APPARATUS FOR DETERMINING THE ORIENTATION OF A CORE SAMPLE TECHNICAL FIELD This invention relates to core drilling and more particularly to apparatus for determining the orientation of a core sample after it has been drilled, separated and extracted from the drill site. BACKGROUND ART Many forms of geological survey depend on core drilling for samples. Some surveying requires that the orientation of a core be determined accurately and reliably. In many situations the angle of the longitudinal axis of the core drill relative to the plane of the earth at the drill site is other than 90°. This is the drilling angle and it may be ascertained in a number of ways. It is sometimes also important to determine, after removing the core for inspection, the rotational orientation or compass orientation of the core sample relative to the surrounding terrain from which it was extracted. A core drill usually comprises an outer tube and an inner tube. The outer tube is rotated and transmits rotational power to a cutting head. A drill bit on the cutting head forms a core which rises up through the inner tube as the drill progresses into the earth or substrate. The inner tube and separated core may be lifted up through the outer tube by lowering a latching body through the outer tube and latching onto a spindle carried by the upper end of the inner tube. The latching body is lowered on a steel cable and engages the upper end of the spindle. Tension on the rod string is translated, by a core lifter within the drill, into a compressive force which acts to fracture the core and separate it from the substrate. The separated core can then be lifted, within the inner tube, by the cable attached to the upper end. DISCLOSURE OF INVENTION According to one aspect of the invention there is provided an upper end for the inner tube of a core drill which includes:- An upper end for the inner tube of a core drill which includes:- (i) a spindle having an upper end and a lower end which carries a shut-off valve and a bearing, (ii) a stub shaft having an upper end and a lower end, the upper end being coupled to the lower end of the spindle with the bearing being within the upper end of the stub shaft, (iii) a centre housing having an upper end and a lower end, the upper end being coupled to the lower end of the stub shaft, (iv) a washer housing beneath the centre housing, (v) a spring between the washer housing and the centre housing, (vi) an impressionable washer at the lower end of the washer housing, (vii) a race track beneath the washer housing having an upwardly facing circumferential track groove, and (viii) a ball in the track facing the washer which, when the upper end is tensioned so as to overcome the bias of the spring, makes an impression on the washer.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an exploded view of the upper portion of the upper end of a core drill inner tube according to one embodiment of the invention, Fig. 2 is an exploded view of the central portion of the upper end of the core drill inner tube, and Fig. 3 is an exploded view of the lower portion of the upper end of the core drill inner tube. BEST MODE FOR CARRYING OUT THE INVENTION The upper end 10 of a core drill inner tube shown in Figs. 1 to 3 has a spindle 11 having at its upper end a threaded portion 12 by means of which the upper end is connected to a latching body when the inner tube is to be raised. Beneath the threaded portion 12 there is a hub 13 and shoulder 13a and beneath the shoulder 13a there is a first shut-off valve 14, a first shut-off valve washer 15, a second shut-off valve 16, a second shut-off valve washer 17, a thrust bearing 18, a spindle bearing 19, having an inner bronze bush 19a, a hanger bearing 20, a nyloc nut 21 and a bronze bush 22 (see Fig. 2) which receives the lower end 23 of the spindle 11. Beneath the spindle 11 there is an extended stub shaft 24 which has an inner recess 25 which receives the bronze bush 22 and an outer recess 26 which receives the hanger bearing 20 and the nyloc nut 21. The stub shaft 24 has a shoulder 27, a threaded end 28, a major portion 29 and a minor portion 30. Located on the stub shaft 24 above the shoulder 27 there is a circlip 31 , thin bronze bushes 32, seals 33, a large bronze bush 38 and a smaller bronze bush 34. The smaller bronze bush 34 seats in the inner recess 35 formed in the centre housing 36 through which the major and minor portions 29 and 30 of the stub shaft 24 pass. The centre housing 36 has an outer recess 37 which receives the bushes 32, seals 33 and the larger bronze bush 38. On the lower side of the centre housing 36 there is an inner recess 39 and an outer recess 40 which is threaded internally to receive the threaded exterior 42 of the washer housing 41 which has an upper recess 43 and a lower recess 44. A compression spring 45 has an upper end which seats in the recess 39 in the centre housing 36 and a lower end which seats against a nyloc nut 47 in the recess 43. The lower recess 44 houses a seal 48, bronze bush 49 and a circlip 50. Indent washer 51 seats on the lower periphery of the recess 44 and is biased away from the race track 52 by a separation spring 53. The race track nut 54 is threaded onto the threaded end 28 of the stub shaft 24. The race track 52 has a groove 55 in which runs a non-magnetic ball (not shown) that is adapted to make an orientation indentation in the aluminium indent washer 51. The lower portion of the upper end shown in Fig. 3 includes a fixed tube cap 56 having an upper recess 57 in which the race track nut 54 sits and a lower recess 58 threaded to engage the inner tube of the core drill. The fixed tube 56 may be replaced by a cap 60, lock nut 61 and tube cap 62. When assembled, the first shut off valve 14 abuts against the shoulder 13a, the thrust bearing 18 abuts against the second shut off valve 17 and the thrust bearing 18 seats inside the recess 19b of the spindle bearing 19. The hanger bearing 20 abuts against the end face 19c of the threaded end portion 19d of the spindle bearing 19 and is held in place by the nyloc nut 21 which is screwed onto the threaded portion 11 a on the lower part 11 b of the spindle 11. The interior of the outer recess 26 of the stub shaft 24 has a threaded portion 26a which engages the threaded end portion 19d of the spindle bearing 19. When a core is being separated by raising the rod string, the separation spring 53 is compressed as the race track 52 is forced towards the indent washer 51. Before contact, the ball in the groove 55 rolls to the lowest point in the groove 55. Eventually, the ball becomes trapped between the groove 55 and the indent washer 51. Further tension in the rod string causes the ball to be driven into the surface of the indent washer 51. This action makes an impression or indent in the washer 51 and the indent is known to be formed at the time the core is separated. When the drill is other than absolutely vertical, the angular (or compass) position of the impression marks the bottom or lowest (vertical) point in the groove 55 and therefore in the plane of the washer 51. Because the indent washer 51 is pinned to the housing 41 , its orientation relative to the housing 41 is known. Thus, the impression can later be related to a reference mark or groove formed on the exterior of the upper end. An advantage of the upper end of the inner tube of the core drill shown in the drawings is that it is a simple matter to adjust and replace the rubber shut-off valves 14 and 16 as all that need be done is to disconnect the stub shaft from the spindle bearing 19 which provides access to the nyloc nut 21. Another advantage is that the tension of the spring 45 can be easily adjusted in the lower portion of the inner end without compressing or slackening the pressure on the rubber shut-off valves 14 and 16. The spring tension is set to put a light landing mark on the disc which allows for the landing mark to always to be clearly recognisable to eliminate confusion. Various modifications may be made in details of design and construction without departing from the scope and ambit of the invention.