Geology and Environment
The ridge of the Holyoke Range was formed 200 million years ago during the late Triassic and early Jurassic periods and is composed of traprock, 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 in the Holyoke Range is on 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 Holyoke Range hosts a combination of microclimates unusual in New England. Dry, hot upper ridges support oak savannas, often dominated by chestnut oak and a variety of understory grasses and ferns. Eastern red cedar, a dry-loving species, clings to the barren edges of cliffs. Backslope plant communities tend to be more similar to the adjacent Berkshire plateau containing species common to the northern hardwood and oak-hickory forest forest types. Eastern hemlock crowds narrow ravines, blocking sunlight and creating damp, cooler growing conditions with associated cooler climate plant species. Talus slopes are especially rich in nutrients and support a number of calcium-loving plants uncommon in Massachusetts. Because the traprock ridges generate such varied terrain, they are the home of several plant and animal species that are state-listed or globally rare, such as the Eastern Diamondback Rattlesnake.
The Holyoke Range is also an important seasonal raptor migration path.
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