boulder choke

{{Short description|Collection of large rocks or rubble that obstructs the passage of a cave or mine}}

File:Clearing a boulder choke in Stile Pot.jpg, Gaping Gill]]

A boulder choke is a collection of large rocks or rubble that obstructs a shaft or passage in a cave or mine. In order to progress through passages cavers often need to negotiate or clear boulder chokes.

Boulder chokes can be formed by rockfall, stream deposition, or mining debris. Due to the nature of their formation boulder chokes may be unstable and can prove difficult to navigate and map due to their complexity.{{Cite web|title = The Cave of The Wild Horses, Kilcorney, Co. Clare, Ireland - The 1983 Extensions|url = http://www.ubss.org.uk/resources/proceedings/vol16/UBSS_Proc_16_3_215-220.pdf|website = www.ubss.org.uk|accessdate = 2015-04-21}}

In order to clear a boulder choke the rocks obstructing the passage may need to be removed, often requiring breaking up first either mechanically or using a small explosive.{{Cite web|url = http://www.outsideonline.com/1909601/how-low-can-you-go|title = How Low Can You Go?|website = Outside Online|publisher = Outside|last1 = Craig|first1 = David|first2 = Jason|last2 = Daley| date=January 2002 |accessdate = 2015-04-21}} The Three Counties System became the longest cave in the UK when cavers succeeded in clearing a route through a boulder choke for {{convert|140|m}} to connect two known cave systems, an exercise that took three years.{{cite news|last1=Wainwright|first1=Martin|title=Potholers break through the final Three Counties link|url=https://www.theguardian.com/uk/the-northerner/2011/nov/07/caving-potholing-three-counties-system-gaping-gill-white-scar-caves-edouard-martel|accessdate=3 April 2017|work=The Guardian|date=7 Nov 2011}}

Boulder chokes can be hazardous, as unstable boulders can unexpectedly move, potentially crushing or trapping cavers.{{Cite web|url = http://cavingnews.com/20111107-three-counties-system-connects-to-create-uks-first-100-kilometre-cave-system|title = Three Counties System Connects to Create UK's First 100 Kilometre Cave System|accessdate = 2015-04-21|website = Caving News|archive-date = 2013-01-16|archive-url = https://web.archive.org/web/20130116222846/http://cavingnews.com/20111107-three-counties-system-connects-to-create-uks-first-100-kilometre-cave-system|url-status = dead}} In May 1988, for example, three cavers were killed in a boulder choke in Ease Gill Caverns.{{cite journal|title=Easegill Tragedy |last=Boothroyd |first=Colin |journal=Descent |date=June–July 1988 |issue=82 |page=12}} In caves prone to flooding, a narrow and contorted path through a boulder choke can fill up with water quickly, preventing progress.{{Cite web|title = Agen Allwedd|url = http://www.mlcmac.talktalk.net/aggy.htm|website = www.mlcmac.talktalk.net|accessdate = 2015-04-21}} In 1972, ten members of the University of Leeds Speleological Association were trapped behind a complex flooded boulder choke in Langcliffe Pot, resulting in a 44-hour-long trip.{{cite journal|last1=Monico|first1=Paul|title=Diving in the Dementor Sump and "The Rescue" 1972|journal=ULSA Explorations Journal II|date=1989|pages=39–42}}