| Computer Code: |
MR |
Preferred Map Code: |
MR |
| Status Code: |
Full |
| Age range: |
Cenomanian Age (KE)
— Turonian Age (KT) |
| Lithological Description: |
Hard to very hard off-white, blocky fractured chalk with numerous nodular chalk beds and thin anastomosing marls. |
| Definition of Lower Boundary: |
The lower boundary is conformable on the highest marl seam of the Plenus Marls Member. This is the Foyle Marl of Mortimore (1986). |
| Definition of Upper Boundary: |
The upper boundary is conformable but difficult to place in the "Transitional" Province where it is usually considered to be at a horizon of less hard flaggy chalks with significant anastomosing marls. This is probably equivalent to the Meads Marls sequence of the Southern Province and Mortimore (1986) places the boundary at the Meads Marl 1 in Sussex. |
| Thickness: |
About 3 m in the classic Chiltern area but noted in the Hitchin Memoir to be between 2 and 7 m (Hopson et al., 1996). A range of 2.7 to 4 m is given in Mortimore (1986). |
| Geographical Limits: |
Known throughout the Southern and Transitional provinces and forms the most readily identifiable positive geomorphological features and associated hard field brash in unexposed ground. |
| Parent Unit: |
Holywell Nodular Chalk Formation (HCK)
|
| Previous Name(s): |
Melbourn Rock [Obsolete Name and Code: Use MR]
(-1708)
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
Ashwell. Full sequence from the Plenus Marls to well above the Melbourn Rock can be seen and was described in the Hitchin memoir (Hopson et al., 1996, p.47). Substitute type section as the original stratotype is obscured below an industrial site. The Melbourn Lime-kiln Pit and a second "a quarter of a mile distant", both at the southern end of Melbourn. The type site is in a car park beneath a modern industrial building developed in the old (?)lime-kiln pit and is now in poor condition and the full sequence cannot be seen. |
| Reference(s): |
| Hopson, P M. 2005. A stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland, with statements on the Chalk of Northern Ireland and the UK Offshore Sector. British Geological Survey Research Report RR/05/01 102pp. ISBN 0 852725175 |
| Hill, W and Jukes-Browne, A J. 1886. The Melbourn Rock and the Zone of Belemnitella plena from Cambridge to the Chiltern Hills. Quarterly Journal of the Geological Society of London, Vol.42, 216-231. |
| Hill, W. 1886. On the beds between the Upper and Lower Chalk of Dover and their comparison with the Middle Chalk of Cambridgeshire. Quarterly Journal of the Geological Society of London, Vol.42, 232-248. |
| Hopson, P M, Aldiss, D T and Smith, A. 1996. The geology of the country around Hitchin. Memoir of the British Geological Survey, Sheet 221 (England and Wales). |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
|
E129
E130
E145
E175
E188
E189
E204
E205
E206
E220
E221
E237
E238
E252
E253
E254
E255
E327
|
