Grim's Ditch: section N of Blythwood House


Heritage Category: Scheduled Monument

List Entry Number: 1002007

Date first listed: 11-Oct-1944


Ordnance survey map of Grim's Ditch: section N of Blythwood House
© Crown Copyright and database right 2018. All rights reserved. Ordnance Survey Licence number 100024900.
© British Crown and SeaZone Solutions Limited 2018. All rights reserved. Licence number 102006.006.
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This copy shows the entry on 16-Nov-2018 at 04:05:01.


The building or site itself may lie within the boundary of more than one authority.

County: Greater London Authority

District: Harrow (London Borough)

National Grid Reference: TQ 11504 90567


A 350m length of Grim’s Ditch running north-east from Montesole Playing Fields towards No.31 Blythwood Road.

Reasons for Designation

Linear boundaries are substantial earthwork features comprising single or multiple ditches and banks which may extend over distances varying between less than 1km to over 10km. They survive as earthworks or as linear features visible as cropmarks on aerial photographs or as a combination of both. The evidence of excavation and study of associated monuments demonstrate that their construction spans at least a millennium from the Middle Bronze Age, although they may have been re-used later. The scale of many linear boundaries has been taken to indicate that they were constructed by large social groups and were used to mark important boundaries in the landscape; their impressive scale displaying the corporate prestige of their builders. They would have been powerful symbols, often with religious associations, used to define and order the territorial holdings of those groups who constructed them. Linear earthworks are of considerable importance for the analysis of settlement and land use from the Bronze Age onwards; all well preserved examples will normally merit statutory protection.

Despite some disturbance in the past, the 350m length of Grim’s Ditch running north-east from Montesole Playing Fields towards No.31 Blythwood Road survives well. It will contain archaeological and environmental information relating to the earthwork and the landscape in which it was constructed.


This record was the subject of a minor enhancement on 25 March 2015. This record has been generated from an "old county number" (OCN) scheduling record. These are monuments that were not reviewed under the Monuments Protection Programme and are some of our oldest designation records.

The monument includes a 350m length of Grim’s Ditch, also known as the Grim’s Dyke, a prehistoric linear boundary surviving as an earthwork and archaeological remains. It is situated on a gentle south-east facing slope at Pinner Green.

The earthwork is denoted by a bank, up to 4m high, with a ditch to the south, about 23m wide and 2m deep. It takes an irregular course running north-east from Montesole Playing Fields for 170m, bearing ENE for 115m before resuming north-east for a further 65m towards Blythwood Road. Partial excavation along part of this length of Grim’s Ditch in 1957 recovered worked flint and Iron Age pottery sherds. OS Maps (1:2500) of 1876, 1896, 1913 and 1932 indicate that this length of Grim’s Ditch originally continued west to Pinnerhill Road whereby it took a southerly course. However these parts have since been destroyed by housing development in the 20th century.

The Grim’s Ditch of Harrow is a linear earthwork which originally ran from west of Cuckoo Hill to Harrow Weald Common, with a possible easterly continuation in the Pear Wood earthwork. Later developments along the route have truncated or destroyed several parts of it. It consists of a large bank with a ditch on the south side and is thought to have been constructed in two phases. The bank was first built from quarried gravel, sand and clay. The ditch was then dug and some of the spoil deposited to the south. It is thought to be Iron Age in origin, although no conclusive dating evidence has been obtained by excavation. Documentary evidence shows that Grim’s Ditch was certainly in existence by 1306. The original purpose of the earthwork is uncertain but it most likely served to demarcate territorial boundaries.


The contents of this record have been generated from a legacy data system.

Legacy System number: LO 76

Legacy System: RSM - OCN


Greater London SMR 052160/00/00. NMR TQ19SW7, TQ19SW6, LINEAR23. PastScape 398424, 398423, 1043166

End of official listing