dirty dot meaning in English

（较小的）污点

I had remarked on one side of the road , at intervals of six or seven yards , a line of upright stones , continued through the whole length of the barren : these were erected , and daubed with lime on purpose to serve as guides in the dark ; and also when a fall , like the present , confounded the deep swamps on either hand with the firmer path : but , excepting a dirty dot pointing up here and there , all traces of their existence had vanished : and my companion found it necessary to warn me frequently to steer to the right or left , when i imagined i was following , correctly , the windings of the road
我曾注意到在路的一边，每隔六七码就有一排直立的石头，一直延续到荒原的尽头。这些石头都竖立着，涂上石灰，是为了在黑暗中标志方向的也是为了碰上像现在这样的一场大雪把两边的深沿和较坚实的小路弄得混淆不清时而设的。但是，除了零零落落看得见这儿那儿有个泥点以外，这些石头存在的痕迹全消失了。