SEDIMENTARY ROCK STRUCTURE- STRIKE AND DIP

Aerial photo showing folded Chuckanut Formation south of Bellingham. The yellow symbols represent strike and dip. The short ticks are dip, with numbers indicating angle of dip. The longer lines indicate strike, using right hand rule.

SEDIMENTARY ROCK STRUCTURE- STRIKE AND DIP

Sedimentary rock units are initially flat lying sequences of deposits. Each layer reflects a change in some aspect of the depositional environment: water velocity, size of the clasts (sediment fragments, be they clay or boulders), degree of rounding of the clasts, etc. Once sedimentary rocks are subjected to tectonic forces, they may be folded, tipped on edge, chopped up by faults, or even overturned. The orientation of the rock beds is described using ‘strike’ and ‘dip’. Dip refers to the downward angle at which the tilted beds now lie. A dip of 90 degrees is vertical. The dip might be stated as “beds dipping 45 degrees to the north”. Strike is more complicated. This is defined as the direction the contacts between the beds point when looking along the contact, and is always a compass direction exactly 90 degrees to the direction of dip. If the beds are folded, this direction will change over some distance. So, it is important that you are using the correct direction when describing the strike. Beds dipping to the north could conceivably be said to strike either to the east or the west. So a convention is necessary, since no direction is given on a map when the dip is recorded. We use the ‘right hand rule’ – when your fingers point down along the dip, your thumb points in the direction of strike. You have to orient your hand so your fingers point down the steepest angle of the tilted beds, the direction a drop of water would run down the rock.

The right-hand rule helps avoid errors when interpreting mapped structures. Click to enlarge.

Examples of field trips on this website that illustrate sedimentary strike and dip include Beach 4 (Olympic coast), Frank’s Beach (Lummi Island), Squires Lake (Alger).