Aquinnah Cliffs Up-Thrust EarthCache

This earthcache will bring you to a perfect spot to view the largest exposures of beds of Cretaceous clay in the Gay Head Cliffs. From here you can see the whole series so far as it is known, beds of lignite, upon which lie the beds of clay and white kaolin sand. The colors of the beds here are strikingly brilliant. Beds of Cretaceous clay may be seen in a section over a mile long. Upon the Upper Cretaceous clay lie the Miocene greensand and some Pliocene sand, which is in turn overlain by Pleistocene deposits. The thickness of the Cretaceous beds exposed here is exceedingly difficult to estimate on account of complicated folding and overthrusting, but it probably nowhere exceeds 100 feet. If you have the time we highly recommend making your way down to the beach below to get an even closer look at this beautiful sight.

The Aquinnah Cliffs were produced when a glacier moving to the northeast thrust various sedimentary layers upward into a landmass. As this up-thrust occurred at the end of the Wisconsin glaciation, the glacier soon melted away, leaving behind both the island of Martha's Vineyard and the impressive cliffs. The appeal of the cliffs is largely due to their colorful nature. What people don't realize is that the dazzling mix of colors is tied to the fact that various clays, sands, and gravels make up their sedimentary layers.

The result of this glacial up-thrust was the exposure of millions of years of geological history, which are still visible today in clear strata. According to dating and the fossil record, the geological layers of the Aquinnah Cliffs originate in the Creatceous period (approximately 100-65 million years ago), the Tertiary or Paleogene/Neogene period (approximately 65-3 million years ago), and the Pleistocene period (approximately 3 million years ago to 11,700 years ago). These eras are primarily characterized by the formation of sedimentary layers, which account for some of the most distinctive features of the cliffs—including the white, kaolintic layers, black, lignite layers, greenish, glauconitic sand layers, and red clay beds. These sedimentary layers formed over time as a result of accumulation of rock and mineral fragments and pressure of subsequent geological layers, but were seemingly porous and susceptible to glacial movement, easily displaced and pushed along by the glacier. Below sea level, the cliffs are supported by Aquinnah conglomerate, which formed during the Lower Pleistocene Era. Because the glacier pushed these geological layers forward and upward, the layers are buckled, bent, and faulted (as shown in the diagram below). Therefore, though we can so clearly see the sedimentary layers, we can also see where these layers have been jumbled by “glaciotectonism.” The result is a disrupted and disjointed, but still somewhat linear geological record and clear evidence of the impact of glacial movement on the Earth’s crust.

In the shadow of the lighthouse, the huge faces of crumbling white sand are thought to be the point bars that formed by the accretion of sand along the inner edge of great bends in the ancient river system. Where these sands give way to clays – and the palette switches to darker, more brilliant iron and organic-rich reds – the ancient river spilled over its banks and settled out onto swampy floodplains patrolled by ferocious theropods. The candy-colored splendor of the clay speaks to the intense climate of the late Cretaceous. Today, red soils are only found near the tropics, but ninety million years ago carbon dioxide was much higher and it was 5 degrees Celsius warmer.

The water surrounding the cliffs sometimes takes on a red, white or grayish tinge. This is the result from clay eroding from the cliffs above. These famous cliffs are rapidly eroding because of the sea level continuing to rise here.

Cliff or coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. Also abrasion commonly happens in areas where there are strong winds, loose sand, and soft rocks. The blowing of millions of sharp sand grains creates a sandblasting effect. The definition of abrasion is grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles.

1. Are you able to distinguish the different layers of rock from your view of the cliffs here. What is the average width of each layer?

2. What colors stand out to you at the time of your visit?

3. Do you see any signs of erosion along the shore line? What color is the water surrounding the cliffs? Is there any type of color tinge in the water due to erosion? If so, what color?

4. To prove that you were here, please post a picture of yourself or your GPS with the cliffs in the background. Any other photos of the area are also greatly appreciated.