Geology
The estuary is unusual in that comparatively little water occupies so large a basin. One theory is that larger rivers such as the Severn and/or Mersey once flowed into the Dee. The current view is that the estuary owes its origin to the passage of glacial ice southeastwards from the Irish Sea during successive ice ages, eroding a broad and shallow iceway through the relatively soft Triassic sandstones and Coal Measures mudstones which underlie the area. The inner parts of this channel were filled by glacially derived sands and gravels long ago and infilling by mud and silt has continued since. It is also thought that prior to the ice ages, the estuary received larger river flows as the upper Severn flowed into the Dee near Chirk. For a period, the Mersey may also have flowed into the Dee by means of a channel which it cut through the base of the Wirral Peninsula.
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