stones into tools and spearheads and worked soapstone into cooking pots.

Over the past two centuries, newer quarry sites provided the familiar "Rock Creek granite" (technically, Kensington Tonalite) used for structures, chimneys, walls and bridges within the Park. At least 17 quarries were in operation in the Rock Creek Park area barely a century ago—and we can visit what remains of some of them along Broad Branch, above the Melvin Hazen tributary and just upstream from Montrose Park.

Geology also helps explain some of the diversity of plant life in the Park. Since the Piedmont and Coastal Plain meet in the vicinity of Rock Creek, coastal vegetation coexists with more mountain-loving plants.

Only occasionally can we observe Rock Creek flowing with the power that once turned mill wheels. Springs and entire tributaries of the creek have disappeared or declined—some of them paved over or replaced by drainage and sewer pipes. Piney Branch, for example, was once an extensive water system in its own right. Today its natural course upstream ends at 16th Street—though its former path is traced northeast by Arkansas Avenue. One sign of progress is the "daylighting" of a 1,600-foot section of Broad Branch that had been diverted into a pipe during the 1930s. In 2014, the stream was brought back into the open air, restoring a natural part of the geology of Rock Creek Park.

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