Beneath It All: The Geology of the Zoo
|Zoo visitors can see across Rock Creek to Mount Pleasant, which sits on the same 200-foot terrace as the Visitor Center.|
As visitors enter the National Zoological Park at Connecticut Avenue, many stop to admire the view northeast to the Mount Pleasant neighborhood. Studying the map, some may notice the hairpin path Rock Creek traces around the Zoo grounds and wonder how it came into being. Later, trudging up the steep walkways, perhaps pushing a stroller filled with baby and accessories, many grumble about Zoo’s hilly grounds. And an occasional visitor may stumble upon an unusual exhibit, which is neither zoological nor botanical, but geological.
Each of these phenomena illustrates how the geology of the region has shaped and continues to shape the Zoo and the city itself.
Washington straddles the boundary between two major geographic provinces—the Piedmont and the Coastal Plain. The rocky, hilly Piedmont stretches all the way from New York City to Montgomery, Alabama, varying in width from 25 miles in the Washington area to 125 miles in North Carolina. Geologically, the Piedmont consists of ancient rock—averaging 400 to 600 million years old—altered (metamorphosed) by millions of years of heat, pressure, folding, and faulting deep in the earth. Much of it has weathered to clay at the Earth’s surface.
To the east, the Coastal Plain consists of much younger sands, clays, and gravels, laid down from 10,000 to 100 million years ago. Over this time span, the rivers and sea alternately built up the thick layer of sediments: When the sea was high, it deposited sediments; at other times, rivers deposited weathered rocks, sands, and clays picked up on their journey through the Piedmont.
The Piedmont’s hard rock resists uniform erosion. Rivers, rain, gravity, and wind eventually eat into even the hardest areas, but cut more quickly and deeply into weak spots like folds, cracks, and other isolated areas of soft rock. This type of uneven erosion makes for a hilly landscape, which contrasts with the low, level Coastal Plain. The sediments of the Coastal Plain were originally deposited in flat layers, and the topography stays fairly smooth because the soft sediments erode easily and uniformly.
Where rivers and streams spill out of the steep Piedmont and into the low Coastal Plain, rapids and waterfalls are common. Thus, the boundary between the two provinces is called the Fall Line or Fall Zone. Many early American colonists settled and founded cities near the Fall Line because the falls marked the boundary of easy river navigation from the ocean. The falls also provided power for mills, and later, for factories. Today, the Piedmont is still one of the most densely populated areas in the country, with New York City, Trenton, Baltimore, Washington, Richmond, Raleigh, and Columbia all lying on or near the Fall Line.
The Fall Line
The Fall Line through Washington roughly runs along 16th Street, a little east of Rock Creek. To the west of 16th Street lie the metamorphic Piedmont rocks, and to the east lie Coastal Plain sediments. This explains the steep hills of Northwest Washington, which contrast with the lowlands of the southeastern part of the city. Other contrasts between the two regions become clear on a map: Rock Creek runs swiftly through a steep, narrow, twisted valley in the resistant rocks of the Piedmont, while the Anacostia River flows slowly through a flat, broad, straight channel in the easily eroded Coastal Plain sediments. And the Potomac widens noticeably at Theodore Roosevelt Island as it crosses the Fall Line into soft Coastal Plain deposits.
Most of the Zoo sits on hard Piedmont rock cut by Rock Creek. This tributary of the Potomac cradles the southern end of the Zoo in its steep-sided valley, making strenuous walking for visitors. Hiking out of Beaver Valley—which channels water into Rock Creek—can tax even the fittest of Zoo visitors on a hot summer day.
Rock Creek is also partially responsible for the impressive view northeast from the Connecticut Avenue entrance: In front of the Visitor Center, visitors can see about a mile across the valley, all the way to the Mount Pleasant neighborhood. The Zoo entrance and Mount Pleasant sit on what was originally the same 200-foot plateau; now Rock Creek bisects the plateau, providing the present view.
Washington is built on a series of such plateaus, or terraces, which are of different ages and elevations. Some date from about five million years ago; others are much more recent. They range from roughly 40 to 400 feet in elevation. Each terrace originated as a wide river bottom—gravel and sand laid down in fairly flat layers by either the Potomac, its tributaries, or their ancestors. Then, as the river cut deeper in response to falling sea levels, or the land rose higher due to local uplift, the flat river bottom was eventually left high and dry. This pattern continues to the present day. Therefore, high terraces, farther away from the present Potomac, are older than the lower terraces near the river.
Most experts recognize at least four distinct terraces in Washington; others count as many as seven. Identification is tricky because the old terraces have been cut through by rivers, eroded by wind and rain, shaken by movements in the earth, and most recently, modified by development.
The terraces extend like broad steps away from the Potomac. Theodore Roosevelt Island and E Street sit atop a 40-foot river terrace. Florida Avenue, itself on a 100-foot terrace, runs along the foot of another, about 170 feet high. Originally named Boundary Street, Florida Avenue formed the northern edge of L’Enfant’s new city, built on the flat and easily farmed Coastal Plain. Today’s drivers are treated to an excellent view of the old part of the city while driving south on 16th Street near Meridian Hill Park, just before descending from the terrace and reaching Florida Avenue.
In the northwestern part of the city, Reno Reservoir perches at 420 feet on the highest terrace in the District. Many springs, some of which contributed to pre-Civil War Washington’s water supply, flow from the walls of this terrace. Water seeps out of the walls where the overlying terrace gravels meet the Piedmont rock below: The rain water percolates down fairly quickly through the gravels, but is forced out when it hits the impermeable Piedmont rock. Though the springs no longer contribute to Washington’s water supply, the city still stores water (drawn from the Potomac) on top of the terrace, in Reno Reservoir. Because it’s the highest spot in the city, water can be transported all over Northwest Washington by gravity.
At the Zoo, the isolated outcrops of terrace gravels that cap hilltops attest to the presence of ancestral Rock Creek or the Potomac here about a million years ago.
|Rock Creek flooded the Zoo during Hurricane Agnes in 1972.|
Modern Rock Creek continues to dictate much of the Zoo’s geology. Not only is it responsible for carving the steep-walled valleys here, but it sometimes affects the park more disastrously—with floods. In 1972, Hurricane Agnes flooded the Zoo, putting the shop building, the boiler room, and the transportation building under seven to eight feet of water. In addition to extensively damaging the shop buildings and their contents, the flood destroyed half the contents in the hay and feed barn and uprooted 173 trees. The waterfowl in the lower ponds came within an inch of being able to float right over the top of their fences. Only one animal, a young wolf, came close to drowning. Then-director of the Zoo, Theodore Reed, stripped off most of his clothes and jumped in the wolf enclosure in time to save the wolf.
Geologists call the Hurricane Agnes flood an 85-year flood, because a flood of that magnitude will occur on an average of about once in 85 years. Data from work done by James O’Connor, City Geologist and Associate Professor of Earth Sciences at the University of the District of Columbia, indicate that Rock Creek’s water level would rise 20 feet above the present bank at the Zoo’s southern entrance during a 100-year flood. Even during a 25-year flood, the water level would rise about 15 feet.
|Rock Creek’s water levels once affected visitors’ access to the Zoo.|
Before the late 1960s, visitors in cars (or buggies) had to pay much closer attention to water levels than today’s visitors do. In order to cross the cement fords located at the southern and northern ends of the Zoo, drivers needed low, or at most, average waters. Otherwise, cars would be swept off the fords and into the creek. Of course, less adventurous motorists could always enter either at Connecticut Avenue or Harvard Street.
Water and the Fall Line have not been the only forces shaping the park. Tectonics, or movements within the Earth’s crust, also affect the Zoo’s geology. Within the Zoo, there are at least seven faults—areas where two blocks of earth have slid past one another. Most of the faults are hidden underground. But the most famous one, located at the Adams Mill Road entrance, is actually on exhibit! Geologist N. H. Darton had a cage put around the exposure of the fault in the 1920s to save it from vandalism and other destructive forces. Since then, it’s been called Darton’s fault. The exhibit cage has done its job as far as human interference goes, but it hasn’t kept out 60 years of natural weathering which has partially obliterated the fault. A big tree root follows the weathered, indistinct fault and helps delineate it.
An interpretive sign within the cage says that sometime within the last 70 million years forces in the Earth pushed old metamorphic Piedmont rocks over much younger Coastal Plain sediments. Geologists now think that the fault occurred much more recently—within the last several million years—and that the upper layer is probably river terrace gravels, not Coastal Plain sediments. Though Darton’s fault occurred relatively recently on the geologic timescale, no earthquakes have been recorded in the District in historic times.
Geologists discovered another fault when they analyzed drill cores taken during the 1960s construction of the Rock Creek Parkway tunnel, at the southern end of the Zoo. When they compared different cores, they found that the elevation of some rock layers didn’t match up from core to core. From this they inferred that a fault ran between the areas where they had taken core samples. In the late 1970s, construction of the Lion-Tiger Building temporarily exposed another fault. And in the early 1980s, construction for Metro’s Red Line uncovered yet another fault just north of the Connecticut Avenue entrance. Most of the faults in the Zoo’s bed rock probably occurred around the same time as Darton’s fault—within the last several million years.
Evidence of other tectonic activity can be seen in the convoluted shape of Rock Creek at the Zoo. The creek follows sets of parallel cracks in the underlying bed rock. Called joints, the cracks were produced millions of years ago by stresses deep in the Earth, but no movement has occurred along their surfaces. (If movement had occurred along the joints, they would be labeled faults.) Joints form in mathematically predictable orientations to different kinds of stresses, resulting in sets of parallel joints for each episode of stress.
Because the crumbled rock within the joints is more easily eroded than the surrounding solid rock, the stream follows these paths of least resistance. Notice that the arms of the creek’s U-shape around the Zoo are parallel. Joints affect the shape of most of the rest of Rock Creek as well—the map illustrates other sets of parallel stretches of the creek.
As early as 1933, R. S. Bassler, Head Curator of Geology at the U. S. National Museum, called the National Zoo a “geologist’s paradise.” More recently, two UDC geologists wrote, “The Zoo’s foundation is a living physical geology text book.” Indeed, the Zoo offers many examples of how different aspects of geology affect landscape. The ancestral Potomac and Rock Creek have left behind the terraces upon which the Zoo and surrounding city are built. Rock Creek, flowing through Piedmont rocks, shapes the Zoo’s grounds and poses flood threats. And tectonics have produced the Zoo’s seven faults and the joints that give Rock Creek its distinctive curve around the southern end of the Zoo.
Next time you visit the Zoo, look at the walls as well as the animals behind them. Many of the animal houses were built with locally quarried rocks. As you examine and compare the building stones used throughout the Zoo, you’ll become familiar with some of the most commonly used varieties in Washington. And you may even find the stone that was used to build your own house.
The Panda House is made of local Potomac bluestone that is more than 500 million years old. Close up you can see grains of shiny mica, rusty garnet, and milky quartz in the metamorphic (altered) rock.
Once you’re familiar with Potomac bluestone, you’ll be able to recognize it as the predominant material in the Elephant House and Mane Restaurant. Many houses in Northwest Washington are also made of this rock, including the Old Stone House in Georgetown, built in 1765.
Although the main structure of the Elephant House is made up of Potomac bluestone, the arched doorways and corners are made of beige-colored Indiana limestone. Look closely at the limestone to see millions of tiny marine fossil fragments. Geologists call the rock fossiliferous limestone because it contains primarily fossils. Indiana limestone, the most popular building stone in the District, is used in the Washington National Cathedral, the Department of the Interior Building, and all the buildings in the Federal Triangle along the north side of Constitution Avenue from the Department of Labor at 14th Street NW to the Federal Trade Commission Building.
You can see more Indiana limestone in the arched doorways of the Small Mammal House. In front of the house, a patio of imported granite encircles the base of the anteater statue. In the polished granite, you can see large crystals of milky quartz, pink feldspar, and black biotite.
As you continue down Olmsted Walk, the Reptile Discovery Center will be on your left. This brick building features twin marble columns made of marble, a metamorphosed limestone that sometimes retains the marine fossils of the original limestone. Look for coin-sized fossils of extinct cephalopods, gastropods, and other sea creatures in the columns—acid rain has etched away pure carbonate from the exposed fronts of the marble, making the fossils stand out in relief.
Next door, the Monkey House showcases another local rock, Kensington gneiss (metamorphosed granite), also about 500 million years old. Kensington gneiss was named after the town of the Kensington, Maryland, where it was first described. Two old Kensington gneiss quarries are located near the Zoo: One is off Broad Branch Road a quarter of a mile north of Brandywine Street; the other is behind the Uptown Theatre in Cleveland Park.
Kensington gneiss, often called “the salt and pepper rock,” contains grains of dark biotite and light-colored quartz and feldspar. Any corner rock of Think Tank reveals how the black, shiny grains of biotite look different on either side of the rock: On one side you should be able to see how the flat biotite grains were aligned in response to pressure and heat during metamorphism.
Although most Zoo building stones are local, the rocks of Lemur Island are from West Virginia. These hard sandstone boulders, which weigh between one and five tons each, hold up well under the constant stress of gravity, rushing water, and inquisitive lemurs. And their gray color blends well with the appearance of Kensington gneiss and Potomac bluestone.
Many other rocks in the Park—like those in the seal exhibit—are not natural rocks, though some can fool a geologist at twenty paces. To make the fake rocks, workers spray real rocks with latex to form a mold, into which they pour concrete and fiberglass. Then, to heighten the illusion, they add cracks called joints.
The Kensington gneiss used in Beaver Valley was excavated during construction of the Zoo and of the Cleveland Park Metro station; it was also used to build the waterfall up the hill in Beaver Valley.
To test your knowledge of Washington’s local building stones, visit the gateways of either the Connecticut Avenue or Adams Mill entrance; both were built in 1932. In addition to Potomac bluestone and Kensington gneiss, the gateposts contain blocks of various sandstones and granites. Slabs of Aquia sandstone from Stafford County, Virginia, also used in the construction of the White House and the Capitol, cap both posts. The gateposts were built to demonstrate the effects of weathering on different kinds of local rocks. The faces of the posts are oriented to the four points of the compass, and each type of rock is oriented in different directions within the gates. For example, a layered rock may deteriorate more quickly if its layers are vertical than if they lie horizontally.
Unfortunately, records were never maintained closely enough to provide a good study of weathering; however, the gateposts do serve to showcase local building stones, allowing easy study and comparison of the rocks. Becoming familiar with some of these stones can add a whole new dimension to a walk through the Zoo or around the city.
Adapted from an article that appeared in ZooGoer in 1988.