Dunes Completely Eroded Away Leaving Black Sand

Mile 12 beyond end of pavement on the wild beach of South Padre Island. The dunes here have been completely eroded away. What remains is black sand. This is magnetite, a common but small component of any normal sand. Magnetite is iron crystal, magnetic, and heavier than the rest of the sand grains so, when the rising sea level erodes the beach and washes sand out into the deep waters of the Gulf, the heavier magnetite particles are left behind.