Detailed Instructions: In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.
Q: Passage: Realignment of the River Glaven means the ruins are now to the north of the river embankment, and essentially unprotected from coastal erosion, since the advancing shingle will no longer be swept away by the stream. The chapel will be buried by a ridge of shingle as the spit continues to move south, and then lost to the sea, perhaps within 20–30 years.The ridge of shingle runs west from Weybourne along the Norfolk coast, before becoming a spit extending into the sea at Blakeney. Saltmarshes can develop behind the ridge, but the sea attacks the spit through tidal and storm action. The amount of shingle moved by a single storm can be "spectacular"; the spit has sometimes been breached, becoming an island for a time, and this may happen again. The northernmost part of Snitterley village was lost to the sea in the early Middle Ages, probably due to a storm.In the last two hundred years, the maps have been accurate enough for the distance from the ruins to the sea to be measured. The 400 m (440 yd) in 1817 had become 320 m (350 yd) by 1835, 275 m (300 yd) in 1907, and 195 m (215 yd) by the end of the 20th century.  The spit is moving towards the mainland at about 1 m (1 yd) per year; and several raised islands or "eyes" have already been lost to the sea as the beach has rolled over the saltmarsh. Landward movement of the shingle meant that the channel of the Glaven, itself excavated in 1922 because an earlier, more northerly course was overwhelmed between Blakeney and Cley, was becoming blocked increasingly often. This led to flooding of Cley village and the environmentally important freshwater marshes. The Environment Agency considered a number of remedial options. Attempting to hold back the shingle or breaching the spit to create a new outlet for the Glaven would be expensive and probably ineffective, and doing nothing would be environmentally damaging. The Agency decided to create a new route for the river to the south of its original line, and work to realign a 550 m (600 yd) stretch of river 200 m (220 yd) further south was completed in 2007 at a cost of about £1.5 million.Managed retreat is likely to be the long-term solution to rising sea levels along much of the North Norfolk coast.  It has already been implemented at other important sites like Titchwell Marsh.
A:
What is causing several raised islands to become lost to the sea?