Geologic Contacts

A geologic contact is where one rock type touches another. There are three types of geologic contact: 1. Depositional contacts are those w...

A geologic contact is where one rock type touches another. There are three types of geologic contact:1. Depositional contacts are those where a sedimentary rock (or a lava flow) was deposited on an older rock
2. Intrusive contacts are those where one rock has intruded another

3. Fault contacts are those where rocks come into contact across fault zones.
Learn in detail about fault here

Following are the some pictures showing each type of geologic contact

Depositional Contacts
1. Angular Unconformity, Siccar Point, Scotland

This place is known as Siccar Point which is the most important unconformity described by James Hutton (1726-1797) in support of his world-changing ideas on the origin and age of the Earth.
Here gently sloping strata of 370-million-year-old Famennian Late Devonian Old Red Sandstone and a basal layer of conglomerate overlie near vertical layers of 435-million-year-old lower Silurian Llandovery Epoch greywacke, with an interval of around 65 million years.

2. Cretaceous Sandstone overlying Conglomerate    Kootenai Formation, SW Montana

Photo Courtesy: marlimillerphoto.com

3. Dun Briste Sea Stack, IrelandDun Briste is a truly incredible site to see but must be visited to appreciate its splendour. It was once joined to the mainland. The sea stack stands 45 metres (150 feet) tall.

Dun Briste and the surrounding cliffs were formed around 350 million years ago (during the 'Lower Carboniferous Period'), when sea temperatures were much higher and the coastline at a greater distance away.  There are many legends describing how the Sea Stack was formed but it is widely accepted that an arch leading to the rock collapsed during very rough sea conditions in 1393. This is remarkably recent in geological terms. 

Photo Courtesy: dunbriste.com 

Fault Contacts

1. Normal Faulting in the Cutler Formation near Arches National Park

Photo Courtesy: travelinggeologist.com

2. Normal Fault in Titus Canyon, Death Valley, California 

Photo Courtesy: travelinggeologist.com

3. Horst and Graben Structure in Zanjan, Iran

Photo Courtesy: Amazhda

Intrusive Contacts 

1. Pegmatite and aplite dikes and veins in granitic rocks on Kehoe Beach, Point Reyes National Seashore, California.

2. Spectacular mafic dyke from Isla de Socorro from Pep Cabré. The Isla de Socorro is a volcanic island off the west coast of Mexico and it is the only felsic volcano in the Pacific Ocean

Photo Courtesy: travelinggeologist.com

3. The margins of this Granite dyke cooled relatively quickly in contact with this much older Gabbro.
Photo near Ai-Ais Namibia

Photo Courtesy: travelinggeologist