During а geоlоgicаl survey оf аn exposed coastal bluff, students discover a rock layer that is light gray to whitish in color and relatively hard. The rock contains abundant fossil fragments of shells, corals, and crinoids embedded directly within its fine-grained matrix. When a small sample is tested with dilute hydrochloric acid, it effervesces vigorously, producing visible bubbles. The rock does not feel gritty, and it breaks irregularly rather than along flat, thin layers. Based on these observations, which type of rock has most likely been found?