greenspun.com : LUSENET : Introductory Geology, Oswego State : One Thread

Can anyone explain what unconformity is better than the book does?

-- Alison Tripi (tripi@oswego.edu), March 28, 2001



I found a website that I think helps describe this more clearly. Here is the picture:


One of the fundamental "laws" of stratigraphy, formulated in the 17th Century by Nicolas Steno, is the law of Original Horizontality. Put most simply: "Sediments are usually deposited in horizontal layers." While there are some clear exceptions to the rule (cross- bedding in sand dunes or the foreset beds in deltas, for example), Steno's observation holds true in almost all cases. Imagine, then, how puzzled James Hutton must have been when he made his famous visit to Siccar Point on the Scottish coast in the 1780s and discovered an angular unconformity.

The exposure shown above, along a stream bank at Coulter's Hell in Wyoming, is similar to what Hutton saw. Sedimentary rocks at the surface are horizontal, as they should be according to Steno, but those below tilt at a high angle. The plane of contact between the upper and lower sediment is the angular unconformity. Hutton correctly deduced that the following sequence of events must have occurred:

The lower sediments were deposited as horizontal layers in a body of water.

The lower sediments were raised above water level and tilted during a tectonic event.

Streams or other erosional forces carved a nearly horizontal surface across the tilted beds.

The land surface subsided (or the water level raised), submerging the erosion surface.

A new series of sediments deposited in horizontal layers on the erosion surface.

The complicated sequence of tilted and horizontal rocks was again uplifted, exposing them to erosion and producing the outcrop we see today.

The revelation for Hutton was that this sequence of events must have taken a very long time. We have no direct knowledge of events that occurred between the end of step 1 and the start of step 5, when sedimentation began anew. The unconformity represents a gap in our understanding of Earth's history at this locality, a hiatus in the rock record. In Hutton's day most people still believed that the Earth was no more than a few thousand years old. Hutton's interpretation of the angular unconformity at Siccar Point, however, meant that there were gaps much longer than that. The Earth, in other words, must be much more than a few thousand years old.

James Hutton's interpretation of an angular unconformity was thus a watershed in the history of geology as a science, often cited as the event that opened the door to our modern view of the Earth.

Here is the website:


Hope this helped!

-- Mei-Ling Chen (mchen@oswego.edu), April 01, 2001.

I found an answer to your question on the web at http://www.m8i.net/ unconf.html What is an unconformity?

An unconformity is not a difficult concept to understand. It merely marks the area betwen two sets of rocks where there is a TIME GAP - or a layer of rocks missing - from a stratigraphy. This is mostly due to erosion. The following diagrams illustrate simply the processes that have formed an unconformity, using the Overstrand unconformity as an example. The principles can be applied to any unconformity, though the erosion type may be different.

-- Christi Stahl (cstahl_7@hotmail.com), April 02, 2001.

a break in the seds record. during this break erosion/uplift has occurred. new deposition takes place.


think how deposition occurs and where the oceans are. think how erosion takes place and where the oceans are.

read the above again.

have fun!

-- al (wildal20@hotmail.com), January 31, 2003.

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