Cloistered in Washington Heights

We were spending Thanksgiving week in NYC, visiting our daughter, and went to see a unique collection of European artifacts from the Middle Ages (~1100 to 1500 CE) in an annex operated by the Metropolitan Museum of Art. They call the castle built in 1935 (this was common during that era) The Cloister. To our surprise and delight, this modern fortress was constructed on a ridge of metamorphic rock call Washington Heights, standing out high above the Hudson River.

Figure 1. The Cloisters “castle” built on top of Washington Heights in Manhattan, NY.

Thick sequences of eugeosyncline sediments were deposited as the Iapetus Ocean was closing about 600 Ma (million years ago), weathered from rapidly eroding volcanoes (e.g. Japan today). Between 541 and 459 Ma, these sediments were buried deeply enough to be heated and semi-melted as the island arc collided with the mainland during the Taconic Orogeny. Figure 2 shows a schematic of the result of this process, which created the Manhattan Formation, the topic of today’s post.

Figure 2. Schematic of subsurface geology alongInterstate 95, from New Jersey to Long Island.

It was a cold November morning when we hiked over the Manhattan Formation (red layer outlined by the rectangle in Fig. 2. The castle is right under the arrow for Washington Heights. When metamorphosed at several miles depth (a guess) under extreme pressure and heat, these immature sediments formed schist containing large garnet phenocrysts.

Figure 3. Outcrop of Manhattan Formation on trail to the Cloisters (Fig 1).

The overall dry gray schist has irregular layers of quartz showing rotation of inclusions (Fig. 4).

Figure 4. Close-up from left side of Fig. 3, showing swirling structure around inclusion (light gray over at upper-center of image). The image is about three inches across.

Hexagonal phenocrysts occur in widely separated areas (Fig. 5).

Figure 5. Image of segment from Fig. 3 revealing many hexagonal phenocrysts of garnet.

The garnet crystals (Figs. 4 and 5) were very large and well formed. The brownish surface indicates they are rich in iron, which is literally rusting (oxidizing).

Figure 6. Detail of garnet (brown hexagon in upper-center of photo), probable albite feldspar (white mineral surrounding the crystal), quartz (gray blebs throughout), biotite mica (black flecks), and orthoclase feldspar (orange-pink flecks and streaks). The central crystal is the size of a quarter.
Figure 7. Photo taken from Washington Heights, showing Hudson River, George Washington Bridge, and a bluff of presumably the same rock on the other side, in New Jersey.

The Manhattan Formation is a hard rock even though it has a preferred plane of inherent weakness as to all schists. For example the plane of weakness in this outcrop (Fig. 3) is approximately vertical, being aligned with original bedding as suggested in Fig. 2 (red layer).

As we recall from previous posts, the closing of the Iapetus Ocean occurred in pulses that began with the Taconic Orogeny and the deformation of the Manhattan Formation. A series of island arcs were thrust onto photo-North America until photo-Europe collided in the Alleghanian Orogeny about 250 Ma.

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