UA Pits slumping, and subsidence
2
Urban Archaeology ...define, record, remove... Urban excavation factsheet 2 PITS, SLUMPING AND SUBSIDENCE © Chiz Harward, 2009, revised 2014 Urban Archaeology contact: [email protected] Pits are a common feature on most sites, and both cause and they are often affected by slumping and subsidence. How the pit is affected can tell you about how the pit was used, and disused. Slumping is a term that is generally used for stratigraphy that has been altered by both actual slumping, slippage, and by subsidence. Slumping is technically a later stratigraphic event to the original context, but is an example of where strict single context recording sometimes needs to be treated flexibly. Zone of blurring and slippage Water/roots/worms Blurring of edges of features Soft fills Pit fill Collapse into adjacent feature Floor slabs above pit hold the pressure for a time Voids/ cracks PRESSURE Soft fills compress PRESSURE PRESSURE Intervening strat falls into adjacent feature due to pressure. Rotational slumping of stratigraphy Filled Pit Wattle or plank lining Compacted primary fills Open pit Timber lining robbed? Sides collapse and fall to base Dumped fills Sides slump but mostly held by the fill PRESSURE Section view Cuts and interfaces are weaknesses which are exploited by water flow, roots and worms; over time a zone of slippage and mixing blurs the real cut edge of a feature. Layers of stratigraphy cut by the feature you are digging will appear in section after removal of the fill. BUT further cleaning is required to remove the slipped/blurred material edge and reveal the clean stratigraphy in section. Be careful not to overclean though! Stratigraphy forms without distortion
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Urban Archaeology factsheet on pits, slumping and subsidence
Transcript of UA Pits slumping, and subsidence
Urban Archaeology ...define, record, remove...
Urban excavation factsheet 2
PITS, SLUMPING AND SUBSIDENCE
© Chiz Harward, 2009, revised 2014 Urban Archaeology contact: [email protected]
Pits are a common feature on most sites, and both cause and they are often affected by slumping and subsidence. How the pit is affected can tell you about how the pit was used, and disused. Slumping is a term that is generally used for stratigraphy that has been altered by both actual slumping, slippage, and by subsidence. Slumping is technically a later stratigraphic event to the original context, but is an example of where strict single context recording sometimes needs to be treated flexibly.
Zone of blurring and slippage
Water/roots/wormsBlurring of edges of features
Soft fills
Pit fill
Collapse into adjacent feature
Floor slabs above pit hold the pressure for a time
Voids/cracks
PRESSURE
Soft fills compress
PRESSURE PRESSURE
Intervening strat falls into adjacent feature due to p r e s s u r e . R o t a t i o n a l slumping of stratigraphy
Filled Pit
Wattle or plank lining
Compacted primary fills
Open pit
Timber lining robbed?
Sides collapse and fall to base
Dumped fills
Sides slump but mostly held by the fill
PRESSURE
Section view
Cuts and interfaces are weaknesses which are exploited by water flow, roots and worms; over time a zone of slippage and mixing blurs the real cut edge of a feature. Layers of stratigraphy cut by the feature you are digging will appear in section after removal of the fill. BUT further cleaning is required to remove the slipped/blurred material edge and reveal the clean stratigraphy in section. Be careful not to overclean though!
S t r a t i g r a p h y f o r m s without distortion
© Chiz Harward, 2009, revised 2014 Urban Archaeology contact: [email protected]
Urban Archaeology ...define, record, remove...
Urban excavation factsheet 2
PITS, SLUMPING AND SUBSIDENCE
Pit filled with unconsolidated material and backfilled. High organic content (eg cess) is compressed over time
Building constructed on levelled ground
Uneven levelling indicates g r o u n d h a d a l r e a d y subsided
Compression of fills
Renewed effort to level area for new building; uneven thickness of levelling indicates contemporary subsidence
Overlying layers start to break up as ground subsides further
1 2
3 4
A Original fill compressed by weight of material above, often organic fills will be compressed most as they decay
B Subsidence of original backfill and levelling
C Earlier features slump sideways into softer pit
D Deposits may break up as they subside. They can rotate or even end up upside down!
E Layers slumping into pit which are thicker over the pit than to the sides. may indicate attempts to consolidate and level contemporary slumping
I Fills in slumped features may be on tilt implying original plane of deposition. They may have slumped themselves
H Slumped features -the base may be tilted, sides may ‘close up’ at top or be overhanging
G Layers which have ‘dropped’ are now discontinuous units but are ideally still dug in phase
F Interfaces between slumped layers can look like cuts, look at other sections to see whether this is the case
I
H
C
A
G
B
G
D
F
E
Sequence of subsidence
Processes of subsidence and slumping
