In the right atrium, the auricle and anterior wall exhibit obvious muscular ridges called

In the right atrium, the auricle and anterior wall exhibit obvious muscular ridges called MUSCULI PECTINATI.

Musculi pectinati or pectinate muscles are identified as specialized intracardiac muscle. The muscular ridges are like combs. These ridges help the right atrium dilate without much walls stress during adverse loading conditions.

The answer would be sand and gravel