---
title: "Unconformity recognition and problems"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:unconformity-recognition-and-problems"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/unconformity-recognition-and-problems
site: BCT Engineering
---

# Unconformity recognition and problems

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2062 Baisakh (old course), 2+2+4 marks
- Video: [What is Unconformity? How it is formed?| Types of Unconformity](https://www.youtube.com/watch?v=i-5wYGE-FXc) — GeoEntire
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/unconformity-recognition-and-problems
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

What is an unconformity? How can it be recognised in the field? What problems are created by the presence of unconformities?

## Short answer

An unconformity is a surface of erosion or non-deposition between younger and older rocks. Field recognition uses a break in succession, irregular weathered surface, basal conglomerate, missing fossils, and different deformation. It causes weak planes, sudden rock quality changes, leakage, and slope instability.

## Answer

An **unconformity** is a surface of erosion or non-deposition that separates younger strata from older rocks and represents a gap (break) in the geological record.

#### Recognition in the field
- A **break in the sequence**: the rocks above and below have different attitude (angular unconformity) or different rock types and ages.
- An irregular, uneven, weathered or eroded surface with a **basal conglomerate** or a layer of fragments from the older rocks just above the surface.
- Missing beds: absence of some formations or fossil zones that should be present.
- Difference in the degree of folding, metamorphism or deformation of the beds above and below.
- Presence of soil (palaeosol), weathered zone, or iron staining at the contact.
- For nonconformity, sedimentary beds directly over igneous or metamorphic rocks; fossils of different ages above and below.

#### Problems created
- A **basal conglomerate** or weathered, soft layer at the unconformity is a plane of weakness and can cause sliding.
- The surface is generally **uneven and weathered**, so foundation depth and rock quality change suddenly.
- **Seepage**: water can move along the unconformity plane, so dams and tunnels may leak.
- Beds above and below have **different strength, attitude and permeability**, so differential settlement and stability problems arise.
- Wrong **correlation of boreholes** and rock depth, since beds are missing.
- **Slope instability**: sliding of younger beds over older tilted beds.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What indicates an unconformity in the field?
   A. Perfectly parallel beds with no break
   B. A basal conglomerate and an uneven weathered surface
   C. Repeated beds in reverse order
   D. Slickensides
   Answer: B. Basal conglomerate shows erosion before deposition. (Key point: Basal conglomerate)

2. What does absence of some formations or fossil zones suggest?
   A. A fold axis
   B. A joint
   C. A gap in the record, an unconformity
   D. No geological event
   Answer: C. Missing beds mean a break. (Key point: Missing beds)

3. What is seen in a nonconformity?
   A. Beds parallel on both sides
   B. Two sets of joints
   C. A fault breccia
   D. Sedimentary beds directly over igneous or metamorphic rocks
   Answer: D. Sedimentary over crystalline rock. (Key point: Nonconformity)

4. Why can dams and tunnels leak at unconformities?
   A. Water moves along the unconformity plane
   B. The plane is impermeable
   C. Unconformities seal beds
   D. There are no beds
   Answer: A. The surface is a water path. (Key point: Seepage)

## Related in Engineering Geology I

- [Fault, parts, recognition, engineering problems](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Afault-parts-recognition-engineering-problems) — Engineering Geology I, chapter 5
- [Folds: recognition and civil engineering influence](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Afolds-recognition-and-civil-engineering) — Engineering Geology I, chapter 5