Pebble count from glacial deposits

Wednesday, May 24, 1989

P.F. Karrow

The purpose in doing a pebble count is to determine the proportions of different types of rock present in order to get some idea of the source of the material – i.e. what regions the ice collected the rocks from. In the case of gravels which may be used for construction, their composition may be economically important since certain rock types such as chert, shale, or schist are undesirable for concrete (chert may cause chemical reactions, and shale and schist cause planes of weakness, both of which weaken concrete). Equipment needed includes a steel knife for testing hardness, a dropper-bottle of 10% hydrochloric acid, and a hand lens. From an exposure of a glacial till or gravel deposit collect 105 pebbles (the extra five are in case you miscount). If you encounter a weathered pebble while collecting, gather the crumbly material into a separate small gab. If you ignore these you will bias the results! Choose sizes about one to three inches in diameter for convenience. The pebble count can be performed outdoors, splitting each pebble with a hammer to reveal a fresh rock surface, or indoors in the lab. In the latter case, pebbles should be well washed, to remove adhering clay, and left to dry. Rocks can be simply classed for this purpose into two groups – sedimentary and crystalline (igneous and metamorphic). All the bedrock in southwestern Ontario, Southeastern Ontario, and near James Bay is water-laid sedimentary rock. Rocks of the Precambrian Shield are crystallized or recrystallized by heat and pressure. The main rock types and their most striking properties are:

Sedimentary:

Sandstone – makes small scratches on knife, sugary fell because of sand grains which compose it. Not common.

Shale – red, gray, green, or black layered clay rock. Easily scratched by knife.

Limestone – effervesces strongly with acid and easily scratched by knife. May contain obvious fossils.

Dolomite – scratched with difficulty by knife blade and reacts weakly with acid where scratched.

Chert – scratches knife and has a class-like (conchoidal) fracture. Worked as "flint" by Indians for projectile points.

Crystalline:

Granite – coarse-grained, light-colored crystalline rock. Scratches knife.

Gabbro – coarse-grained, black crystalline rock.

Gneiss – like granite but with prominent parallelism of crystals.

Schist – usually a large amount of parallel mica flakes giving a shiny glitter. Easily scratched by knife.

When 100 pebbles have been identified as to rock type the results can be expressed as percentages and compared to a map of the bedrock geology of Ontario, which will show the distribution of various rock types. It will be seen that the local bedrock is the most abundant. How far did the farthest-travelled types come to reach your area? Is the deposit you analysed good for making concrete?

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