Metacomet Ridge - Geology

Geology

The Metacomet Ridge is the result of continental rifting processes that took place 200 million years ago during the Triassic and Jurassic periods. The basalt (also called traprock) crest of the Metacomet Ridge is the product of a series of massive lava flows hundreds of feet thick that welled up in faults created by the rifting apart of the North American continent from Eurasia and Africa. Essentially, the area now occupied by the Metacomet Ridge is a prehistoric rift valley which was once a branch of (or a parallel of) the major rift to the east that become the Atlantic Ocean.

Basalt is a dark colored extrusive volcanic rock. The weathering of iron bearing minerals within it results in a rusty brown color when exposed to air and water, lending it a distinct reddish or purple–red hue. Basalt frequently breaks into octagonal and pentagonal columns, creating a unique "postpile" appearance. Extensive slopes made of fractured basalt talus are visible at the base of many of the cliffs along the Metacomet Ridge.

The basalt floods of lava that now form much of the Metacomet Ridge took place over a span of 20 million years. Erosion and deposition occurring between the eruptions deposited layers of sediment between the lava flows which eventually lithified into sedimentary rock layers within the basalt. The resulting "layer cake" of basalt and sedimentary rock eventually faulted and tilted upward (see fault-block). Subsequent erosion wore away many of the weaker sedimentary layers at a faster rate than the basalt layers, leaving the abruptly tilted edges of the basalt sheets exposed, creating the distinct linear ridge and dramatic cliff faces visible today. It is possible to envision this by imagining a layer cake tilted slightly up with some of the frosting (the sedimentary layer) removed in between. Evidence of this layer-cake structure is visible on Mount Norwottuck of the Holyoke Range in Massachusetts. The summit of Norwottuck is made of basalt; directly beneath the summit are the Horse Caves, a deep overhang where the weaker sedimentary layer has worn away at a more rapid rate than the basalt layer above it. Mount Sugarloaf, Pocumtuck Ridge, and Mount Toby, also in Massachusetts, together present a larger "layer cake" example. The bottom layer is composed of arkose sandstone, visible on Mount Sugarloaf. The middle layer is composed of volcanic traprock, most visible on the Pocumtuck Ridge. The top layer is composed of a sedimentary conglomerate known as Mount Toby Conglomerate. Faulting and earthquakes during the period of continental rifting tilted the layers diagonally; subsequent erosion and glacial activity exposed the tilted layers of sandstone, basalt, and conglomerate visible today as three distinct mountain masses. Although Mount Toby and Mount Sugarloaf are not composed of traprock, they are part of the Metacomet Ridge by virtue of their origin via the same rifting and uplift processes.

Of all the summits that make up the Metacomet Ridge, West Rock, in New Haven, Connecticut, bears special mention because it was not formed by the volcanic flooding that created most of the traprock ridges. Rather, it is the remains of an enormous volcanic dike through which the basalt lava floods found access to the surface.

While the traprock cliffs remain the most obvious evidence of the prehistoric geologic processes of the Metacomet Ridge, the sedimentary rock of the ridge and surrounding terrain has produced equally significant evidence of prehistoric life in the form of Triassic and Jurassic fossils, in particular, dinosaur tracks. At one site in Rocky Hill, Connecticut, more than 2,000 well preserved early Jurassic prints have been excavated. Other sites in Holyoke and Greenfield have likewise produced significant finds.