By now it is probably well known the Castle Rock is a glacial feature called a crag and tail. However, at Johnston Terrace, a survey done by the BGS shows a unique feature called a contact. I've done a visit myself and it is challenging to see micro features of the contact, but generally the contact can be identified from the sidewalk.
As with any ECs of mine, simple answers will suffice as I do not issue PhDs. A good attempt at the answers + a photo of your visit will be good enough, but most important is that you learn a little bit more about our planet.
- The image above has a red redacted area. This is the contact. To the left is basalt (340myr), to the right are layers of sandstone and mudstone of the Ballagan formation (350myr).
- Look to the left of the contact. s it bumpy or flat? Are the flat surfaces full of cracks?
- The red area marked - is it bumpy ot flat? Is it cracked or smooth?
- Based on the answers above, what kind of contact do we have here?
- What other possible features could you find?
- Why is this feature a contact?
Contacts, in the geological sense, are sections where two different rocks touch each other. There are 3 types of contacts.
Depositional Contacts - A generic rule of depositional contacts are horizontal. These contacts are formed when layers are deposited onto the original rock.
Dun Briste Sea Stack with it's layers. Each layer was deposited on top of the older rock.
Intrusive Contacts - Commonly seen when igneous rocks intrude the older rock. Commonly called dykes. Some common signs of intrusive contacts include evidence of heat alteration or a chilled margin
Basaltic dike intruding volcanic mudflow deposits, eastern Oregon. Note the baked and altered edge of the country rock.
Note the rock on the right (dyke) - the crystal size closer to the contact is smaller because it cools faster.
Fault Contacts - When two rocks "collide". Common features include cut off surfaces. You can also commonly find the rock surface highly fractured, polished surfaces and slickenlines.
Faults
Slickenlines