Northern Manhattan IslandThe complex geology of Manhattan Island is best exposed in parklands along its northern end (Figure 16). Several major rock units comprise the stratigraphic section beneath this portion of the city: the Middle Proterozoic Fordham Gneiss, the Cambrian Manhattan Formation, and of the Cambrian and Ordovician Inwood Marble. Outcrops of these formations disclose the complex, northeastward-trending structure of the region.
Figure 17 is a simplified cross section along I-95 extending from the Palisades of New Jersey eastward across the northern tip of Manhattan Island, to sites along the Cross Bronx Expressway. The complexity of the structure is a result of a series of great folds and the great thrust faults associated with the Taconic shear zone, Cameron's Line. To the east of this boundary, the Hartland Formation was, until recently, undifferentiated from the Manhattan Formation to the west. It is equivalent in age and very similar in physical appearance to the Manhattan Schist. Today, the Hartland Formation is considered to be comprised of oceanic crust and eugeoclinal materials that were highly metamorphosed and thrust westward during the accretion of the Iapetus terrane during the Taconic Orogeny.
The differences in fracturing and weathering of each of the rock units is responsible for the lay of the land. The general shape of the landscape is related to the erosion of river valleys before and between periods of glaciation. Much of the modern landscape in Manhattan and the Bronx only has a thin veneer of soil developed after the most recent episode of glaciation. The influence of weathering on the landscape is perhaps best illustrated by the location of the Hudson River along the western margin of Manhattan and the Bronx. The Hudson River follows the unconformable boundary between the underlying crystalline basement rocks and the younger westward-dipping sedimentary and volcanic strata of the Newark Basin.
On Manhattan Island the weathering character of each of the metamorphic rock
units also influences the local topography. Driving northwestward along Harlem
River Drive, the roadway crosses the densely urbanized Harlem lowlands, underlain
by Inwood Marble. The marble is both softer and more soluble than the other
rock units, and therefore, is worn down closer to sea level. Continuing westward,
the roadway climbs an escarpment rising to Washington Heights, the highest area
on Manhattan Island. On the south side of the highway in Highlands Park, massive
exposures of schist and gneiss of the Manhattan Formation reveal the complex
faulted and contorted structure of the bedrock. The bedrock beneath Washington
Heights, the upland area on the eastern end of the George Washington Bridge,
also consists of the more erosion-resistant Manhattan Formation. A scenic location
to look at the Manhattan Formation in this area is around the The
Cloisters Museum in Fort Tryon Park.
As discussed previously, there are many spectacular exposures of bedrock along
the highways throughout the Bronx, but they are nowhere safe for casual study.
(It is illegal to stop; but as many Bronx Expressway travelers know, major traffic
jams are a frequent occurrence. During such delays there is typically plenty
of time to contemplate the highway outcrops from your car window!) Fortunately,
there are many exceptional places to study the geology in the Bronx.
The geologic map in Figure 17 is extremely simplified. The rocks in the central and eastern portion of the Bronx are tightly folded and probably broken by numerous thrust faults. However, the similar character of the schist and gneiss throughout the area make detailed mapping practically impossible. Although Cameron's Line may be well defined throughout portions of Connecticut, the location of the suture is poorly defined within the Bronx. The north-to-south trace of the Bronx River possibly defines one trace of this irregular boundary. To the east of the Bronx River, the bedrock consists of the gneiss and schist of the Hartland Formation. Follow these links to Highlands geology sites in the Bronx:
The URL is: https://gotbooks.miracosta.edu/gonp/nyc/highlands/manhattan.htm
Last modified: 3/11/2019