Question

The igneous intrusion along the bedding plane of the coal seam, as indicated by P, is known as

• A. dyke
• B. washout
• C. sill
• D. seam split

Hint:

Compare how the intruded rock sits relative to the layering: does it run parallel to the bedding or cut across it?

Answer 1

The Correct Option is C

Step 1: Understanding the Question.
The figure shows layers of shaly sandstone, shale, coal and sandstone lying one above another, and a body of igneous rock marked P that has pushed in and spread out along the boundary between two of these layers, following the bedding rather than cutting across it. The question asks what this kind of intrusion is called.

Step 2: Key Formula or Approach.
Igneous intrusions are classed by how they sit relative to the layering (bedding) of the rock they invade. A body that forces its way in and spreads out parallel to the bedding planes is called a sill. A body that cuts straight across the bedding, climbing through different layers, is called a dyke instead. Washout and seam split are not igneous features at all; they describe erosion or splitting of the coal seam itself by sedimentary processes.

Step 3: Detailed Explanation.
In the figure, P sits right at the contact between the coal seam and the sandstone above it, and the intruded rock body spreads sideways along that contact rather than crossing up through the shale and shaly sandstone layers above. That sideways, bedding parallel geometry is the defining shape of a sill.
Option (A) dyke is wrong because a dyke would show as a near vertical body cutting across the layering, not hugging one bedding plane.
Option (B) washout is wrong because a washout is a channel cut into the coal seam by a later stream, filled with sandstone, and has nothing to do with molten igneous rock.
Option (D) seam split is wrong because a seam split is where a single coal seam divides into two thinner seams separated by non-coal rock, again a sedimentary feature with no igneous intrusion involved.
Only option (C) sill matches an igneous body that intrudes and spreads along the bedding plane of the coal seam.

Step 4: Final Answer.
An igneous intrusion that runs parallel to and along the bedding plane of a coal seam is called a sill.
This corresponds to option (C).