Stratigraphy and Stratification

Since sediments are deposited, it follows that the topmost layer in any succession of strata is the youngest. Also, any particular stratum in a sequence can be dated by its position in the sequence relative to other strata. This is the Law of Superposition. This principle applies to all sedimentary rocks except, of course, those that have been overturned by folding or where older strata have been thrust over younger rocks. Where strata are overfolded, the stratigraphical succession is inverted. When fossils are present in the beds concerned, their 

Diachronism of a lilhological boundary and the migration lime of a fossil assemblage. The tbssiliferous horizon may be regarded as a lime plane if the localities (a), (b) and (c) are not far distant. As a mle, lime planes cannot be identified. 

An unconformity represents a break in the stratigraphical record and occurs when changes in the palaeogeographical conditions lead to a cessation of deposition for a period of time. Such a break may correspond to a relatively short interval of geological time or a very long one. An unconformity normally means that uplift and erosion have taken place, resulting in some previously formed strata being removed. The beds above and below the surface of unconformity are described as unconformable. 

Types of unconformities (a) nonconformity or heteroiithic unconformity, (b) anguiar unconformity, (c) disconformity and (d) paraconformity. 

Angular unconformity between highly folded Horton Flags, Silurian, below and almost horizontal Lower Carboniferous Limestone above, Helwith Bridge, North Yorkshire, England. 

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