See NS4178 : Auchenreoch Glen
for the context.
This nodule is lying in the bed of the burn; many similar examples can be seen there. They have eroded out of the steep southern side of the glen, where they occur in layers; those layers form part of a distinctive series of sedimentary deposits, known as the Ballagan Beds: for the details, see NS4178 : Ballagan Beds in Auchenreoch Glen
, which also shows some of these layers.
However, the stone is of more than geological interest. In the past, it was of economic importance in local agriculture. Ure's "General View of the Agriculture in the County of Dumbarton" (1794) contains a detailed discussion of the various rocks burned for lime in the county, and he describes in some detail the manner in which this process was carried out. He lists the three main kinds of limestone used in the county:
(1) The most familiar form of limestone, which is of marine origin (most of the stone of this kind that was used locally was quarried at Cumbernauld and East Kilpatrick).
(2) "Moor limestone", named after the situation in which it is usually found. This is described as having a course gritty surface, and as being "quite destitute of marine productions". This kind is evidently cornstone, a limestone of terrestrial origin: NS4480 : Cornstone outcrop
(3) "Cam-stone (glen-stone)", "mostly found in the bottom of glens" and "quite destitute of marine productions. It contains a considerable quantity of clay" and "lies in thin strata embedded in till. Some natural sections in the sides of glens, in the parish of Dumbarton, exhibit to one view more than a dozen of these strata from three inches to a foot in thickness". This a good description of the Ballagan Beds seen here and elsewhere (see the link in the second paragraph).
It is clear from these descriptions that the last-named type of stone is cementstone, of the kind shown in my photograph; it is an impure limestone that was precipitated out during extended periods of drought. The work just cited mentions that a great deal of it was burnt "for manure" (in other words, for treating fields), but notes one peculiarity of this stone, namely, that it must be slaked while still red-hot in the kiln; if allowed to cool slowly, it will not break up into lime. Several people with pitchers would be employed in pouring water into the kiln, which would emit "very loud explosions" during this operation. Lime-kilns were built near streams in order to provide ready access to water; see Link
for the remains of various local lime-kilns.