The Rock

Shale Vs. Clay

Clay is a naturally occurring material composed primarily of fine-grained minerals. Clay deposits are mostly composed of clay minerals, which harden when fired or dried; they also may contain variable amounts of water trapped in the mineral structure. Clay minerals are typically formed over long periods of time as rocks are weathered through chemical means. Clay deposits are most often found near sedimentary depositional environments, such as large lakes, and marine deposits. Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals, as well as quartz and calcite minerals. When clay sediments that have formed in or near marine environments are buried, the compacted clay becomes shale as Iron Oxides help to settle the particles together. In short, the difference between clay and shale is that clay is a naturally occurring mineral, while shale is composed of several minerals including clay.

Shale

The shale at the quarry is gray, layered, and does not have visible crystals. It is quit soft, and can be easily crushed between fingers, or be broken off from other pieces of shale. The following video will demonstrate the weak durability of the shale, and will give you an idea of its feel and texture.

Shale is composed of several minerals, though clay is the most abundant mineral that makes up shale. Among the clay minerals, quartz sand, gypsum and pyrite minerals are sometimes compressed and hardened to form shale.

A close-up picture of a rock with a fossil 

Formation

Shale is formed through a process called compaction, as sediments and fine grains which compose the rock are consolidated at the rock bed over the years. The particles which form shale, being of fine nature, tend to float in water long after the other dense particles settle down. With the tendency of these fine particles to settle in slow moving water, shale rock formation is most often observed in lakes, river deltas, flood plains and even in deeper regions of continental shelf, where water is relatively calm. It is due to this process of compaction that the clay minerals settle and are combined in calm water, forming shale. 

The image above is is a document found in one of the buildings at the Don Valley Brickworks site, explaining observations made by geologist A.P. Coleman. In short, clay, shale, and sand sediments had been trapped under the Toronto ground by a tropical sea more than 400 years ago, making clay and shale very abundant in the quarry as it was the North Slope of the glacier. The shale itself, which is not a natural mineral, was formed when the clay was compressed by the tropical sea more than 400 years ago, creating the many layers of shale that attracted the Taylor brothers to the site.