As explained at
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Both of these stones are visible in this photograph: the inscription is on the left, and the coat of arms is on the right. See also:
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By 1922, both of those stones had become so worn by the elements as to be almost indecipherable, but the County Council had them cleaned in that year and placed in a higher and more prominent part of the bridge [Eunice G. Murray, "The Church of Cardross and its Ministers" (1935)].
This bridge crosses the rather small Auchenfroe Burn, and only these old stones and the iron railings distinguish what now otherwise looks much the same as the adjacent sections of road. The Auchenfroe Burn is "formed by the union of the Kilmahew and Wallacetown Burns. When it joins the Clyde it is called Cardross Burn" [David Murray, "Old Cardross" (1880); this David Murray was the father of the Eunice G. Murray cited above]. On the naming of the burn here, and on the approach to the shore, see also
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For the story behind the bridge, see, for example "Cardross: The Village in Days Gone By" (Arthur F. Jones, 1985). As the tale is usually recounted, the young Jean Watson took a piece of meat for her destitute mother from the laird's store, not realizing that it had been set aside for the laird himself. She then fled, but had to stop at the Auchenfroe Burn, which was in spate; she is said to have vowed that, if ever she was able, she would build a bridge over the burn. After she had married a shipbroker named Moore, who subsequently prospered financially, she was able to fulfil her vow.
As the author notes, there are several variations of this story, which has a certain air of legend about it. However, what is beyond question is that a fund (later known as the Moore Mortification Fund) was set up by Jean Moore, née Watson, for the benefit of the poor of the parish who lived between the Auchenfroe Burn and Keppoch. See the work just cited for further details.