Fossils and Geologic Time Topics

Geologic Time (Part 2)

Relative Dating (continued)


Unconformities are surfaces in rocks caused by erosion or non-deposition that represent missing time.  Rocks beneath an unconformity are older than the unconformity.  Rocks above it are younger.

Unconformity exposed in a Fairfax County quarry. The arrow points to the unconformity. (Photograph by Stan Johnson)

MapUnconformities represent gaps in the geologic record.  This Fairfax County unconformity represents about 500 million years that are not represented between the top of the granite and the bottom of the Coastal Plain sand deposit.  The unconformity is the boundary between the sediment and the granite.

Geologic History As The Order of Events

• Superposition, unconformities, and cross-cutting relationships allow geologists to determine the sequence of events in rock masses.  

Sequence of Events diagram
Diagram by Phyllis Newbill

The relative order of events is as follows (read up from the bottom):

8.  Layer 12 was deposited.  

7. The rocks eroded again, forming the upper unconformity.

6. Dike B intruded.

5. The fault broke the rocks.

4. Layers 7-11 were deposited. 

3. Layers 1-6 and Dike A eroded, forming the lower unconformity.

2. Dike A intruded.

1. Layers 1-6 were deposited and folded.