page_title: Holyoke Range
section_title: Geology and environment
hierarchical_section_title: Holyoke Range / Geology and environment
caption_attribution_description: English: from USGS Bedrock Geology of Massachusetts, 1983. Showing the bedrock of the Holyoke and Mount Tom Ranges; labels added. For Holyoke Range
context_page_description: The Holyoke Range or Mount Holyoke Range is a traprock mountain range located in the Connecticut River Valley of Massachusetts. It is a sub-range of the narrow, linear Metacomet Ridge that extends from Long Island Sound near New Haven, Connecticut north through the Connecticut River Valley of Massachusetts to the Vermont border. A popular hiking destination, the range is known for its anomalous east–west orientation, high ledges and its scenic character. It is also notable for its unique microclimate ecosystems and rare plant communities, as well as significant historic sites, such as the Mount Holyoke Summit House and the Horse Caves.
context_section_description: 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 timber rattlesnake.
  The Holyoke Range is also an important seasonal raptor migration path.
caption: Bedrock geology. Purple = basalt; surrounding brown & blue-grey = sedimentary rock
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