In this pictograph of a mountain, the height is much greater than the width. Hence, it is an ideograph for the adjective high.
Illustrative Text Reference:
Harappa: Seal impression: J-228 A: Madho Sarup Vats, 1940: Excavations at Harappa: Being an Account of the Excavations at Harappa carried out between the Years 1920-21 and 1933-34: Volume II: Plate C: Sealings: Image 689: Government of India Press, Calcutta.
Image Credit:
Indus Script Sign Number 2290: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.