Here's why:
* Lack of Preferred Orientation: Conglomerates generally lack a preferred orientation of their clasts. This random arrangement prevents the development of a continuous, planar weakness that defines cleavage.
* Stronger Clast Bonds: The individual clasts are often cemented together strongly by the matrix material. This makes it difficult for the rock to break along a consistent plane.
However, there are some situations where a semblance of cleavage can occur:
* Weak Cementation: If the matrix is poorly cemented, the conglomerate might fracture along the weakest points, often around the clasts.
* Stress-Induced Fractures: Under significant stress, conglomerates may develop fractures that are somewhat planar. However, these fractures are usually irregular and not considered true cleavage.
Instead of cleavage, conglomerates tend to exhibit:
* Fracturing: The most common mode of failure, breaking along irregular, non-planar surfaces.
* Jointing: Development of fractures with no significant movement, often occurring due to regional tectonic stresses.
In summary, conglomerates generally lack a well-defined cleavage due to their heterogeneous composition and lack of a preferred orientation of their clasts. However, under certain conditions, they can exhibit fracturing or jointing.
