Geology of the New York City Region

5. New York Botanical Gardens and the Bronx Zoo

A trip to the New York Botanical Gardens and the Bronx Zoo can fill a weekend. In addition to the plants and animal exhibits there are numerous notable geologic features worthy of examination. Both parks charge admission and parking fees, however the botanical garden sometimes offers free admission on certain weekdays, and only charges a small fee to access the grounds. Special exhibits within the park charge an additional fee at each site. The zoo costs more, and can be particularly crowded on the weekends during the warmer months. The botanical gardens has limited parking that can become full especially during the spring flowering and during fall color seasons. Both parks are also accessible via the subway and Metro North.

Just east of the main entrance to the New York Botanical Gardens there are several large rocky exposures of Fordham Gneiss. Outside the entrance to the "Native Plant and Rock Garden" area is an unusual large gneiss boulder, an erratic, which has been broken in half (Figure 22). The boulder was probably split by the growth of a tree within a small crack. As the tree grew, its roots eventually wedged the boulder into pieces, but the tree has long-since vanished. A walk along Azalea Way affords views of barren rock hills of Fordham Gneiss among the parkland's forest. Paths follow along the Bronx River and between botanical theme exhibits throughout the park.

Split glacial erratic in New York Botanical Gardens
Figure 22. A glacial erratic of Fordham Gneiss in the New York Botanical Gardens. The large boulder was probably split by the physical weathering action of tree roots. The tree has long since vanished.


The Bronx Zoo Park

Like the other parks in the city, Bronx Zoo Park landscape has been modified to take advantage of the glaciated terrain. Most of the erratics have been moved or utilized in the construction of the open-air animal staging areas. The grizzly bear site has a particularly large and scenic glacially polished and grooved outcrop strewn with glacial erratics; with the addition of the bears makes it an interesting photo stop (Figure 23). Just outside of the World of Darkness exhibit building is a large glacial erratic named the Rocking Stone. It consists of a granitic gneiss and schist similar to the glacially polished rock it is sitting on.

Manhattan Schist in bear pen in The Bronx Zoo Park
Figure 23. Glaciated outcrops and erratics of schist and gneiss of the Manhattan Formation are integrated into the grizzly bear pen and other exhibits throughout the Bronx Zoo Park.

Near the Pelham Parkway entrance station to the Bronx Zoo Park on the west side there are several large outcrops of granitic schist with numerous small quartz veins. One large outcrop is highlighted with a plaque stating "Geology Tour Stop 1." With the help of Zoo employees we were able to dig up one of the original brochures from the early 1970s. The brochure called the outcrop "Manhattan Schist." In appearance it is, however, the problem is that the Inwood Marble, which occurs between the Manhattan Formation and the Fordham gneiss, appears to be missing. Not far away on the east side of the Bronx River (in Bronx River Park on Morris Park Avenue) are outcrops of brown-weathering gneiss of the Hartland Formation. The variation of the gneisses in the botanical park, the schist in the zoo, the missing marble, and the gneiss in the park on the east side of the Bronx River suggests a complex structure in the vicinity of the Bronx River. The north-to-south trend of the Bronx River valley presumably follows the trace of a thrust fault associated with Cameron's Line. The zone of weakness in this area in which the thrust fault moved westward may be within the softer Inwood Marble.

It is important to remember that all the rocks in the Bronx were at one time many miles below the earth surface, and that the rock masses we observe today are only remnants of great mountain ranges that formed in the region during Late Ordovician time, and continued to evolve during the Silurian and Devonian periods that followed. Most radiogenic dates derived from rocks in the New York City region record a date of Late Devonian time. This corresponds to the last significant stage of metamorphic thermal heating that the rocks in the New York City region experienced during the following Devonian Acadian Orogeny. Although Cameron's Line is mapped in the eastern portion of the Bronx, the great thrust faults associated with this continental suture probably moved oceanic crustal rocks far westward into New Jersey. Portions of the thrust sheet associated with Cameron's line may be present along the western edge of Manhattan and westward under portions of the Newark Basin region. Through the ages, the land has worn down as erosion stripped away these rocks. The resulting sediments were incorporated into sedimentary rock formations throughout the Appalachian Basin and into sediments beneath shallow seaways that existed during much of Paleozoic time across the Midcontinent and beyond. (The term, Appalachian Basin, refers to all of the sedimentary and igneous deposits that accumulated intermittently during the Paleozoic and Early Mesozoic along the eastern margin of North America. These materials now represent the extensive rock formations that have been faulted, folded and metamorphosed, and subsequently exposed by erosion throughout the extent of the Appalachian Mountains.)

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Last modified: 3/11/2019