Conglomerate Rocks:
A conglomerate is a sedimentary rock that that contains rounded clasts which are greater than 2 millimeters in diameter. (A clast is a rounded rock fragment resulting from the break down of a larger rock). These clasts can be sedimentary, metamorphic or igneous rock fragments, but the overall rock structure is sedimentary. The material that holds all of the particles together is often hardened sand or mud.
Conglomerate rocks can be formed in several ways, but most of them have common features. Something begins the process by depositing clast in a particular location. This could be done by a moving stream, in oceans or even by glaciers. Over thousands of years the spaces between the clast fragments get filled with calcium carbonate or other minerals to form the matrix that binds everything together. With time the minerals harden to form a material like concrete which can be hard to distinguish from modern concrete.
Below you will see 2 pictures. The one on the left is concrete using pebbles as a fill material, and on the right is a natural conglomerate.
It appears that the conglomerates in this area were not formed here. If you look around the area you will see a large number of rocks about the same size as the conglomerates you are looking at. They are randomly distributed around the hillside, and most of them appear to be sandstone, not conglomerates (although there are quite a few conglomerates mixed in. This type of formation is typical of an area where retreating glaciers deposited rocks that had been picked up in other areas while the glacier was advancing. It would seem to be a good assumption that the conglomerates in this area were deposited in this way.
If that is the case, how were these conglomerates formed in the first place? During the Cambrian period the global sea levels were high, and the ocean stretched over most of the northeastern United States, including most of Pennsylvania. The ocean eventually receded leaving behind a variety of sedimentary rocks suck as shale, limestone, sandstone and conglomerates. So, as unlikely as it may seem, these sedimentary rocks in this area were likely formed by an ancient ocean, then picked up by an advancing glacier only to be deposited at this site.
There are two ways that conglomerates can be classified – texture and the size of the clasts.
There are two classifications for texture – Paraconglomerates and Orthoconglomerates.
Paraconglomerates are characterized by a matrix with at least 15% grains less than 2 mm in size.
Orthoconglomerates contain a denser concentration of clast with less than 15% grains less than 2 mm in size.
Below you can see an example of each type:
Clast size is another way of classifying conglomerate. Below is a chart used for this type of classification.
• Granule conglomerate 2–4 mm
• Pebble conglomerate 4–64 mm
• Cobble conglomerate 64–256 mm
• Boulder conglomerate >256 mm
Logging requirements: Email the answers to the questions below to the owner.
1) At waypoint 1 you will find a conglomerate rock which is sticking out of the ground.
- Give the approximate length, width and thickness of this rock.
- How would you classify this rock by clast size as described above?
2) At waypoint 2 you will find a conglomerate rock which has kind of an unusual shape.
- How would you describe the shape?
- How many exposed sides are there on this rock?
- Give the approximate length of each side and the approximate overall height of the rock.
- How would you classify this rock by clast size as described above?
3) Optional: If you like to commemorate your visit with a photo I’d encourage you to walk around the area, and take a photo of another example of a conglomerate that you find in the area. You can include those photos in your log, but DO NOT INCLUDE PHOTOS OF THE TWO ROCKS ABOVE!!