Sleeping Giant, a fault-block ridge that formed 200 million years ago during the Triassic and Jurassic periods, is composed of traprock, also known as basalt, an extrusive volcanic rock. Minor earthquakes have also been measured by seismographs and reported by residents. 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 Sleeping Giant. 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 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. One way to imagine this is to picture a layer cake tilted slightly up with some of the frosting (the sedimentary layer) removed in between.[5]
