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The Lleiniog Sand and Gravel Formation, of late Devensian age, has been divided into four Members. The sediments are interpreted as the sequential products of the wasting of a single ice lobe. Member I formed where subglacial streams debouched into a pro-glacial lake and is noteworthy for the occurrence of a bedform, hitherto undescribed but which may be common in many outwash sequences; Member 2 consists of channel sands and gravels deposited by pro-glacial streams; Member 3 has the characters of a melt-out till deposited after a readvance; and Member 4 accumulated in pools of meltwater on the irregular surface of Member 3. Clasts above granule grade are dominantly of locally-derived Carboniferous rocks although a far travelled component from northern England and southern Scotland can be distinguished as well as a small but constant input of Lower Palaeozoic rock types from the Conwy area. The sand grade is dominated by quartz grains released from Permo-Triassic sandstones. Clay grade material has been derived from Carboniferous shales. 相似文献
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