Talcott Mountain - Geology

Geology

Talcott Mountain is a high point on the Metacomet Ridge, a fault-block ridge that formed 200 million years ago during the Triassic and Jurassic periods. It is composed of trap rock, also known as basalt, an extrusive volcanic rock. Basalt is a dark colored rock, but the iron within it weathers to a rusty brown when exposed to the air, lending the ledges a distinct reddish appearance. Basalt frequently breaks into octagonal and pentagonal columns, creating a unique "postpile" appearance. Huge slopes made of fractured basalt scree are visible beneath many of the ledges of Talcott Mountain; they are particularly evident along the base of the lower tier of cliffs west of Heublein Tower. The basalt cliffs are the product of several massive lava flows hundreds of feet deep that welled up in faults created by the rifting apart of North America from Eurasia and Africa. These basalt floods of lava happened over a period of 20 million years. Erosion occurring between the eruptions deposited deep layers of sediment between the lava flows, which eventually lithified into sedimentary rock. The resulting "layer cake" of basalt and sedimentary sheets eventually faulted and tilted upward. Subsequent erosion wore away 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. The best way to imagine this is to picture a layer cake tilted slightly up with some of the frosting (the sedimentary layer) removed in between.