Here's why:

* Sandstone is a sedimentary rock formed from cemented sand grains.

* Shell fragments are common components of sand, especially in coastal areas.

* Feldspar and quartz fragments are also common constituents of sand, derived from the erosion of igneous and metamorphic rocks.

While other sedimentary rocks like conglomerate or breccia might contain shell fragments, they typically have larger, more angular fragments. Sandstone is characterized by its sand-sized, rounded grains.