Diverticulosis of the colon. Endoscope view of diverticula in the wall of the sigmoid colon, the part of the large intestine that joins to the rectum. Diverticula are small pouches (dark) in the lining of the colon. Their formation is thought to be linked to a low-fibre diet and they are very common in the elderly. Usually they cause no symptoms, although some may cause abdominal cramps and constipation or diarrhoea.