The Dump! EarthCache

This earthcache will introduce you to two geological features located in the trails off Handy Street, Attleboro: bedrock/outcrop and glacial impact (striations and erratics).  

When you are standing at ground zero you'll notice that beneath your feet is an outcrop of bedrock. This is one of many outcrops that can be found throughout the trails, many of which have evidence of striations.

(Bedrock is the solid rock that lies under the surface of the ground you walk on.  When its out in the open like the one you stepped on, it’s called an outcrop.  Bedrock in Massachusetts can be as much as 250-500 million years old.)

The lines going across the outcrop are called glacial striations [strigh-aye-shuns].

(Glacial striations are scratches or gouges cut into bedrock by glaciers. The depth and width of striations depends on the size of the rock lodged in the glacier, and the resistance of the bedrock to being gouged.  Striations were created by the the Laurentide Ice Sheet over 20,000 years ago.)

Glacial striations typically look like multiple, straight, and parallel lines and they indicate the direction the glacier was moving.  As the glacier moved along Earth’s surface it picked up everything from small rocks to boulders the size of a house. These rocks and boulders became stuck in the glacier and acted as scrapers on the landscape which is why, today, the exposed bedrock has those parallel scratch marks.

Above: Glacial striations on bedrock outcrop.

As you step forward and away from the outcrop, you’ll notice several huge boulders.  You may be wondering how they got there because they are so huge. Well, those are perfect examples of erratics.  Once the glacier melts, the rocks and boulders that it picked up earlier are left behind in a new location, often far from where the glacier first picked them up.

(Rocks and rock fragments left behind (dumped) by the glacier that differ from the local bedrock are call erratics.)

BUT WAIT, there’s MORE! Not all erratics are alike, and you’ll notice some differences among the erratics at this site.  Some of them are solid rock fragments that had broken away from a larger rock source.  Others are made up of small fragments of rock, glued or cemented together by a mixture of calcium carbonate, iron oxide, silica and/or hardened clay.

(The smaller rock fragments are called clasts and resulting rocks are called conglomerate rocks).

[Conglomerate rocks are rock fragments (clasts), etc., that are held together or cemented by a mixture of calcium carbonate, iron oxide, silica and/or hardened clay.]

WAIT!  There’s even MORE!  Not all conglomerate rocks are the same.  The following characteristics are used to classify and categorize conglomerate rock:

• The composition of the clasts. If all the clasts are the same type of rock or mineral), the rock is categorized as monomictic conglomerate. If the clasts are made up of two or more rocks or minerals, the rock is a polymictic conglomerate.

Above: Monomictic Conglomerates  

Above: Polymictic Conglomerates

• The size of the clasts. Rock comprised of large clasts is cobble conglomerate. If the clasts are pebble-sized, the rock is called pebble conglomerate. If the clasts are small granules, the rock is called granule conglomerate.
• The amount and chemical composition of the cement that binds the clasts . If the clasts do not touch each other (lots of cement between the rocks), the rock is paraconglomerate. Rocks in which the clasts touch each other is called orthoconglomerate.

To earn your smiley for this Earthcache, please send the CO the answers to the following questions. It will not be necessary to wait for a response from him to log your find. He will contact you if there are any concerns or questions with your responses.

DO NOT PROVIDE ANSWERS IN YOUR LOG SO THAT OTHERS WILL NOT HAVE ACCESS TO YOUR ANSWERS.

1. Using your GPS or compass determine which of the following best describes the direction in which the GLACIER was moving?
   1. N-S
   2. W-E
   3. NE-SW
   4. NW-SE
   5. N-E
   6. N-W
   7. S-E
   8. S-W
2. What do you estimate the length of the longest striation you see in the outcrop to be?
3. In the middle of the outcrop are the striations with the deepest gouges.  One of them has the distinction of being the deepest.  How deep is that gouge?
4. The erratic in the middle of the dump is the highest and longest of the bunch.  Approximately how long and how high is it (in feet)?
5. All the erratics are conglomerates, but there are different types among the bunch.
   1. What best describes the type of the erratic closest to the trail?
   2. What best describes the type of the erratic measured in question 4?
6. If you choose to post of photo of you at the location (not required) please do not include any object mentioned in the previous questions.