Mount Holyoke sits prominently at the end of a range of basaltic rock that cuts east-west across the broad, flat, Connecticut River Valley. The mountain formed some 200 million years ago when lava flowed from the valley floor, cooled and was upended. More recently, glaciers left their signature, scouring the mountain's jagged edges smooth in some places, exposing bedrock or leaving till, sand, clay or muck in others.
Vistas from the Mount Holyoke Range reveal this geological legacy. Development and farmland compete for the deep fertile soils of the Connecticut River Valley. To the east and west are the hilltowns, nearly cleared of trees for farmland in the 1800’s, now blanketed in forest.
Most sources agree that the New England landscape originated with the accretion of many smaller land masses, roughly 450 to 250 million years ago. Indeed, Massachusetts is largely an assemblage of colliding land masses of the supercontinent Pangea. Rocks found in Western and Central Massachusetts tell the story of these collisions. Gravel, mud, beach sands and microscopic marine exoskeletons were squeezed and heated. Gravel and mud are now metamorphic gneiss and schists. Sands and skeletal remains are today’s quartzite and marble.
The rocks of the present-day Connecticut River Valley tell a different story. Around 220 million years ago, Pangea began to rift apart. The valley of today was once a lowland basin, edged by towering 4,000 foot highlands. Over time, the highlands eroded. Sand, mud and pebbles washed into the basin and ultimately hardened into sedimentary rock.
As highlands wore down, lava simultaneously oozed onto the valley floor from vents deep within the earth. It flowed in enormous sheets, hundreds of feet thick in places, and hardened into a rock called basalt. As the earth’s crust shifted and moved, the basalt sheet tilted. What was originally flat is now upended, hundreds of feet higher than the valley floor. The Mt. Holyoke Range’s ridge and its southern slopes are basalt. Beneath and protected by it, lie the sedimentary rocks that originated from the eroding highlands.
Rifting created our present-day Connecticut River Valley and laid down its bedrock some 220 million years ago. The Ice Age, however, in more recent geological time left its mark on many of the Valley’s surface features. The southward-spreading glacier scooped up everything in its path, much as a snowplow does. Soils and rock were incorporated into the river of ice. Within the glacier, some rock disintegrated back into sediments – sand, silt, and clay. Others fractured into pieces – gravel, pebbles, cobbles and boulders of various sizes.Many of these features can be seen at Skinner State Park and Holyoke Range State Park: this Self-Guided Geology Walking Tour brochure will guide you to some of them.
As the climate warmed, torrents of meltwater formed enormous rivers. Filled with rocks and sediments, they tumbled down the barren hills. Their waters filled glacial Lake Hitchcock, which covered much of the valley floor. The larger boulders sank like stones where river met lake. The finer sediments drifted farther from shore, and slowly settled to the bottom. Eventually the lake drained. The Valley now has deep, rich soils, in contrast to the thinner rocky soils of the hilltowns.