The ridge that forms the spine of the M&M Trail across Provin Mountain, East Mountain, and the Mount Tom and Holyoke ranges was formed 200 million years ago during the late Triassic and early Jurassic periods and 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 talus are visible beneath many of the ledges; they are particularly visible along the Metacomet-Monadnock Trail on Bare Mountain. The basalt ridges 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. 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. One of the best places to view this layer-cake structure is just beneath the summit of Mount Norwottuck. 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. The Horse Caves are located a short distance off the M&M Trail via the Robert Frost Trail. The sedimentary rock of the Connecticut River Valley is also well known for its fossils, especially dinosaur tracks, which have been discovered in several locations near the ridges that the M&M Trail traverses.