In storms waves sweep shells across the spits and into the mud flats behind.
A chenier is a beach ridge made of gravel, shell or sand, resting on top of mud and separated from the shore by a belt of intertidal mudflats. The southwest corner of the Firth of Thames is fringed by a 15 km-long and up to 2 km-wide coastal plain that accumulated in the last 4000 yrs. It is one of the best examples in the world of a combined gravel and shell chenier plain that has built up by the progressive addition of gravel and shell beach ridges on top of tidal mud flats. The gravel beach ridges form the narrower northern part of the plain.
The southern 2/3 of the plain is made of shell ridges/spits. The shell ridges originated intertidally, as sand and shell banks, during major northeast storms, which swept away the mud and concentrated the coarse material into a heap. These offshore banks were driven shoreward by later storms and eventually attached to the existing shell beach at their northern ends. From here, they developed into narrow shell spits that grew southwards, parallel to the coast, separated from the previous shell beach by a 50-200 m-wide depression. In the shelter of the shell spit, this elongate depression progressively filled with intertidal mud that may have been colonised by mangroves and eventually by high-tidal salt marsh, thereby adding new land to the plain. During high spring tides, some storm waves washed over the shell spit spreading feathery lobes of shell into the mud depression. These characteristic feathery lobes can still be seen in many places on the grazed older parts of the chenier plain today.
Between 7000 and 4000 years ago, when sea level was 1.5-2 m higher than today, the coastline at Miranda was up to 2 km further west, right up against the foot of the hills. Between 4000 and about 1000 yrs ago, as sea level dropped to its present level or slightly lower, much of the shell chenier plain, between Miranda and Kaiaua, was constructed. Each successive shell ridge was added seaward of the preceding one and the plain built out across the intertidal mud. The 4000 yr-old chenier up against the hill is 1.5-2 m higher than the 1000 yr-old one, and those of today. Together they document the decline in sea level since 4000 yr ago. In the last 1000 yr there has been little growth of the gravel plain, but in the south, additional shell spits have been added, as the chenier plain has grown southwards rather than seawards. This is believed to be the result of the stable and more recently rising sea level.