Hanging Hills - Geology

Geology

The fault-block Hanging Hills were formed 200 million years ago during the Triassic and Jurassic periods and are 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 the Hanging Hills; they are particularly visible along the base of East Peak where it plunges into Merimere Reservoir. 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 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. Subsequent scour by moving glacial ice plucked away the basalt from the steep southern end of the crest of the broken ridge, creating overhanging cliffs. In the Meriden region, numerous northeast-trending normal faults offset the volcanic flows and intervening sedimentary rocks. Several of these faults break the Metacomet Ridge north of Meriden. Stream erosion and glacial ice carved canyons along these faults, dividing the ridge into the finger-like promontories of the Hanging Hills. Merimere Reservoir was built in the fault-controlled valley between East Peak and South Mountain.