GeologyRocks

Devonian

The fourth period of the Palaeozoic. Spans 409-353Ma. The rocks of this period are referred to as Old Red Sandstone (ORS) as they are often red coloured sandstone. The name originates from the county of Devon in South West England. The colonisation of land was also well under way as shown by the famous Rynie Chert deposit.

Epochs:

• Late Devonian 371-353Ma
• Middle Devonian 380-371Ma
• Early Devonian 409-380Ma

Directional selection

Fitness increases/decreases with trait magnitude. E.g. selection on bill size in Darwin’s medium ground finch – Geospiza fortis. Population crashes in 1977, 1980 and 1982, due to droughts, selected for increased bill depth due to abundance of hard seeds of Tribulus cistoides. Deep beaks are needed to crack these seeds and therefore large bills were selected for.

Disconformity

An unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

Disruptive selection

Individuals with extreme trait values have greatest fitness. E.g. selection on bill size in the black-bellied seedcracker – Pyrenestes o. ostrinus. This species of African finch exhibits non-sex related polymorphism in bill size. Members of the species have either one of two bill morphs; bills are relatively small or relatively large. The morphs feed on different sized seeds and this is directly related to natural selection on bill size.

Drumlin

A glacial deposit; an elongate shaped hill. The longest dimension is parallel to the movement of the ice flow (see arrow).

Dunham Classification

A classification of limestone based on depositional textures. The classification consists of mudstone (<10% grains, >90% mud), wackestone (>10% grains, mud supported), packstone (grain supported, but contains mud), grainstone (grain supported, no mud) and boundstone (grains bound in situ, i.e. reef).