Cave, Arch and Stack

• Hydraulic action is when the sheer force of water forces air into cracks causing the rocks to be dislodged 
• Abrasion is when the rocks, sand and shingle are thrown against cliffs break/scrape rocks off 
• Solution is when salts and acids in the water dissolve minerals in the rock 
• Headland with weaknesses such as joints, faults or cracks is eroded by the sea to form firstly caves 
• In some cases, a blowhole can form in the roof of the cave as compressed air is pushed upwards by the power of the waves, causing vertical erosion
• Over time, erosion on both sides of the headland cuts through the back wall and enlarges the cave to create an arch 
• Continued erosion at the foot of the headland and the effects of vibrations on the roof of the arch weakens it, eventually resulting in the collapse of the arch roof, leaving a stack isolated from the headland