While on a field trip with your geology class, you stop at an outcrop of sandsto

Question

While on a field trip with your geology class, you stop at an outcrop of sandstone. An examination with a hand lens shows that the sandstone is poorly sorted and rich in feldspar and quartz. Your instructor tells you that the sediment was derived from one of two sites in the area:

Site #1: A nearby exposure of weathered basaltic lava flows.

Site #2: An outcrop of granite at the previous field trip stop up the road.

Select the most likely site and explain your choice. What name is given to this type of sandstone?

Explanation / Answer

Site 2: An outcrop of granite at the previous field trip stop up the road.

Granite contains three main minerals – quartz, alkali feldspar, and plagioclase feldspar. Sandstone commonly consists of sand sized grains of quartz, feldspar, and other minerals. It may also include organic matter or rock fragments. Poorly-sorted sediments have grains of varying sizes, and are evidence of sediments that have been deposited fairly close to the source area, i.e., have not undergone much transport.

The name of this type of sandstone is Greywacke or Graywacke generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix.

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