Siltstone and shale are sedimentary rocks formed in ancient marine environments. They are "mudrocks" composed of silt and clay particles slowly deposited through suspension in calm waters. Silica and calcium carbonate from marine creatures provides the cement necessary to eventually form the rock. As the marine environment dries during various epochs of climate change, sedimentary rock is left behind.
Siltstone and shale are a type of sedimentary rock called clastic rock, which forms when "clasts" -- that is, fragments of other rocks or minerals -- are deeply buried and compacted in a depression. In the case of siltstone and shale, the clasts are silt and clay particles. Over time, the buried sediment becomes cemented and forms sedimentary rock. Geologists can date sedimentary rocks relative to each other, because older rock is buried beneath younger rock.
Silt and Clay
Clastic sedimentary rocks are deposited in three ways: by water, glaciers and wind. Although siltstone and shale are similarly formed in water, it is necessary to be able to distinguish between silt and clay particles to identify siltstone and shale. Silt and clay are both tiny particles that have weathered away from rocks and minerals. Silt is intermediate in size between the larger grains of sand and the smaller clay particles. To be classified as silt, the particles much be smaller than .06 millimeters in diameter, (.002 inches) and larger than clay particles, which are smaller than .004 millimeters in diameter (.0002 inches).
Shale Depositional Environment
Shale is formed in an environment that consists of calm water: for example, water near the shores of large lakes or continental shelves at sea edges. The calmness of the water enables suspended particles like clay to eventually sink and settle in the bottom of the lake or sea. Silica and calcium carbonate from marine life, particularly from shells, also settle with the clay particles, and over time they form cement for the clay particles to "lithify" -- that is, become rock -- and form shale. When extensive organic material such as from plankton and plants becomes embedded with the shale, oil shale can form.
Siltstone Depositional Environment
Siltstone is deposited in a similar environment to shale, but it often occurs closer to the shoreline of an ancient delta, lake or sea, where currents cause less suspension of particles. Siltstone commonly occurs adjacent to sandstone deposits -- that is, near beaches and delta edges where sand is deposited. Silt, hence siltstone, occurs in the water adjoining the sandy beaches and deltas, which filters the smaller silt particles. The siltstone will grade into shale in deeper water where the suspended clay particles are more abundantly deposited. In either case, calm waters are needed for the suspension and sorting of silt and clay. Thus, sandstone, siltstone and shale are interrelated rocks that are distinguished by particle size.