Help please as to what was/is the purpose of these stone pieces at Golden Acre Park?

[Leodian]

During a walk at Golden Acre Park (GAP) on November 14 2016 I noticed some stones firmly fixed in a large piece of rock at the old weir at the dam end of the lake at GAP. I wonder if anyone knows what their purpose was/is? They might have been put there to deter people from getting on top of the rock but there seems no obvious reason for a deterrent there as there does not seem to be anything below (unless the rock covers something, but there seemed to be no obvious sign of that). The first photo shows the stones location at what will likely be part of the weir that remains (note the vertical groove that I wonder might have held a 'gate' to control water level). The second photo looks down on the stones. The third shows the location by the footbridge at that end. Old maps label the site as a 'weir'.

AtOldWeirDamEndGoldenAcreParkLeedsNov142016(1).jpg (188.08 KiB) Viewed 7200 times

AtOldWeirDamEndGoldenAcreParkLeedsNov142016(2).jpg (181.55 KiB) Viewed 7200 times

AtOldWeirDamEndGoldenAcreParkLeedsNov142016(3).jpg (213.34 KiB) Viewed 7200 times

[sparky415]

I've walked past that hundreds of times. One assumes it's linked to the bridge, but there is no stream there....at one time (before the war) the lake was far bigger and this was probably some sort of sluice to regulate the level..