--- title: "Primary and secondary structures; strike and dip" subject: "Engineering Geology I" chapter: 5 key: "semester_3/engineering_geology_i:primary-and-secondary-structures-strike" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/primary-and-secondary-structures-strike site: BCT Engineering --- # Primary and secondary structures; strike and dip Engineering Geology I · Chapter 5: Structural Geology - Asked in: 2078 Bhadra, 2+2+2 marks; 2075 Chaitra, 2+2+3 marks (asked 2 times) - Video: [Structural Geology - Lesson 1 - Part 1 of 4](https://www.youtube.com/watch?v=YN2LBvt73OI) — C. Gunn-Rosas - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/primary-and-secondary-structures-strike - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5 ## Question How do you differentiate primary geological structures and secondary geological structures? Describe the relationship of strike and dip of geological planes with illustrations. How do you find out the strike line when the dip direction is measured? ## Short answer Primary structures form during rock formation (e.g., bedding); secondary result from later deformation (e.g., folds). Strike is the horizontal line on an inclined plane, always perpendicular to dip direction. To find strike from dip direction, add or subtract 90° to get the bearing, then convert to quadrant bearing. ## Answer #### Primary and secondary structures | Point | Primary structures | Secondary structures | |---|---|---| | Formation | During the formation of the rock | After the formation, due to later stresses | | Examples | Bedding, ripple marks, mud cracks, cross bedding, vesicles, columnar joints | Fold, fault, joint, cleavage, foliation, unconformity | | Rocks | Igneous and sedimentary | All rocks | | Study gives | Environment of deposition and way up | Deformation history | #### Relationship of strike and dip - **Strike** is the direction (bearing) of a horizontal line on an inclined plane. - **Dip** is the maximum inclination of the plane, measured at **right angles** to the strike, in the vertical plane. - Hence, strike and dip direction are always 90° apart, and the dip direction is perpendicular to the strike line. - Strike is the contour line of the plane (all points on it are at the same level), and dip is the line of steepest descent. - Any other direction on the plane has a smaller inclination, called the apparent dip. ```text N | strike line (horizontal) | / | / W --------+--------- E / | \ / | \ / | v dip direction S (90 deg to strike) Plan view; the dip amount is the angle seen in a vertical section ``` The attitude is written as strike direction/dip amount and dip direction, for example N30°E/40°SE. #### Strike from the measured dip direction 1. Measure the dip direction (bearing of the line of steepest descent) with a compass. 2. Strike = dip direction + 90° or dip direction - 90° (both give the same line, in opposite senses). 3. Convert to a quadrant bearing. Example: dip direction N60°W (azimuth 300°). Strike = 300° - 90° = 210°, which is S30°W; the same line read the other way is 300° + 90° - 360° = 30°, which is N30°E. The strike is **N30°E (S30°W)**. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Strike and dip direction are? A. 90 deg apart B. Parallel C. 45 deg D. 180 deg Answer: A. Dip is perpendicular. (Key point: Relation) 2. Strike is also the? A. Line of steepest descent B. Contour line of the plane C. Plunge D. Trend Answer: B. Same level. (Key point: Strike line) 3. Any other direction gives? A. Larger B. Same C. Smaller apparent dip D. Zero always Answer: C. True dip is max. (Key point: Apparent dip) 4. Dip direction N60W gives strike? A. N60E B. N30W C. N60W D. N30E Answer: D. 300-90=210, i.e. N30E. (Key point: Calculation) ## Related in Engineering Geology I - [Primary versus secondary geological structures](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aprimary-versus-secondary-geological-structures) — Engineering Geology I, chapter 5, asked 4× - [Strike-dip relationship and apparent dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrikedip-relationship-and-apparent-dip) — Engineering Geology I, chapter 5, asked 2× - [Dip direction from strike N55°E, dip 30°](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adip-direction-from-strike-n55e) — Engineering Geology I, chapter 5 - [Strike when dip direction is N40°W](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrike-when-dip-direction-is) — Engineering Geology I, chapter 5 - [Dip amount, strike; fold identification criteria](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adip-amount-strike-fold-identification) — Engineering Geology I, chapter 5 - [Strike of limestone bed, dip N40°W, 64°](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrike-of-limestone-bed-dip) — Engineering Geology I, chapter 5 ## Related in other subjects - [Primary and secondary energy at 45% exceedance](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aprimary-secondary-energy-45pct) — Hydro Power, chapter 3 - [Primary and secondary energy (102 m head)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aprimary-secondary-energy-102m) — Hydro Power, chapter 3 - [Primary/secondary energy and pondage volume](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aenergy-and-pondage-volume) — Hydro Power, chapter 3 - [Primary and secondary energy with 10% environmental flow](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aenergy-with-environmental-flow-950) — Hydro Power, chapter 3