I.G.Kenyon

A Bed
A layer of rock separated from the layer above and below by a bedding plane
A bed represents a single unbroken episode of sediment accumulation
Beds vary in thickness from
1cm to many tens of metres
Beds 2 to 5cm thick are called flags or flagstones
Beds may occur in uniform thicknesses over large areas or pinch out laterally

Beds & Bedding Planes, Blue Hills, Near St.Agnes
Bedding Planes
One bed
£1 coin for scale

Bedding Plane
Defines the top or bottom of a bed
Represents a change in the nature of sedimentation a change in the rate or type of sedimentation a pause where no sediment is deposited a period of erosion where some sediment is removed

Lamination
A layer of sediment <1cm thick
Common in argillaceous rocks such as siltstone and shale
Individual laminations may be just 1mm thick or even less
If the sedimentary unit is >1cm thick it is a bed

Laminations in Devonian Mylor Beds, Porthleven
1cm
Laminations here are <1mm thick
Difference in colour explained by variation in amount of organic/carbonaceous matter incorporated into the sediment
Mineral content mainly clay minerals such as kaolinite, illite and serecite

The Law of Superposition
First proposed by Nicolaus
Steno in the 17 th Century
If one bed of sediment lies on top of another, then the one above must be the younger
This assumes the beds have not been overturned due to earth movements
Sedimentary structures collectively known as ‘way-up criteria’ can be used to decide if the beds have been overturned or not

Graded Bedding
A bed which displays a fining upwards sequence from the base.
3cm
3cm
The fining upward sequence may be produced in several ways

The Formation of Graded Bedding 1
Progressive settling of grade sizes from coarse to fine in comparatively calm bodies of water
Example-greywackes on the continental slope, where a poorly sorted sediment is deposited rapidly
The larger, denser rock fragments and sand size particles sink first, followed by the smaller and lower density silt and clay particles
Greywackes are deposited by turbidity currents which are often initiated by minor seismic events

Formation of Graded Bedding by Turbidity Currents
Cross section

Graded Bed with an Erosional Base
Irregular surface with laminations of shale beneath truncated in places
Fining upwards
Represents an abrupt change from the much finer grained sediment underneath

Cross Bedding
Also known as Current Bedding and False Bedding
If very large scale it is termed Dune Bedding
If very small scale it is termed Cross Lamination
In each case the sediment is being moved and accumulated at an angle to the principal bedding direction
Produced by a uni-directional current of wind or water moving sediment as a series of asymmetrical ripples or dunes

The Formation of Cross Bedding
Topset beds are truncated
Erosion surface
Foreset beds
2m
Bottom set beds are preserved
Layers curve in towards the horizontal (asymptotically) at the base of a cross bedded unit
Erosion surface-truncated topset beds
Foreset beds
10cm
Bottom set beds

Dune Bedding – Large Scale Cross Bedding
2m

Large Scale Cross Bedding – Dune Bedding
Topset beds are truncated
Palaeo-wind direction indicated by yellow arrows
Foreset and bottom set beds preserved
People for scale

Herring Bone Cross Bedding
Upper Unit
Middle Unit
Lower Unit
Penknife for scale
Represents a current reversal through 180 °. Blue arrows indicate the direction of sediment movement in each of the 3 units above

Cross Lamination (Very small scale cross bedding)
Truncation/erosion surface of topset beds
Pen top for scale
Current direction
Approximate base of cross laminated unit
Individual laminations 2 to 4mm thick
Fine sandstone unit, Compass Point near Bude

Included/Derived Fragments
Younger upper series
Derived fragments from older lower series unconformity
Older beds may be eroded before the deposition of the next bed in the sequence
The eroded fragments are then included as clasts in the bed above
1m
Lower older series
Younger upper series
Lower older series
Derived fragments

Mud Cracks
Formed when sediment is exposed to the atmosphere
Common in tidal flats, mudflats and playa lakes
Mud cracks form as desiccation polygons
The sediment dries out and shrinks as water is evaporated from it
Contraction centres develop and a polygonal pattern of cracks develop
Analogous to columnar jointing in cooling lavas
Note how the edges curl up to accentuate the V shaped gap between them
30cm

Mudcracks
The mud cracks are widest at the surface tapering to a point at a depth of 0.5 to 2.0 cm
Often later infilled with finer, wind blown sediment of a different colour or calcareous material if in a playa lake

Mud Cracks and Rain Pits
Rain pits formed by impact of raindrops on an exposed sediment surface. They appear as small rounded depressions up to 1cm in diameter, sometimes with a small raised rim.
Rain pits mark the top of the sediment

Ripple Marks-Symmetrical
Mark the top of the bed and imply the sediment was under the influence of wave action
Minibus key for scale
Carboniferous sandstones, Compass Point near Bude

Ripple Marks- Asymmetrical
Formed by a uni-directional current such as a river or the wind, the downstream or downwind side will have the steeper face

Ripple Marks- Asymmetrical
Asymmetrical ripples in the flood sediment covering the footpath by the
River Ouse, York. The direction of current is from top right to bottom left.