narrow furrow between two walls with different s of abrasion

canyon cave cavern cliff concept ecosystem eifel environment erosion europe exploration forest formation geologic geology green groove grotto high hiking hill land massif massive mountain nature old outdoor park pattern plateau ravine reserve rock rocky rough rural sandstone stone summer surface texture tourism travel valley wall wild wilderness woods narrow furrow two walls shapes abrasion huge moss-covered formations teufelsschlucht rocks background sunny day irrel germany