Previous SiteNext Site

Mourne Plain Complex - Tullyframe PitDown
No picture
Summary Full report
Site Type: Pit
Site Status:
District:
Grid Reference: J32633168
Rocks
Rock Age: Quaternary (Late Midlandian)
Interest
Other interest: No data, Glacial deposit, Glaciomarine deposit, Glaciotectonic, ice lobe, moraine, outwash

Summary of site:

In its mid-section, the White Water is confined between Knockchree in the east and Rocky Mountain to the west. Tullyframe Pit is located in this narrowing, to the west of the river, at its confluence with an un-named stream draining the south eastern slopes of Finlieve.

The section exposed in this large sand pit is 530 m long and 23 m deep over an area of around 7.5 hectares and these deposits are situated on the northern fringe of the Cranfield Moraine (site 441). There are four facies (associations of sediment) in this thickness. The first is a tabular sequence of silt, sand and gravel beds. The sands become coarser upwards and ripple-marked surfaces with thin mud coverings occur at intervals. At the top the gravels are crudely bedded with uneven grain sizes and little matrix and contain the occasional thin seam of coarse sand. Isolated cobbles dot the topmost level where there are also cobble clusters. Sediments under the isolated cobbles are distorted. The second facies consists of indistinctly bedded, massive diamicts. Diamicts are beds with a mixture of widely differing grain sizes from clay at one extreme to boulders at the other and are most often associated with glacial activity. The deposit's maximum thickness is around 8 m with boulder lines and sandy partings loosely defining beds which are more clearly demarked with increased distance from the moraine crest. It is also cut by cobble and boulder-filled channels. The third facies is predominantly tabular beds of rhythmically bedded silts, grading into sand with ripple-marked surfaces. It totals 3.2 m in thickness and beds are less than 20 cm thick. There are dish and pillar structures up to 8 cm across, caused by the discharge of excess pore water soon after deposition. The top of the section is marked above an erosion surface by 1.8 m of stratified mud to pebble gravel. The fourth and final facies occupying the top 3 m of the pit is a crudely stratified diamict with a sandy matrix containing isolated large cobbles and boulders. There is also a channel filled with 2 m of stratified gravel. Lenses of chaotic gravels, partly supported in finer sediment but partly matrix-free, interrupt the diamict. In the top metre, isolated cobbles and boulders become more frequent and the diamict contains recognisable folds up to a metre high but declining in amplitude to the north. The overall picture to emerge from Tullyframe Pit is of the discharge of sediment-laden meltwater from the east end of the Cranfield Moraine into a body of standing water, though the temporary lake involved had a short and erratic history. The first facies is dominated by sediment washing out into the lake along a wide front with varying rates of discharge, sediment types and patterns of flow accounting for the rhythmic nature of the sands and silts. Debris pulses created the sands and gravels in the sequence while the larger, isolated cobbles and boulders were dropstones released from floating masses of melting ice. The diamicts of the second facies were laid down largely as dense debris flows plunging across the bed of the lake as dense, cold masses settling within a short interval, hardly giving time for gravity to sort the large particles. Winnowing of fine debris between flows left the boulders as discrete layers separating them. The third facies reflects a more limited sediment supply but the rhythms show deposition immediately in front of the moraine probably from cold density flows across the lake bed. Such sediments usually trap large quantities of water and the dish and pillar structures record the release of the excess. Turbid flows settle under gravity but the massive sands are the product of general outwash from the moraine, probably across its entire front. Debris flows of dense sediment formed the diamicts of facies four and the leading fronts of these flows (noses) probably carried the largest rock fragments. The folds at the top of this facies association were formed as saturated but cohesive sediment beds slumped. At its maximum the lake level would not have topped the 90 m crest of the moraine and the eastern shore would have been defined by the lower slopes of Knockchree Mountain, the western by the eastern flank of Rocky Mountain. How far up-valley it reached is not clear. There are no notches on the valley walls to mark shoreline erosion that might have defined the area of the lake more clearly, almost certainly because it was so short-lived. All available evidence suggests that it would not have been smaller than 1.2 square kilometres and was quite possibly much larger.


Previous SiteNext Site

|Home|