In geology, which of the following is not considered a primary sedimentary structure found within sedimentary rocks?

Introduction / Context:
Sedimentary rocks preserve many features that tell geologists about the environment in which the sediments were deposited. These features are called sedimentary structures. Knowing which features are true sedimentary structures and which are simply physical properties of the rock is an important part of introductory geology.

Given Data / Assumptions:

• The question asks which option is not a sedimentary structure.
• Options include graded bedding, rock colour, ripple marks, mud cracks and cross bedding.
• We assume basic knowledge of common sedimentary structures.

Concept / Approach:
Sedimentary structures are features formed during or shortly after sediment deposition, such as bedding, cross bedding, ripple marks and mud cracks. They reflect processes like current flow, settling of particles, drying and tidal action. Rock colour, however, is not classified as a structure. Colour may give clues about composition or oxidation conditions but is treated as a physical property, not a structure formed by depositional processes. Therefore rock colour is the feature that does not belong in the list of sedimentary structures.

Step-by-Step Solution:
Step 1: Recall examples of classic sedimentary structures taught in textbooks. Step 2: Identify graded bedding as layering where particle size changes from bottom to top, which is a recognised structure. Step 3: Recall that ripple marks are small ridges formed by moving water or wind, and that mud cracks form when wet mud dries and contracts. Step 4: Recognise that cross bedding is inclined layering within beds, formed by shifting dunes or ripples. Step 5: Notice that rock colour does not describe a three dimensional depositional feature but simply a visual property.

Verification / Alternative check:
Look at any geological field guide or diagram of sedimentary structures. You will see terms like bedding planes, cross bedding, ripple marks and mud cracks. Colour is often listed separately under description of rock but not under structures. This independent check confirms that rock colour is not a sedimentary structure.

Why Other Options Are Wrong:
Option A, graded bedding, is a true sedimentary structure where grain size varies through the bed, usually finer upwards. Option C, ripple marks, are small wave like features on bedding surfaces formed by moving fluids. Option D, mud cracks, form polygonal patterns when muddy sediments dry and shrink. Option E, cross bedding, represents inclined layers within beds created by migration of ripples or dunes.

Common Pitfalls:
A frequent mistake is to think that any observable feature in a rock, such as colour, must be a structure. In geology, structures specifically refer to shapes, orientations and patterns produced by physical processes. Properties like colour and hardness are important but belong to a different descriptive category. Remembering this distinction helps you quickly identify rock colour as the odd one out.

Final Answer:
The option that is not a sedimentary structure is Rock colour.