SIXTH SUPPLEMENTAL INDENTURE

Exhibit 4.28

SIXTH SUPPLEMENTAL INDENTURE

This SIXTH SUPPLEMENTAL INDENTURE (this “Supplemental Indenture”), dated as of December 2, 2013, among ACI Terminal, LLC, a Delaware limited liability company (the “Guaranteeing Subsidiary”), Arch Coal, Inc., a Delaware corporation (the “Company”), the other Guarantors (as defined in the Indenture referred to below) and UMB Bank National Association, as trustee under the Indenture referred to below (the “Trustee”).

WITNESSETH

WHEREAS, the Company, certain of the Guarantors and the Trustee have heretofore entered into an Indenture, dated as of June 14, 2011 (as amended, supplemented, waived or otherwise modified, the “Indenture”), providing for the issuance of the Company’s 7.000% Senior Notes due 2019 (the “2019 Notes”) and the Company’s 7.250% Senior Notes due 2021 (the “2021 Notes” and together with the 2019 Notes, the “Notes”); the related First Supplemental Indenture, dated as of July 5, 2011, among the Company, certain of the Guarantors and the Trustee; the related Second Supplemental Indenture, dated as of October 7, 2011, among the Company, certain of the Guarantors and the Trustee; the related Third Supplemental Indenture, dated as of July 2, 2012, among the Company, certain of the Guarantors and the Trustee; the related Fourth Supplemental Indenture, dated as of July 31, 2012, among the Company, certain of the Guarantors and the Trustee; and the related Fifth Supplemental Indenture, dated as of July 26, 2013, among the Company, certain of the Guarantors and the Trustee; and

WHEREAS, the Indenture provides that the Company shall cause any Restricted Subsidiary which becomes obligated to Guarantee the Notes, pursuant to the terms of Section 4.12 of the Indenture, to execute and deliver a supplemental indenture to the Indenture pursuant to which such Restricted Subsidiary shall Guarantee the obligations of the Company under each series of the Notes and the Indenture in accordance with Article Ten of the Indenture with the same effect and to the same extent as if such Restricted Subsidiary had been named in the Indenture as a Guarantor; and

WHEREAS, pursuant to Section 9.01 of the Indenture, the Company, the Guarantors and the Trustee are authorized to execute and deliver this Supplemental Indenture, without the consent of any Holder of the Notes.

NOW, THEREFORE, in consideration of the foregoing and for other good and valuable consideration, the receipt of which is hereby acknowledged, the parties mutually covenant and agree for the equal and ratable benefit of the Holders of the Notes as follows:

1.   CAPITALIZED TERMS.  Capitalized terms used herein without definition shall have the meanings assigned to them in the Indenture.

2.   AGREEMENT TO BE BOUND.  The Guaranteeing Subsidiary hereby becomes a party to the Indenture as a Guarantor and as such will have all of the rights and be subject to all of the obligations and agreements of a Guarantor under the Indenture.  The Guaranteeing Subsidiary agrees to be bound by all of the provisions of the Indenture applicable to a Guarantor,

including Article Ten thereof, and to perform all of the obligations and agreements of a Guarantor under the Indenture.

3.   GUARANTEE.  The Guaranteeing Subsidiary hereby agrees, on a joint and several basis with all the existing Guarantors, to fully and unconditionally Guarantee to each Holder of the Notes and the Trustee the Obligations pursuant to and as set forth in Article Ten of the Indenture on an unsecured senior basis.

4.   NO RECOURSE AGAINST OTHERS.  No past, present or future director, officer, employee, incorporator, stockholder, member, manager or partner of the Guaranteeing Subsidiary, as such, shall have any liability for any obligations of the Company, the Guaranteeing Subsidiary, or any other Guarantor, under the Notes, any Guarantee, the Indenture or this Supplemental Indenture or for any claim based on, in respect of, or by reason of, such obligations or their creation.  Each Holder of the Notes or any Guarantee by accepting a Note waives and releases all such liability.  The waiver and release are part of the consideration for issuance of the Notes and the Guarantee.

5.   NEW YORK LAW TO GOVERN.  THIS SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

6.   COUNTERPARTS.  The parties hereto may sign any number of copies of this Supplemental Indenture in counterparts, all of which together shall constitute one and the same agreement.

7.   EFFECT OF HEADINGS.  The Section headings in this Supplemental Indenture are for convenience of reference only and shall not be deemed to alter or affect the meaning or interpretation of any provisions hereof.

8.   RATIFICATION OF INDENTURE; SUPPLEMENTAL INDENTURES PART OF INDENTURE.  Except as expressly amended hereby, the Indenture is in all respects ratified and confirmed and all the terms, conditions and provisions thereof shall remain in full force and effect.  This Supplemental Indenture shall form a part of the Indenture for all purposes, and every Holder of Notes heretofore or hereafter authenticated and delivered shall be bound hereby.  The Trustee makes no representation or warranty as to the validity or sufficiency of this Supplemental Indenture or with respect to the recitals contained herein, all of which recitals are made solely by the other parties hereto.

[SIGNATURE PAGES FOLLOW]

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IN WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture to be duly executed as of the date first above written.

Signature Page to Sixth Supplemental Indenture

Signature Page to Sixth Supplemental Indenture

Signature Page to Sixth Supplemental Indenture

Signature Page to Sixth Supplemental Indenture

Signature Page to Sixth Supplemental Indenture