Conglomerate is a type of sedimentary rock that contains large rounded clasts (similar to aggregate in concrete). The space between the clasts is generally filled with smaller particles and/or a chemical cement that binds the rock together.
Conglomerate can have a variety of compositions. As a sedimentary rock it can contain clasts of any rock material or weathering product that is washed downstream or down current. The rounded clasts of conglomerate can be mineral particles such as quartz or they can be sedimentary, metamorphic or igneous rock fragments. The matrix that binds the large clasts together can be a mixture of sand, mud and chemical cement.
Conglomerate often begins with a deposit of clasts. The finer size sand and clay which fill the spaces between the larger clasts is often deposited later on top of the large clasts and then sifts down between them to fill the spaces. The rounded shape of the clasts reveals that they were tumbled by running water or moving waves before becoming a part of the conglomerate.
The Copper Harbor Conglomerate varies between 300 and 5,500 feet thick. It overlays and interfingers with the top of the Portage Lake Volcanics. If you look closely you will see layers of volcanic material in between the layers of conglomerate.
Chart: Geology east of 89º from U.S. Geological Survey Open-File Report 99-149 by William F. Cannon, Michele E. McRae, and Suzanne W. Nicholson 1999. Geology west of 89º after Bornhorst (1994).
Prior to logging cache, email the answers to the following questions:
- What size are the largest clasts in the conglomerate?
- What is the average size?
- In what direction does this ridge run?
This is a rocky beach area, but beautiful sunsets can be viewed from here if you are lucky.
