3 .sx SEDIMENTOLOGY .sx ( a ) Unit 1 .sx This unit consists predominantly of gravelly lithofacies represented by massive to crudely horizontally stratified , medium to coarse sandy gravels ( facies Gm [Miall , 1977] ) and subsidiary planar cross stratified sets of fine gravel ( facies Gp) .sx The sandy matrix is grey in colour .sx The unit has a maximum observed thickness of 2.8 m ( Section 1 , Fig. 4) .sx The base of the unit was not seen .sx The gravels typically form laterally persistent sheets ( 0.2-0.7 m in thickness ) which lie on erosional planar or occasionally concave-up surfaces .sx Internally , the gravels display well marked fining-upwards sequences ranging upwards from coarse clast-supported gravels at the base to finer matrix-supported gravels ( Section 1 , Fig. 4) .sx Individual sheets are separated by lenses of planar cross-stratified sets ( up to 0.4 m high ) of medium to fine sand ( facies Sp) .sx These sets often display reactivation surfaces , marked occasionally by the presence of a pebbly foreset .sx Also noted from within Unit 1 are several ice-wedge casts .sx In contrast to the grey sands within the basal part of Unit 1 , towards the top of the unit there are occasional beds of generally structureless red-brown sands similar to those of Unit 2 .sx Eighteen palaeocurrent measurements taken from the planar sand foresets within Unit 1 range from 32 degree to 220 degree , with a mode in the 60-75 degree range .sx ( i ) Interpretation .sx The basal sands and gravels of Unit 1 represent a high energy fluvial environment .sx It is suggested that the gravels formed as longitudinal/transverse bars at high stage in channels with high width to depth ratios .sx The majority of the gravels ( facies Gm ) probably moved at high stage as gravel sheets ( Eynon and Walker , 1974) .sx As high stage flows waned the coarse gravels at the base of each sheet were deposited .sx These gravels then acted as a nucleus for further deposition and progressively finer gravels were deposited as flow strength declined .sx This process of fining-upwards deposition may have been assisted by the vertical growth of the bar itself , which led to decreasing flow depth ( Hein and Walker , 1977) .sx Occasionally these gravel sheets developed avalanche faces ( facies Gp ) possibly due to the waning of the flood event resulting in the development of separation eddies in the lee of the bar , or due to the migration of the bar into deeper water areas or areas of flow expansion ( Hein and Walker , 1977) .sx During reduced flows sedimentation would have been restricted to lateral channels and channels cut along the upper surfaces of the bars ( facies Sp) .sx The presence of reactivation surfaces within the sands suggests that energy conditions were variable within the lower stage flows .sx The predominantly grey matrix associated with the lower cross-stratified sands of unit 1 suggests a source area for the sediment predominantly from the Jurassic rocks to the south of the site ( ) .sx This conclusion is supported by the palaeocurrent measurements which suggest a flow from the south-west .sx The presence of intraformational ice-wedge casts suggests that deposition was , at least in certain areas , discontinuous as the development of ice wedge casts is not possible close to the active river channel ( French , 1976) .sx Similar sequences to that described above have been recorded from many terrace gravel sites within southern Britain ( Bryant , 1983 ; Dawson , 1985 ) and are attributed to periglacial river systems where flood generation is largely the result of spring snowmelt .sx It seems probable that Unit 1 represents a similar environment .sx figure&caption .sx ( b ) Unit 2 .sx This unit is predominantly a well-sorted red-brown medium to fine sand with occasional gravels .sx The unit has a maximum observed thickness of 3.2 m ( Section 1 , Fig. 4) .sx The bounding surface between Units 1 and 2 is erosional .sx The lower parts of Unit 2 , Unit 2a , comprise variable sequences of planar cross-stratified ( facies Sp ) sands , ripple laminated sands ( facies Sr ) and large scale trough cross-stratified sands ( facies St) .sx The planar cross-stratified facies usually comprise isolated sets up to 20 cm thick and occasionally cosets up to 50 cm thick .sx Rolled clay balls ( 1.5 cm diameter ) of Mercia Mudstone lying upon foreset laminae of the sands ( facies Sp ) are present in places .sx Seven palaeocurrent measurements taken from the planar cross-stratified sands in Sections 1 and 2 range from 80 degree to 195 degree with a modal class range of 165-180 degree .sx The trough cross-stratified sets are up to 1m thick , although only thinner sets were recorded from the accessible sections ( Section 3 , Fig. 4) .sx These lithofacies tend to lie on concave-up erosional bases cut , in places , into the underlying Unit 1 .sx Cyclic sedimentary sequences occur within the upper parts of Unit 2 .sx Unit 2b ( Section 4 , Fig. 4 ) comprises basal horizontally-bedded sands with alternating coarser and finer beds ( facies Sh ) rising into ripple-drift dominated sands ( facies Sr) .sx Within each cycle the angle of climb of climbing ripples steepens up-sequence , commonly being near-vertical at the top with both stoss and lee side preservation ( transition from type A to type B of Allen , 1982) .sx Occasionally the cycle culminates with near vertical type S climbing ripples .sx Finally , fine-grained laminae cap each cycle , the uppermost of which marks the contact of Unit 2 with Unit 3 .sx This bounding surface appears to be conformable throughout the whole exposure , preserving the upper surface of the ripples .sx ( i ) Interpretation .sx Unit 2 represents a lower energy environment than that of Unit 1 .sx The observed occurrence of the red-brown sands of Unit 2a within the underlying Unit 1 suggests that , at least initially , deposition of Unit 2 was pene-contemporaneous with deposition of the upper parts of Unit 1 .sx The sands of Unit 2a ( Sections 2 and 3 , Fig. 4 ) display sedimentary structures consistent with fluvial deposition and given their association with the underlying Unit 1 are interpreted as representing a relatively lower energy fluvial environment .sx The large trough cross-stratified sets possibly represent dunes occupying the deeper parts of the active channel , indicated by the underlying concave - up base .sx The planar cross-sets possibly developed by the migration of linguoid or transverse bars with ripple cross-lamination developing in shallower areas of the active channels ( Miall , 1977) .sx Palaeocurrent measurements from Unit 2a suggest a flow dominantly from the north-west .sx This flow pattern is consistent with a sediment source to the north and west within the Triassic sands ( Bromsgrove Sandstone and Kidderminster Formation ) in the area around Birmingham ( ) .sx The cyclic sediments of Unit 2b ( Section 4 , Fig. 3a ) are interpreted as turbidites deposited within a lake .sx Within each cycle parallel-laminated sands pass rapidly upwards into ripple drift sands , indicating a progressive decrease in flow velocity .sx The succession from type A to type B climbing ripples is interpreted as the result of increasing sedimentation rates in response to the continuing trend of a decrease in flow strength ( Allen , 1972) .sx Where present type S climbing ripples represent very high sedimentation rates .sx The draped silt/clay laminations which complete each cycle indicate further decline in flow strength , with sediment settling out from suspension .sx This declining flow strength cycle is typical of turbidity currents and is considered to record the B-C-D transition of a typical Bouma sequence ( Bouma , 1969) .sx Comparable sedimentological sequences to that within Unit 2b at Snitterfield recorded from Lake Malaspina ( Gustavson , 1975 ) and glacial Lake Hitchcock ( Gustavson et al. , 1975 ) were considered to represent sedimentation on or near to the pro-delta slope .sx Thus a similar depositional environment is envisaged for Snitterfield with the cyclic nature of sedimentation being controlled by turbidity currents down the delta slope .sx The lack of a full Gilbert-type delta sequence , in particular the lack of delta foresets , suggests that either there was not enough time for a complete delta sequence to develop , or that not enough sediment was arriving at the site to build a substantial delta .sx ( c ) Unit 3 .sx The lower part of Unit 3 consists of rhythmitic coarse-fine sands and silts .sx The nature of the rhythmites changes upwards through the profile .sx At the base of the sequence each consists of a sand layer , grading upwards into finer silts which occasionally grade upwards into finer-grained , graded clay layers .sx Towards the top of the rhythmite section the coarse sand layer becomes less prominent and the clay layers thicken .sx Within the upper parts of Unit 3 the rhythmites give way to a more massive clay .sx The rhythmites described below were recorded from Section 4 ( Fig. 4 ) and are shown in detail in Fig. 5 .sx The exact thickness of each of the sand layers varies throughout the deposit , but generally their thickness decreases upwards from about 1 cm in the lowest rhythmites to less than 1 mm half way up the recorded section , before disappearing completely towards the top of the recorded section .sx figure&caption .sx The sand layers within the lower rhythmites display ripple cross-stratification , with individual foreset laminate often being less than 1 mm in thickness , although still exhibiting normal internal grading .sx Ripples reach a maximum length of 2.5 cm with recorded amplitudes of up to 6 mm ( Fig. 5) .sx The ripple forms are generally well-preserved due to the draping of the overlying silts .sx Climbing ripple sequences are also observed within the sands .sx The sands frequently display parting lineations , which show sorting of coarser and finer grains into ridges and troughs .sx These are interpreted as current lineations and , as such , are consistent with the palaeocurrent directions recorded from Unit 2 .sx The base of the sand layers frequently shows scour of the underlying bed to depths of 1-2 mm .sx Multiple graded beds of silt occur between the sand and clay layers , silt always appearing to form the major part of the rhythmite .sx The silt layers progressively decrease in thickness relative to that of the clay layers up through the sequence .sx The silt layers consist of normally graded beds with occasional sand partings , especially in the lower part of the sequence .sx The clay layers , where present at the base of Unit 3 , are thin ( 3 mm or less ) but gradually become thicker towards the top of the rhythmite section with recorded thicknesses of up to 10 mm .sx Scattered clasts of quartz and quartzite occur throughout Unit 3 .sx The largest recorded was 11 mm diameter .sx Typically , the clasts are less than 4 mm in maximum observed dimension .sx Structures including faulting , folding and dewatering structures , are present throughout the whole rhythmite section ( lower Unit 3) .sx Faulting is predominantly normal ( extensional ) and near vertical , with movement on the scale of millimetres , although reverse ( thrust ) faulting is also present .sx The deformation does not appear to extend upwards into the more massive clays ( upper Unit 3 ) and thus it is considered that the faulting is syndepositional .sx Additionally dewatering fissures are present with associated disruption of the surrounding sediment .sx Numerous trace fossils are present within Unit 3 , mainly in the form of short narrow burrows ( 4 mm long , 2 mm wide ) which often combine to form dendritic networks .sx The burrows are both horizontal , along bedding planes , and vertical , across multiple graded units and appear to have no preference as to grade of sediment within the rhythmite .sx ( i ) Interpretation .sx Unit 3 displays the rhythmitic dark/pale sequence recorded by Shotton ( 1953) .sx It is suggested that it is unlikely that the rhythmites represent annual glacial varves as interpreted by Shotton ( 1953 ) and it is considered more probable that they represent small scale turbidites .sx Douglas ( 1980 , p.282 ) proposed a similar interpretation for rhythmites within the Bosworth Clays and Silts of western Leicestershire based upon the presence of multiple graded silt beds and small-scale cross-bedding within the silt bands .sx During deposition of the lower part of Unit 3 the higher velocity flows were sufficient to transport sands , develop migratory bedforms and cause scour of the underlying beds .sx The presence of current lineations suggests flow velocities sufficiently great to form upper phase plane beds ( Leeder , 1982) .sx Such features have been recorded from within Pleistocene lacustrine sequences in Ireland ( Cohen , 1979) .sx The nature of sedimentation at the beginning of each rhythmite indicates an initial rapid increase in flow strength , followed later by a more gradual decline .sx This flow pattern is consistent with that found in turbidity currents and is similar to that responsible for the sands of Unit 2b .sx Thus these sediments are considered to be distal equivalents to the sands of Unit 2b .sx