Answer;
This is because most likely some time a ago there use to be Ammonites living there and as they died there, they got fossilized.
Step-by-step explanation;
-Fossils of a marine animal called Ammonite are found in large numbers in the Kali Gandaki river in Nepal. Ammonites were sea animals having shells - either straight or coiled. When the Tethys sea disappeared, they were caught in the shale layers of clay and transformed into fossils. This is one of the proofs that the Himalayas were indeed once under water.