About a million years ago, the earth cooled enough so that massive ice sheets, known as “continental glaciers” formed in Northern Canada. The ice sheets did not melt during the summer and got larger and thicker as each winter passed. Eventually, the ice at the bottom of the ice sheets started to move toward New England. Brookline, Massachusetts, was changed significantly as the glacier that passed over it was hundreds of miles long and it pushed and flattened anything in its path.
The glacier repeatedly advanced and retreated through Brookline until about 16,000 years ago. At times the town was covered with ice that was several thousand feet deep. Each time the glacier advanced it scraped the surface of the earth taking with it debris composed of pieces from the size of dust particles to boulders. It pushed through the town leaving hundreds of tons of debris piled up. As it went it created “drumlins”, These hills are made up mostly or entirely of glacial debris called “till”, Brookline has seven major drumlins: Corey Hill, Fisher Hill, Single Tree Hill, Walnut Hill, Larz Anderson Park Hill, Aspinwall Hill, and Mount Walley.
In addition to the till, glaciers left deposits of sand, gravel, and rocks. One of the rocks that is seen around Brookline's, Lost Pond, is "Brighton melaphyre", often referred to as "Brighton amygdaloidal". This "porphyritic"igneous melaphyre consists of dark-colored "aphanitic" groundmass and "phenocrysts" of various kinds." More specifically, "The rock is greenish, brownish, or purplish "amygdaloidal" melaphyre of basaltic character composed of augite, labradorite, and olivine with accessory pyrite, magnetite, and hornblende.... In some masses of the rock amygdules are notably abundant, but other masses contain very few." (1) Here is a picture of a type of melaphyre that shows some amygdules:
When the glacier passed through Brookline, Massachusetts it also created other geological features like “kettle holes”when blocks of ice separated from the main glacier. In Brookline, the conditions were right for these ice blocks to be buried. When they eventually melted, they created holes or depressions that filled with water. These holes become “kettle ponds.” The picture below shows how a kettle hole is formed as the buried ice blocks slowly melted(2):
Lost Pond is a kettle hole that has developed into a kettle pond and then a “quaking bog"” or “schwingmoor”on the side where you will be approaching. In the database of Home Ground, Robert Morgan describes a quaking bog as “an area of sphagnum moss, rushes, sedges, and decaying vegetation, the whole mass of which is floating on a pool of water. The surface appears solid and stable, until trusted with the weight of a step. What seems to be firm ground then shivers, sinks, and rises, like a natural trampoline or waterbed. If the first shimmy of this rich root mass underfoot is not heeded, one might easily break through the entangled mat into water and loose mud below, as if one had stepped into quicksand.”
You may wonder how this condition formed at Lost Pond. It began with the acidic kettle pond full of poor nutrients that allowed sphagnum moss to grow. This moss is known as the “bog builder” since it becomes waterlogged and holds many times it weight in water. As the sphagnum moss died, decayed, and decomposed, it turned into “peat” The peat formed a floating mat that grew in from the edge of the pond. When this mat is stepped on it undulates and thus, Lost Pond is considered to be a "quaking bog.
To get an idea of Brookline's surface compostion today, take a look at this MassGIS (Office of Geographic Information) map:
GLOSSARY
Drumlin - an oval or elongated hill believed to have been formed by the streamlined movement of glacial ice sheets across rock debris, or till. (Britannica.com)
Till - clay containing pebbles, cobbles, and sometimes boulders. (Google.com)
Melaphyre - a porphyritic rock consisting of phenocrysts of feldspar in a dark groundmass; broadly : a porphyritic igneous rock with dark-colored aphanitic groundmass and phenocrysts of various kinds. (Merriam-Webster Dictionary)
Porphyritic - a rock that has a distinct difference in the size of the crystals with at least one group of crystals obviously larger than another group. (e-Study Guide for Physical Geology by Diane Carison)
Aphranitic - a fine-grained igneous rock having such compact texture that the constituent minerals cannot be detected with the naked eye.
(Merriam-Webster Dictionary)
Phenocryst - a relatively large and usually conspicuous crystal distinctly larger than the grains of a the groundmass of a porphyritic igneous rock.(e-Study Guide for Physical Geology by Diane Carison)
Amygdaloidal - a volcanic rock in which rounded cavities formed by the expansion of gas or steam have later become filled with deposits of various minerals. (Dictionary.com)
Peat - a highly organic material found in marshy or damp regions, composed of partially decayed vegetable matter: it is cut and dried for use as fuel. (Dictionary.com)
Quaking Bog - an area of sphagnum moss, rushes, sedges, and decaying vegetation, the whole mass of which is floating on a pool of water. (Robert Morgan, Home Ground, Language for an American Landscape)
Sedimentary Rocks - Sedimentary rock types are created when sediment compresses. ... A river flows through a canyon and picks up a bunch of silt. That sediment and silt runs downstream and deposits where the river ends. It could be in a flood plain or a valley, but we're using a coastline as an example. When that material gets to the beach, it sits there. Now if you watch this happen over millions of years, more and more sediment builds up and compacts. That compacted sediment eventually becomes a type of rock. Examples of sedimentary rock include sandstone, amber, anthracite, and limestone. (geography4kids.com)
Igneous Rocks - Igneous rocks are the ones that were superheated and originally liquid. They come from the center of the Earth! Not really the center, but they often start their lives below the crust and then get pumped out. (geography4kids.com)
To get credit for this EarthCache you must email the cache owners with the answers to the following questions:
1. What is till?
2. In relation to the ice flow, what direction is the long axis of a drumlin?
3 .What is the approximate shape of the kettle pond? Why do you think it is this shape?
4. What is the diameter of the pond?
(Don't know? Check the map at the kiosk)
5. According to the Surficial Geology map, what is Brookline mainly composed of?
6. Take a look at the rocks on the trail by the kettle pond. Do you think they are sedimentary or igneous? Explain your answer.
Optional - Take a picture of yourself standing on the platform that is floating in the kettle pond at Ground Zero and post it to the Gallery.
The ecosystem around the pond is fragile, please do not step off the boardwalk.
(1) USGS Bulletin 837-840, "Geology of the Boston Area", pg. 42
(2) Eliot School Science Curriculum, Needham, MA