den wick

glacial deposits

 

 

 

 

 

 

 

 

 

 

 

 

Striated flagstone surface

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cliff tops emerging from beneath till

 

Scara Taing

Significance: the cliff sections at Scara Taing  (HY 369330) are important for the exposures of three superimposed tills and subjacent striated bedrock surfaces which provide evidence of fluctuating patterns of ice flow during the Devensian cold stage.

The lowest till, up to 3 m thick, is a brown, massive, matrix-supported diamict with dominantly pebble-sized clasts in a slightly silty sandy matrix. Local rock types predominate, derived from the Rousay Beds, with striated clasts of grey-coloured fine and brown-coloured coarse sandstone and dark grey silty sandstone. The diamict appears to include a few crumbs of shell. Striae on the underlying sandstones trend 296˚. The lower till is overlain by up to 4 m of red brown matrix-supported, massive diamict. The contact between the two tills is sharp and erosive. The middle till contains scattered pebbles in a silty sand matrix, including shell fragments. Clasts derived from the Rousay Beds are again dominant but the till includes increased numbers of red sandstone clasts derived from the Eday Beds and occasional chalk pebbles. The upper till is up to 4 m thick and has a sharp lower contact. The diamict is more rubbly than the underlying units and contains many subangular blocks of locally-derived, grey Rousay sandstones. The diamict appears generally massive but, in places, shows crude horizontal beds of gravelly diamict. Striae on the underlying sandstones trend 308˚. The upper 1.5 m of the unit has been strongly cryoturbated and includes erected and frost-shattered clasts. The upper till forms hummocky terrain inland, with elongate drift mounds trending SE-NW.

It is unique to see three superimposed till units at a single site on Orkney (Rae, 1976). The tills can be correlated on the basis of lithology and stratigraphic position with units elsewhere on the islands. A lower brown till, resting on bedrock, occurs at a number of sites on Shapinsay and East Mainland, and is succeeded by a widespread red brown and locally shelly till. Hummocky moraine, representing the latest phase of ice sheet glaciation, occurs at a number of localities in West Mainland, including Evie on the southern shore of Eynhallow Sound (Wilson et al. 1935).

The tills at Scara Taing appear to reflect the sequence of glaciation during the Devensian cold stage. The lack of any interbedded sediments at Scara Taing and other multiple till sites (Rae, 1976) suggests that all the tills may date from the last (Late Devensian) glaciation. The earliest phase recorded is that represented by the lower till, when ice moved NW out of Eynhallow Sound. Although no subjacent striae were recorded, the middle till, with its increased content of Eday Beds clasts and shell, probably represents a shift in ice flow towards the W. This is consistent with the observation of an older set of E-W striae, beneath SE-NW striae, at Grain (HY 384326) (Wilson et al. 1935), 1.5 km NE of Scara Taing. Regional patterns of striae and erratic carry also suggest that the red brown till was deposited by ice moving from ESE to E (Rae, 1976). The upper till relates to a late stage in glaciation when ice again moved NW out of Eynhallow Sound. The ice was active and still capable of significant erosion at this stage, as shown by the abundance of locally-derived blocks, but the distance of transport was small. The associated oriented hummocky moraine, which rises to c.50 m O. D. on the flanks of the Brae of Moan, suggests the presence of an ice margin no more than a few km W of the present shoreline.