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Where rocks touch: geologic contacts

Another delightful posting from my friend Marli Miller.Thanks, Marli. I’ve blogged here earlier myself about the famous unconformity at Siccar Point, and the depositional contact at the Giants Causeway between a later lava flow, and the paleosol formed by weathering of the one before it.

geologictimepics

Geologic contacts are the surfaces where two different rocks touch each other –where they make contact. And there are only three types: depositional, intrusive, or fault. Contacts are one of the basic concerns in field geology and in creating geologic maps –and geologic maps are critical to comprehending the geology of a given area. For those of you out there who already know this stuff, I’ll do my best to spice it up with some nice photos. For those of you who don’t? This post is for you!

Depositional contacts are those where a sedimentary or volcanic rock was deposited on an older rock (of any type). Intrusive contacts are those where igneous rocks intrude older rock (of any type). Fault contacts are… faults! –surfaces where two rocks of any type have moved into their current positions next to each other along a fault.

In a cross-sectional sketch they may…

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