The fossil-bearing beds are included in cycles repeated about 110 times during the 10.5 million years depositional history of the Orcadian Lake. These cycles range from 5m to 20m in thickness and each has four phases:
Phase 1. Black finely laminated siltstone and mudstone generally 0.5m to 2m with a high carbonate and organic carbon content. Formed in deep waters, below the level where surface wave action disturbs the bottom sediments. The laminations, each one to two millimetres thick, consist of dark and light coloured bands that may be related to seasonal variations in biological activity in the lake. This bed commonly contains abundant fish remains and appears to have been formed when the lake was at its deepest.
Phase 2. Carbon-rich (bituminous) silty mudstone with thin, irregular laminae of siltstone and sandstone. These beds are characterized by Gypsum pseudomorphs and stromatolite (blue-green algae) forming sheets, mounds or orange-weathering limestone within the rock. These are signs that as the lake became shallower, bottom currents began to stir up the lake floor, and the influx of sediment increased.
Phase 3. The overlying beds in the cycle are lithologically similar to those of Phase 2, but are characterized by the presence of abundant sand-filled polygonal mudcracks. At this stage, the lake was so shallow that its bottom was periodically exposed as an extensive mudflat.
Phase 4. The uppermost beds in the cycle are usually ripple-laminated, fine-grained sandstones and siltstones with desiccation cracks. The sands were deposited by wide rivers flowing into a shallow lake and then reworked in a sand-flat environment, which occasionally dried out to form a desert floor.