Little Neston, being an extension of the Flintshire Coalfield across the Dee estuary. These Coal Measures rocks were formerly exploited by a small mining operation. The strata have a slight, generally easterly dip and are cut by numerous extensional faults most of which are aligned broadly north–south. For the most part the bedrock is poorly exposed being covered by superficial deposits of Quaternary age. Notable exposures of the Helsby Sandstone occur at Bidston Hill and at Red Rocks at the northwestern tip of the Wirral along with the tidal islands at Hilbre. Elsewhere Mercia Mudstone rocks outcrop prominently at Caldy Hill, Thurstaston Common and Heswall Dales. Much of the Wirral is covered by a mantle of glacial till, a legacy of the last ice age. Wind-blown sands cover the northern coastal margin. Low ground behind these sand are reclaimed tidal flat deposits which also extend into the heavily modified Birket which occupies a buried bedrock channel. This channel and others beneath the Dee and Mersey estuaries were formed in part by the southeasterly movement of Irish Sea Ice during successive ice ages. Low cliffs of the Kinnerton Sandstone at Burton Point are part of a relict shoreline, the Dee estuary having silted up during the post-glacial period. The former coast can be traced from Blacon northwest to Burton Point and thence to Parkgate where spring tides still reach the historic coastline. A well developed glacial drainage channel, known as the Deva Spillway cuts across the base of the peninsula between the two estuaries on either side of the Wirral and is interpreted as having played a major part in the deglaciation of the region in late-glacial/post-glacial times. Physical geography Although it has been stated that "it is difficult to find any work in which there is a written description of the exact area defining the Wirral Peninsula", historian Stephen Roberts defines it as "the peninsula which is bounded by the Dee and Mersey estuaries, Irish Sea and... the route of the Shropshire Union Canal between Ellesmere Port and Chester". This definition extends the original hundred slightly further east, to the River Gowy. The Shropshire Union Canal joins the Mersey at Ellesmere Port and the Dee at Chester. This canal technically makes the peninsula an island. In the north of the peninsula, the River Fender, Arrowe Brook and Greasby Brook drain into The Birket, which itself flows into the River Mersey