Fiord, fyawrd, also spelled fjord, is a long, narrow, winding inlet or arm of the sea. Fiord is a Norwegian word, applied to the deep bays and inlets along the ragged and mountainous coastline of Norway. Geologists believe that rivers cut these fiords, and glaciers deepened them millions of years ago. Most fiords have steep, rocky walls with thick woods and foaming, roaring waterfalls. Most fiords also have shallow sills (underwater ridges) at their mouths that become more deeply submerged further inland. Small stretches of fertile farmland lie below some of the fiord walls.