6 July 2013 - Leader: Peter Robinson
The first stop on this interesting trip was at Harwood Dale Moor SSSl where a recent temporary excavation exposed a section through the soil. A dark organic rich soil, black and peaty at the base was underlain by a light grey gley soil with iron staining in irregular patches. This older soil was considered to possibly be a plinthite formed in a warm (subtropical) climate during the Ipswichian interglacial.
We then drove to view polygons in Broxa Forest. This example of patterned ground of Devensian age is made visible by the vegetation. Slight mounds of heather and mosses are surrounded by narrow, poorly drained strips characterised by bog cotton and sphagnum. These were originally noted by Dimbleby who worked for the Forestry Commission and left the area open with few trees in order to better preserve the site.
We ended the morning by a visit to Seive Gill, now dry but formerly an impressive tributary melt water channel leading into the Forge valley. The vertical sided gorge below the former waterfall exposes the Hambledon Oolite.
After lunch at the Forge Valley Inn we walked up onto the ridge formed by the West Ayton gravel terrace and discussed the origin of this feature. The gravels do not contain erratic pebbles but are composed entirely of Jurassic lithologies from the North Yorkshire Moors. They were probably deposited by the melt water from the Forge Valley being constrained to flow westwards between the base of the Corallian dip slope and the ice tongue in the Vale of Pickering. In the afternoon the quarry manager showed us around Hanson's Sand and Gravel Quarry near Wykeham.
The coarser washed and graded gravels were full of erratics including andesites (? from Cheviot), granites, schists, and Carboniferous limestones clearly indicating that they were deposited in the melt out moraine of the ice tongue. Most of the sequence is under water but post glacial sediments were seen above the water table.
Below the modern soil was a peaty layer with lignite and well-preserved leaves underlain by a pale grey clay with roots.