page_title: British anti-invasion preparations of the Second World War
section_title: Lines and islands
hierarchical_section_title: British anti-invasion preparations of the Second World War / Field fortifications / Lines and islands
caption_attribution_description: Near other fortifications. Just north of Waverley Mill Bridge. On nature reserve, accessible. OS reference SU869457 Photographed on 01-May-06.
context_page_description: British anti-invasion preparations of the Second World War entailed a large-scale division of military and civilian mobilisation in response to the threat of invasion by German armed forces in 1940 and 1941. The British Army needed to recover from the defeat of the British Expeditionary Force in France, and 1.5 million men were enrolled as part-time soldiers in the Home Guard. The rapid construction of field fortifications transformed much of the United Kingdom, especially southern England, into a prepared battlefield. Sea Lion was never taken beyond the preliminary assembly of forces. Today, little remains of Britain's anti-invasion preparations; only reinforced concrete structures such as pillboxes and anti-tank cubes are commonly found.
context_section_description: The primary purpose of the stop lines and the anti-tank islands that followed was to hold up the enemy, slowing progress and restricting the route of an attack. The need to prevent tanks from breaking through was of key importance. Consequently, the defences generally ran along pre-existing barriers to tanks, such as rivers and canals; railway embankments and cuttings; thick woods; and other natural obstacles. Where possible, usually well-drained land was allowed to flood, making the ground too soft to support even tracked vehicles.
  Thousands of miles of anti-tank ditches were dug, usually by mechanical excavators, but occasionally by hand. They were typically 18 feet (5.5 m) wide and 11 feet (3.4 m) deep and could be either trapezoidal or triangular in section with the defended side being especially steep and revetted with whatever material was available.
  Elsewhere, anti-tank barriers were made of massive reinforced concrete obstacles, either cubic, pyramidal or cylindrical. The cubes generally came in two sizes: 5 or 3.5 feet (1.5 or 1.1 m) high. In a few places, anti-tank walls were constructed – essentially continuously abutted cubes.
  Large cylinders were made from a section of sewer pipe 3 to 4 feet (91 to 122 cm) in diameter filled with concrete typically to a height of 4 to 5 feet (1.2 to 1.5 m), frequently with a dome at the top. Smaller cylinders cast from concrete are also frequently found.
  Pimples, popularly known as Dragon's teeth, were pyramid-shaped concrete blocks designed specifically to counter tanks which, attempting to pass them, would climb up exposing vulnerable parts of the vehicle and possibly slip down with the tracks between the points. They ranged in size somewhat, but were typically 2 feet (61 cm) high and about 3 feet (91 cm) square at the base. There was also a conical form.
  Cubes, cylinders and pimples were deployed in long rows, often several rows deep, to form anti-tank barriers at beaches and inland. They were also used in smaller numbers to block roads. They frequently sported loops at the top for the attachment of barbed wire. There was also a tetrahedral or caltrop-shaped obstacle, although it seems these were rare.
  Where natural anti-tank barriers needed only to be augmented, concrete or wooden posts sufficed.
  Roads offered the enemy fast routes to their objectives and consequently they were blocked at strategic points. Many of the road-blocks formed by Ironside were semi-permanent. In many cases, Brooke had these removed altogether, as experience had shown they could be as much of an impediment to friends as to foes. Brooke favoured removable blocks.
  The simplest of the removable roadblocks consisted of concrete cylinders of various sizes but typically about 3 feet (0.91 m) high and 2 feet (61 cm) in diameter; these could be manhandled into position as required. However, these would be insufficient to stop armoured vehicles. One common type of removable anti-tank roadblock comprised a pair of massive concrete buttresses permanently installed at the roadside; these buttresses had holes and/or slots to accept horizontal railway lines or rolled steel joists (RSJs). Similar blocks were placed across railway tracks because tanks can move along railway lines almost as easily as they can along roads. These blocks would be placed strategically where it was difficult for a vehicle to go around – anti-tank obstacles and mines being positioned as required – and they could be opened or closed within a matter of minutes.
  There were two types of socket roadblocks. The first comprised vertical lengths of railway line placed in sockets in the road and was known as hedgehog. The second type comprised railway lines or RSJs bent or welded at around a 60° angle, known as hairpins. In both cases, prepared sockets about 6 inches (152.40 mm) square were placed in the road, closed by covers when not in use, allowing traffic to pass normally.
  Another removable roadblocking system used mines. The extant remains of such systems superficially resemble those of hedgehog or hairpin, but the pits are shallow: just
caption: A section of the River Wey incorporated into GHQ Line as an anti-tank barrier
exif: {"Image Make": "FUJIFILM", "Image Model": "FinePixA204", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Digital Camera FinePixA204    Ver1.00", "Image DateTime": "2006:06:01 12:46:19", "Image YCbCrPositioning": "Co-sited", "Image Copyright": "", "Image ExifOffset": "252", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail Orientation": "Horizontal (normal)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "1240", "Thumbnail JPEGInterchangeFormatLength": "10319", "Thumbnail YCbCrPositioning": "Co-sited", "EXIF ExposureTime": "1/220", "EXIF FNumber": "7/2", "EXIF ExposureProgram": "Program Normal", "EXIF ISOSpeedRatings": "100", "EXIF ExifVersion": "0220", "EXIF DateTimeOriginal": "2006:06:01 12:46:19", "EXIF DateTimeDigitized": "2006:06:01 12:46:19", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "13/5", "EXIF ShutterSpeedValue": "39/5", "EXIF ApertureValue": "18/5", "EXIF BrightnessValue": "27/4", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "18/5", "EXIF MeteringMode": "Pattern", "EXIF LightSource": "Unknown", "EXIF Flash": "Flash did not fire, auto mode", "EXIF FocalLength": "6", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "1600", "EXIF ExifImageLength": "1200", "Interoperability InteroperabilityIndex": "R98", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "EXIF InteroperabilityOffset": "1092", "EXIF FocalPlaneXResolution": "3077", "EXIF FocalPlaneYResolution": "3077", "EXIF FocalPlaneResolutionUnit": "3", "EXIF SensingMethod": "One-chip color area", "EXIF FileSource": "Digital Camera", "EXIF SceneType": "Directly Photographed", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF SceneCaptureType": "Standard", "EXIF Sharpness": "Normal", "EXIF SubjectDistanceRange": "0"}