Geoscience Reference
In-Depth Information
Steno used these stages to represent each of the six days of the Creation
week, and so may have been constraining an earthly chronology of 6,000
years. The six stages can be summarised as follows: in stage one, hori-
zontal strata are deposited in the sea; stage two sees the regression or
retreat of the sea leading to a drying out of the land and the formation of
cavities beneath; in stage three, mountains and valleys are formed
through the action of fires and water; stage four is marked by a return
of marine conditions and the deposition of fossiliferous layers of sedi-
ment; in stage five another regression occurs and erosion of the rocks by
river water takes place; finally in stage six the present-day landscape is
formed by fire, water and collapse of the rock layers.
A colleague of mine in Dublin, the noted historian of geology
Gordon Herries Davies, wondered why Steno's study of a small and
seemingly insignificant area of limestone and their contained fossils
in a part of Italy should be so influential. Why were Steno's findings
important in the history of the development of geological thought?
The answer lies in his observation that the nature of the Earth's
history and development could be deciphered through an examination
of the rock succession. In this he recognised unconformities, which
represent a break in the stratigraphical record, and where often hori-
zontal layers are found deposited on earlier tilted and eroded rocks.
These are called angular unconformities, although horizontal uncon-
formities do occur in the geological record, but the breaks in the
sequences are naturally harder to spot. Although he illustrated such
structures it is unclear whether Steno actually understood their
significance as recording breaks in the geological succession. Steno
did, though, recognise the importance of superposition, a principle
more often associated with the English geologist William Smith
(1769-1839), in which it is understood that in a sequence of rocks
the underlying horizons are older than those above them, unless of
course it can be demonstrated that the whole sequence has been over-
turned. Steno also recognised that horizons (layers of rock) found
at either side of a valley that corresponded in terms of their position
in the sequence and lithology were once connected by a lateral and
