15Brennan, Martin. The Stones of Time: Calendars, Sundials, and Stone Chambers of Ancient Ireland. Rochester, VT: Inner Traditions, 1994. 46-8.
Many authors have referred to the decorated stone at Cairn X1 as the "Calendar Stone." This stone may be seen on our webpage in Du Noyer's illustration. One writer presents the statistics of passage tomb art and astronomical observations: "On a national scale, just 138 known passage tombs (64%) have extant passages, of which sixteen (11%) exhibit evidence for solstice alignment, being equally divided between the summer and the winter solstice. There is a strong association between solar alignments and the location of megalithic art in passage tombs. Approximately 35 (16%) passage tombs are decorated, of which about twelve exhibit kerb art, including five with decorated entrance stones, all in the Boyne Valley: Knowth 1 (both tombs), Knowth 13, Knowth 15, Newgrange and Dowth South. For that reason, artwork on the entrance kerbstone of site Xl at Loughcrew would place this site in exceptional company. It should also be noted that the rayed design featured on the decorated orthostat at site Xl is known elsewhere only at Knowth and Newgrange, occurring a number of times in both complexes, most spectacularly on kerbstone 15 at Knowth. (O'Sullivan, Muiris, Frank Prendergast, and Geraldine Stout. "An Intriguing Monument." Archaeology Ireland 24.1 (2010): 22.)