Welcome to the Mt. Tom Range, part of the scenic Holyoke Range in western Massachusetts. This EarthCache highlights a fascinating geologic contact zone where two very different rock formations meet: the dark, fine-grained Hampden Basalt and the lighter, sedimentary layers of the East Berlin Formation. This boundary represents a dramatic shift in ancient environments, where volcanic lava flows once overlapped sediment deposited in a rift valley during the early Jurassic period. At this exact spot on Mt. Tom, you can observe how the basalt flowed over and cooled atop the sedimentary rocks, creating a clear and accessible contact that tells a story of fiery volcanic activity meeting a tranquil lake or river system. Studying this interface helps us piece together the geologic history of the Connecticut River Valley and the ancient processes that shaped this landscape over 200 million years ago.
The East Berlin Formation, composed primarily of reddish and gray sedimentary rocks, was deposited in low-energy environments such as shallow lakes and rivers. These sediments accumulated during the Late Triassic to Early Jurassic, forming siltstones and sandstones rich in features like ripple marks and mud cracks—evidence of repeated wetting and drying cycles in a rift valley setting.
Later, during the Early Jurassic, tectonic activity intensified, and lava erupted through fissures in the thinning crust. This lava flowed across the landscape, cooling into thick sheets of fine-grained volcanic rock now known as the Hampden Basalt. At this site, you can observe the contact point where these two geologic environments—quiet water and active volcanism—meet directly in the rock record.
This contact is more than just a boundary between two rock types; it's a record of dramatic geologic change. Look closely, and you may see signs of thermal alteration where the hot lava baked the sedimentary layer beneath it. This zone can help us understand the sequence of events preserved in the stratigraphy of the Mt. Tom area.
What to Look For:
- A distinct color and texture change at the boundary between the dark gray basalt and reddish or purplish sedimentary layers.
- Vesicles or columnar jointing in the basalt, indicating cooling patterns.
- Horizontal layering of the sedimentary rock, indicating past water activity.
This is an example of the Hampden Basalt:
A key feature of Hampden Basalt are the small pores, or vesicles. These are caused by gas in the magma which bubbled up during a volcanic eruption, and then were preserved due to rapid cooling and solidifying.
This is an example of the East Berlin formation:
Educational Context:
This contact is an important feature in understanding the stratigraphy of the Mt. Tom Range. Formations like the East Berlin and Hampden Basalt are the fundamental units used in geologic maps and cross-sections, tools geologists use to reconstruct Earth’s history. As part of the Mesozoic Era, these formations span a time period from about 201 to 190 million years ago, recording the environmental and tectonic evolution of the Connecticut River Valley.
To Claim the Cache, Email Me the Answers:
- Describe the differences you observe in color, grain size, and rock type above and below the contact.
- Based on your observations, what sequence of geologic events do you think occurred here?
- Include a picture of the contact point that you're describing with you or a belonging of yours.
*Please do not post a picture of the contact to the log so that others can find it on their own. Other pictures are great! Thank you.*
Site Notes:
There is a parking lot right next to GZ, it's marked as a waypoint. The short path down to GZ is also marked as a waypoint, just go to the back left corner of the parking lot and you'll see it. Be careful when descending to the stream because this isn't a well maintained path, it's more like the suggestion of a path, however you shouldn't need to do any bushwhacking. Below is a picture in case you can't find it. This cache is accessable all year, but depending on the time of year, you may need to wade in the stream! Late summer to early spring should be fine, but it depends on the current precipitation. I suggest going when it hasn't rained for a little bit unless you like wading. Enjoy this little waterfall and grab Partok's Union Hall Waterfall cache on your way out!
To learn more about the geography of Mt. Tom check out these other Earthcaches:
Lake Hitchcock: An Introduction
Dinosaur Footprints Earthcache
Basalt Towers
Ripple in Still Water
References:
Professor Luce Ward’s GEO102 class at Smith College, Fall ‘24