Blue Hills

The Blue Hills Reservation contains over 125 miles of trails and terrain to explore. The hills are also home to The Great Blue Hill Observation Tower, known locally as Eliot Tower, and the oldest weather observatory in North America (1884).

1-mile hike: Start at Blue Hills Trailside Museum (3097-3099, Canton, MA 02021). Hike for 0.5 miles to the top. Enjoy views from Elliot Tower. Visit the Weather Observatory (it offers tours, marker #30 on map above).

3-mile hike: Start at Blue Hills Reservation Headquarters (695 Hillside St, Milton, MA 02186) (Near #34 on map). Hike along the skyline trail for 1.5 miles to Elliot Tower.

15-mile hike (full skyline trail): Start on Willard Street in Quincy (42.234087, -71.029228). Follow skyline trail (from #44 to #30 on map).

The Blue Hills create an East-West ridge, rising from 160 feet to just over 600 feet at its summit. 440 million years ago (during the supercontinent Gondwana), however, this area looked nothing like it does today: a large, conic volcano dominated the surrounding landscape, much like those today in the Pacific Northwest (1). This volcano, like most others, formed when one plate subducted under another. The subducted rock, exposed to intense heat, melted and then rose to the surface (5).

Once the volcano erupted, it spewed ash and magma onto the surrounding areas. This erupted magma (now called lava), once exposed to the air, cooled rapidly. This rapid cooling process gave the minerals little time to crystalize (4). As a result, the rock making up the Great Blue Hill -- called Blue Hills Granite Porphyry -- has only small, fine-grained minerals (3).

In the areas surrounding the Blue Hills, the bedrock consists of granite. During volcanic activity, some magma erupts from the volcano and cools quickly, like that forming the rhyolite of the Blue Hills. However, some magma also remains below the volcano in the magma chamber. This magma cooled much slower than the extrusive magma, and, as such, crystals can form in the rock. The slow-cooling counterpart of rhyolite is granite, which means much of the bedrock around the Great Blue Hill is Quincy Granite (2). One such area where this granite is found is in the Quincy Quarries. Visit our "Quincy Quarries" subpage to learn more.

On our way up the trail, we found many outcroppings of rock. This rock was a type of rhyolite called Blue Hills Granite Porphyry. We know it was rhyolite because we could not see large crystals of minerals; rather, we saw very small grains of quartz, mica, and feldspar. Often the grains were too small to differentiate.

In many of the outcroppings, we observed joints or veins in the rock. These were caused by pressure changes, stress, and movement.

At the top of one hill along the skyline trail, we observed two large boulders that seemed out of place. Darker in color than much of the surrounding rhyolite, we predicted that a glacier may have transported them. Boulders moved great distances from their original locations are known as glacial erratics (3).