Cook Inlet mudflats are made of fine glacial silt. During low tide the mud flats can be hundreds of yards wide yet during high tide they are completely underwater. 
 
Alaska’s Turnagain Arm is part of the Cook Inlet and boasts the second largest tidal range in North America, up to 38 feet. At low tide, large mud flats are visible. It is also one of the few places you can see a bore tide, which is a rush of seawater into a narrow inlet. Bore tide waves can be 6-10 feet tall and rush at speeds of 10 to 15 miles per hour.