Top right - a sleeper or corpse.
Bottom right - 'The Toilet' in the primitive conditions immediately behind the lines men wash with tin cans and mend their clothes (or check for fleas and lice). The British Army was stingent in ensuring a continual rotation among the infantrymen who only spent a few days in the front lines before moving to the rear for training or rest and recouperation. The Royal Garrison Artillery operating the larger guns may have been further behind the lines but they could be constantly in action for far longer periods before being relieved due to the eternal shortage of big guns. Carrick must have had a fairly hard war which perhaps may have contributed to his early retirement. The fact that he produced only one sketch book in the Summer of 1918 suggests that he was, as an ordinary gunner, simply too busy to sketch and draw in 1916 and 1917. Perhaps it was only once he was elected an Associate of the Royal Scottish Academy while still in the trenches in 1918 that he may also have found it easier to gain permission to make such sketches despite his humble rank.
For those interested there are more of Carrick's wartime sketches in the 'Castle Rock - Royal Artillery' section of this site.