Lake District rock bars

in Rock

The succession of small basins with intervening breaks coinciding with rock bars or morainic ridges is a feature of many valleys which once witnessed the passage of ice. Another similar ungraded valley is that of nearby Kent mereLake District Honeymoon Hotel
. Here a much bigger lake once existed in the vicinity of Kent mere Hall in which diatomaceous* deposits gradually accumulated and led in time to its infilling. A major break in the long profile of the valley occurs north of Kent mere Church  where a pronounced rock bar, its edge plucked and accentuated by moving ice, is developed at the junction of the soft and well cleaved Brow gill Slates with the harder volcanic rocks. Against this bar, the glacier rested during its retreat stage and laid down a great boulder moraine just north of the church. Upstream a further rock step occurs near Kentmere Reservoir , although the building of the dam has masked much of this glacially derived feature.

Compared with the outlying valleys like Kentmere and Long Sleddale, Great Langdale experienced a much fuller and more intense glaciation over a long period. Lying in the lee of the highest peaks like Scafell and Great Gable, the valley head never lacked the heavy snowfall to nurture active glaciers. The area also felt the full force of outwardly moving ice streams when the whole central region was blanketed by a more or less continuous ice dome. Even after the Ice Age had 'officially' ended, its upland corries once more saw the growth of small glaciers for a short time about 8800 B.C. As at the head of Borrowdale and elsewhere, these grew and ultimately spilled over and moved slowly down into the trough end of Great Langdale at Mickleden to a height of only 400 ft.

There the glacier laid down the same agglomeration of hummocky drift, with hillocks and intervening marshy hollows occurring over a wide area . This was but the final dying phase of glacial activity in the valley. Earlier a more intense glacier action had already transformed what was originally a small V¬shaped valley eroded by normal river action into the Ushaped trough we know today. With its steep rocky sides, scree slopes, abrupt combe end and marshy floor broken only by rock bars, it fulfils all the requirements of a typical glaciated valley. Microfeatures like roche moutonees, great boulder trains left behind after the ice melted and striations* on the exposed rock surfaces, all occur as further evidence of intense local ice activity.

Many of the major features seen by those visiting Great Langdale date from a tinle when a great glacier occupied the whole valley during the Last glaciation. There is evidence to suggest that at its maximum stage of develop¬ment the upper surface of the glacier lay at a height of about 1,400 ft. At this time the snout lay well beyond the mouth of Great Langdale. While advancing in the direction of Ambleside and the head of Windermere it was constantly fed by ice accumulating in its source region around Bowfell. In this active state the sole of the glacier gouged out hollows in the valley floor; these later became the sites of lakes like Elterwater. The present lake is very much the shrunken remnant of a larger original feature. Infilling has taken place and its irregular reedy margins show that the process is still going on and in time no doubt the whole lake will disappear.

This has already been the fate of a similar lake which once occupied the valley floor upstream from Chapel Stile. The glacier, for all its great erosive powers when at the height of its activity, never quite succeeded in removing the harder rock bars which lay across its path as at Chapel Stile and Skelwith Bridge. Both rock bars coincide with beds of a toughened volcanic ash. The ice, by plucking at the well cleaved slates and jointed lava beds on either side of the rock bars, tended to accentuate the features rather than remove them. With such prominent obstacles in its path the river has been forced to cut deep gorges through the rock bars. That at Skelwith Bridge is well known and much visited. Certainly after heavy rain the waterfalls make impressive viewing when seen from the footpath which wends its way up through the wooded glade of the gorge.

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Adrian vultur writes for Lake District Honeymoon Hotel

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Lake District rock bars

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This article was published on 2011/01/11