Almscliff Crag

© Craven & Pendle Geological Society

Almscliff Crag © P. Kabrna 2004

Tor Formation

For the origin of tors see Brimham Rocks.

General setting

The lower reaches of the Wharfe Valley between Otley and Wetherby are characterised by a broad plain. The E-W running hills that flank this valley reveal, in a series of sporadic outcrops, a series of Millstone Grit (Namurian, Upper Carboniferous) sandstones dipping generally southwards. The valley itself is underlain by a 250 m-thick shale succession, of Arnsbergian to Alportian age, that lacks all but a few minor sandstones. The ease of erosion of this part of the succession has, not surprisingly, caused the Wharfe to flows eastwards, deviating from its southwards passage in its upper reaches in the Yorkshire Dales.

The resilient sandstones of the Kinderscout Grit Group form a major ridge running for 30 km unbroken between Otley and Ilkely, on the south side of the Wharfe Valley, and there are many famous local landmarks along its length, such as Otley Chevin and the Cow and Calf. In contrast, the northern ridge, formed of the Almscliff Grit has very few natural exposures except for Almscliff Crag which stands out like a sore thumbb from the otherwise gentle slope. The reasons behind this exposure essentially relate to the fact that the sandstone greatly increases its thickness for around 150 m along strike at this location.

Paul Wignall pointing out growth fautls: Almscliff Crag © P. Kabrna 2004

Lithostratigraphy

The lower part of the Namurian stratigraphy of the Pennines, and northern England in general, is characterised by a series of south-westward directed deltaic advances into a linked series of subbasins. As was shown by Reading and Ramsbottom nearly 40 years ago each successive deltaic sandstone body tended to step-over the previous one so that the maximum thickness of each successive sandbody lay to the SW of the previous one. This advance was also marked by a long-term transition from turbiditic to deltaic sandbodies. The Almscliff Grit (known as the Marchup Grit in adajcent exposures to the west) is the oldest deltaic sandstone body in the area (Pendleian Stage, E1c zones) and it is regionally not very extensive; the Almscliff Crag location is essentially its southern-most limit, although the sandbody is more extensive to the west and north west where it is known as the Grassington Grit Formation i.e. a braidplain facies.

History of Research at Almscliff Crag

The geological survey memoir of the Leeds district, surveyed in 1936-1937 by W. Edwards, has the Almscliff Crag location bounded by two major NW-SE trending faults that extend into the Wharfe Valley (note that, at this time, the Grit was also assigned an E2 age). However, subsequent study by Ian Chisholm revealed that this unusual exposure could be more readily explained by local thickening of the delta sandstone body by a large listric growth fault (Chisholm 1981). Key evidence for this reinterpretation came from the local change in dip from a gentle southerly dip to a steeper north-easterly one in the NE corner of the outcrop. This was said to reflect a pronounced rollover anticline developed in the hanging wall of a major growth fault that trends around the NE side of the exposure (see map). Utilising a series of cross sections along the dip slope, Chisholm was also able to demonstrate that the 65 m thickness of the Almscliff Grit locally thickens to 100 m at the Crag, the extra 35 m occurring in the base of the Grit. Finally, at location 6 (see map), Chisholm noted that broad grooving occurs on a SW-dipping plane which he thought to be minor faults with soft sediment slickensides related to the movement on the main fracturee (p.67). In fact, this may be the actual main listric fault surface.

Almscliff Crag was re-examined by Andy Sims during the course of his PhD research at the University of Leeds (1989). He found considerable additional evidence to support Chisholmms interpretation. Most of this is found on the main NE face where a whole series of synthetic and antithetic faults are well displayed. These faults can clearly be seen to have been developed prior to cementation movement appears to have occurred by grain rolling, in zones up to 5 cm wide, rather than grain breakage. There is also evidence for fluid escape up several of the faults and several broad patches of homogenised sediment may point to more general fluid escape. Finally, none of the faults extend to the top of the outcrop clear evidence for a syn-sedimentary history.

Final thoughts

The Chisholm-Simms model undoubtedly explains many of the features of Almscliff Crag but, in the general context of growth fault records, this exposure is unusual. All workers are generally agreed that the Almscliff Grit records truly fluvial deposition probably in a proximal delta-top location. However, the vast majority of growth faults, both ancient and modern are developed on the upper delta slope and commonly affect and control the occurrence of distributary mouth bar deposition. The gravitational instability and the loading of distributary sands on less dense lower delta slope siltstones is clearly the key to this developmental locus. It is possible that the Almscliff delta had an abrupt seaward termination and was steep-fronted, so that the transition from delta top to basin occurred in just a few kilometres. The fact that an oil company borehole (Weeton-1), taken only 4 km to the south of Almscliff, found no evidence for the Grit would tend to support this.

© Paul Wignall Ph.D. (University of Leeds)